Animal Diversity- I
(Non-Chordates)
Phylum Platyhelminthes
Ranjana Saxena
Associate Professor,
Department of Zoology,
Dyal Singh College,
University of Delhi
Delhi – 110 007
e-mail: ranjanasaxena1957@gmail.com
Contents:
PLATYHELMINTHES
DUGESIA (EUPLANARIA)
Fasciola hepatica
SCHISTOSOMA OR SPLIT BODY
Schistosoma japonicum
Diphyllobothrium latum
Echinococcus granulosus
EVOLUTION OF PARASITISM IN HELMINTHES
PARASITIC ADAPTATION IN HELMINTHES
CLASSIFICATION
Class Turbellaria
Class Monogenea
Class Trematoda
Class Cestoda
PLATYHELMINTHES
IN GREEK:PLATYS means FLAT; HELMINTHES means WORM
The term platyhelminthes was first proposed by Gaugenbaur in 1859 and include all flatworms.
They are soft bodied, unsegmented, dorsoventrally flattened worms having a bilateral
symmetry, with organ grade of organization. Flatworms are acoelomate and triploblastic.
The majority of these are parasitic. The free living forms are generally aquatic, either marine or
fresh water. Digestive system is either absent or incomplete with a single opening- the mouth,
anus is absent. Circulatory, respiratory and skeletal system are absent. Excretion and
osmoregulation is brought about by protonephridia or flame cells. Ammonia is the chief
excretory waste product. Nervous system is of the primitive type having a pair of cerebral
ganglia and longitudinal nerves connected by transverse commissures. Sense organs are poorly
developed, present only in the free living forms. Basically hermaphrodite with a complex
reproductive system. Development is either direct or indirect with one or more larval stages.
Flatworms have a remarkable power of regeneration. The phylum includes about 13,000
species.
Here Dugesia and Fasciola hepatica will be described as the type study to understand the
phylum. Some of the medically important parasitic helminthes will also be discussed. Evolution
of parasitism and parasitic adaptations is of utmost importance for the endoparasitic
platyhelminthes and will also be discussed here.
Dugesia (Euplanaria)
HABIT AND HABITAT:
Dugesia is a free living inhabitant of cool and clear water of freshwater ponds, lakes, streams
and shallow water rivers. They are gregarious i.e. they live in groups attached to the
undersurface of leaves, logs, rocks and other debris during day-time. They become active in
dark. Dugesia are worldwide in distribution.
MORPHOLOGY:
Body of Dugesia is thin, flattened, leaf-like and oval with a definite polarity. A full grown
Dugesia measures about 50mm in length and is greyish, brownish, or blackish in color. The
dorsal surface is darker in color than the ventral surface. Ventral surface is covered with cilia
that helps in locomotion. However, a narrow strip all along the margin of the ventral surface is
non-ciliated and is known as the adhesive zone. It helps in adhesion. The anterior end of the
body is differentiated into a broad, blunt and triangular head that bears two lateral projections
called the auricles. Present on the mid-dorsal line of the head are two black eye spots (Fig. 1).
A small neck like constriction separates the head from the main body. Mouth is present in the
middle of the body, on the mid-ventral surface. In sexually mature worms the genital aperture
is present on the ventral surface a little behind the mouth. Numerous microscopic excretory
apertures are situated on the dorsal surface (Fig.1).
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BODY-WALL:
The body-wall is made up of an outer epidermis and inner muscle layer. The two layers are
separated by a basement membrane. The space between the muscle layer and gut is filled up
with parenchymal cells (Fig. 2).
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EPIDERMIS: The epidermis is made up of a single layer of large cuboidal epithelial cells. It is
ciliated on the ventral surface. Interspersed in between the epidermal cells are sensory cells,
adhesive glands and mucus gland cells. The mucus gland cells provide a mucus coating,
forming a slime track on which the animal crawls. They are more abundant on the ventral
surface in the anterior part of the body. Adhesive gland cells secrete a sticky substance that
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helps in the attachment of the body to the substratum, cementing of eggs and capturing the prey.
Both mucus and adhesive gland cells are seated deep in the mesenchyme and have long narrow
ducts which pass through the muscle layer, and the basement membrane and finally open on the
surface of the epidermis. Present in the epidermal cell, mostly on the side, are many rod shaped
hyaline bodies known as rhabdites. Rhabdites are secreted by rhabdite gland cells present
below the epidermis (Fig. 2). The exact function of rhabdites is not known but it is believed that
they help in capturing the prey, in locomotion and give protection to the body. When the
rhabdites are discharged they come in contact with water and swell and form a thick, opaque
adhesive layer around the body which gives protection to the animal.
BASEMENT-MEMBRANE: Just beneath the epidermis is a thin structureless basementmembrane. The basement membrane not only provides surface for the attachment of epidermal
cells, but it also acts as a partition between the epidermis and the muscle-layer. The basementmembrane bears pigments and helps in maintaining the general form of the body. Basement
membrane serves as the elastic membrane.
MUSCLE –LAYER: Beneath the basement-membrane is the muscle layer. It consists of outer
circular muscle fibres and inner longitudinal muscles. Also present are oblique or diagonal
fibres arranged in a vertical manner. The longitudinal muscles on the ventral surface are more
strongly developed than on the dorsal surface. Dorsoventral muscles are present between the
dorsal and ventral surface.
PARENCHYMA OR MESENCHYME: Lying between the muscular layer and the alimentary
canal is the parenchymatous tissue which are loose connective-tissue cells that act as a
packing material. Its fluid filled spaces provide turgidity to maintain the body form. It contains
free wandering amoeboid cells that remain in the formative state. These formative cells bring
about regeneration of damaged or lost parts. Mesenchymal cells also take up the circulatory
function by conducting food and other metabolic products from one part of the body to another.
LOCOMOTION:
Although Dugesia lives in water it does not swim but moves about by gliding. While gliding
the head is slightly raised. The cilia present on the ventral surface move in the backward
direction and help the organism to move forward over a slime track. The slime track is secreted
by the mucus gland cells present in the epidermis. The mucus affords a grip to the cilia and also
protects them from injury by the substratum.
Sometimes the animal also crawls. Crawling is brought about by the movement of the muscles.
Elongation of the body is brought about by the contraction of the circular and oblique muscles.
The anterior end of the body then gets firmly fixed onto the substratum by mucus. The
longitudinal muscles then contract and pull the animal forward. The longitudinal muscles
contract alternately on the right and left side of the body. Thus, the head also bends alternately
on the right and left as the animal moves forward in a wavy manner. Dugesia can change its
direction with the help of the oblique muscles.
DIGESTIVE SYSTEM:
Dugesia has an incomplete digestive tract with a single opening – the mouth. Anus is absent in
them. Alimentary canal consists of mouth, pharynx and intestine (Fig. 3).
MOUTH: Mouth is a small oval aperture situated on the mid ventral surface of the body. In the
absence of the anus, the mouth serves the function of both ingestion and egestion.
PHARYNX: Mouth opens into a cylindrical pharynx through a small buccal cavity. The thick
walled muscular pharynx lies in a pharyngeal cavity or pouch bounded by a muscular sheath
called the pharyngeal sheath. The pharynx can be everted out through the mouth and helps in
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feeding. When in retracted condition the pharynx remains enclosed in the muscular
sheath.
INTESTINE: The pharynx leads into the intestine which divides into three branches. One of
these branches run forward along the middle line upto the head and the other two run
backwards upto the posterior end. All the three branches give off numerous lateral ramifying
branches. Thus, the intestine with its ramifying branches form a network and occupy a major
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part of the body. All the branches end blindly as there is no anal aperture. The much branched
intestine increases the surface area for the digestion, absorption and distribution of food.
Columnar epithelial cells line the inner walls of the intestine.
FEEDING AND DIGESTION:
FOOD: Dugesia is carnivorous. It feeds on dead or living organisms, mostly the crustaceans,
worms, insect larvae and snails. It can also subsist on body fragments of larger animals, living
or dead.
INGESTION: Dugesia can detect its prey from some distance with the help of chemoreceptors
present on the sides of the head. On detecting the food, it moves towards it, creeps over it with
the head slightly raised and entangles the prey in slimy secretions of mucus glands and
rhabdites. It now holds the anterior end of its body over the prey and immobilizes it. Pharynx is
then everted through the mouth which encloses the food. The food is then ingested by the
peristaltic action of the pharyngeal wall. Smaller prey is ingested as such, while the larger ones
are first broken down into smaller particles by the pumping action of the pharynx aided by the
digestive juices secreted by the pharyngeal glands and then ingested.
DIGESTION: Digestion is both extracellular and intracellular. In the pharynx, the food is
broken down by the pumping action of the pharynx and is then acted upon by extruded
digestive juices. The digestive juices include pharyngeal enzymes and endopeptidase enzymes
of gland cells of the intestine. This is extracellular digestion. Partially digested and liquefied
food is then pumped into the intestine by peristaltic action. Intracellular digestion takes place
in the phagocytic cells lining the intestine. Digested food diffuses through the walls of the
intestine into the mesenchyme. Mesenchyme helps to distribute the digested food to all parts of
the body in the absence of the circulatory system. Reserved food is stored in the form of fat and
sometimes as protein globules in the epithelial cells of the intestine. Undigested food is egested
through the mouth as there is no anus.
Dugesia can live without food for long periods. During this period they obtain their
nourishment by dissolving their reproductive organs, parenchyma and muscles. The missing
body parts are regenerated when they start feeding again.
RESPIRATION:
Special respiratory organs are lacking. Exchange of respiratory gases takes place through the
general body surface by diffusion.
EXCRETORY SYSTEM:
Excretion is brought about by specialized cells called flame cells or protonephridia. The
excretory system consists of two networks of longitudinal canals running throughout the
length of the body, one on each side. These open to the outside by several minute pores called
nephridiopores present on the dorsal surface. Each pair of trunk is coiled and is connected to
one another by transverse vessels in the head region. Each longitudinal net work gives off
many branches which in turn divide into extremely fine capillaries (Fig. 4A). Many of these
capillaries bear flame cells or flame bulb at its tips. Flame cells are modified mesenchymal
cells. Each flame cell is large and gives out numerous branched protoplasmic processes
(resembling pseudopodia) in the surrounding mesenchyme. In the centre of the cell is a
conspicuous bulbous cavity or cell lumen containing a bunch of cilia whose beating motion
gives the cell a flickering appearance like a candle flame and hence the excretory cells are
known as flame cells (Fig. 4B). Each cilia arises from a basal granule. The cytoplasm is
appressed to one side and contains a rounded nucleus, some excretory globules and vacuoles.
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PHYSIOLOGY OF EXCRETION:
The excretory waste material enters the cavities of the flame cells from the mesenchyme by the
simple process of diffusion. The continuous beating of the cilia causes hydrostatic pressure
which keeps the fluid waste circulating through the longitudinal trunk. The walls of the lateral
excretory canals are also ciliated at least in parts which further keeps the fluid moving through
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them. As the fluid waste passes through the longitudinal excretory canals, useful substances are
reabsorbed by the walls of the canals, while the excretory waste material along with excess of
water is expelled out through the nephridiopores. A considerable amount of waste is also
expelled out through the mouth and general body surface.
NERVOUS SYSTEM:
Dugesia has a primitive type of centralized nervous system. It consists of a pair of cerebral
ganglia at the anterior end, a little behind the eyes. The two cerebral ganglia join to form a
bilobed brain. The brain is made up of connecting transverse fibres and nerve cells. Numerous
nerves extend forward and laterally from the brain to the head, eyes and auricles. Posteriorly,
the brain gives out two pairs of longitudinal nerve cords which run backwards giving rise to
numerous transverse branches to both the external and internal parts of the body. These lateral
transverse branches anastomose medially with branches of opposite sides to form transverse
commissures (Fig. 5A). This gives them a ladder- like appearance and hence the nervous
system is sometimes called as the ladder type of nervous system.
In addition to the centralized nervous system, Planaria also possesses a sub-epidermal nerveplexus like cnidarians.
Brain receives the stimuli from the sense organs and conveys them to different parts of the
body.
SENSE ORGANS: Sense organs consists of a pair of eyes, some scattered tangoreceptors,
auricular organs, and rheoreceptors.
EYES OR OCELLI: A pair of eyes are present as rounded dark spots near the anterior end on
the dorsal surface. It consists of a cup shaped pigment screen. Inside the cup are many sensory
cells that act as light receptors (Fig.5B). Spatial arrangement of pigment cells and light sensitive
cells render the animal capable of crude discrimination of direction of light. The pigment cup
serves as a shield and light can enter only through its opening to stimulate the photosensitive
expanded ends of sensory cells. The light stimulates the sensory cells which transmit the
impulse to the brain by optic nerve. The eye of Planaria lacks a lens and refractive apparatus.
No image formation takes place in them. They can only perceive the difference between light
and dark.
If the eyes are removed from Dugesia they can still react to light but slowly and less accurately.
This is because it possesses some light sensitive cells over the general body surface.
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AURICULAR ORGANS: A few sensory cells are found sunken in grooves or pits in the head
region and are called auricular organs. They are well provided with nerves, are devoid of
gland cells and rhabdites. These are organs for chemical sense. They help in detecting food and
directing the animal towards it.
TANGORECEPTORS: These are sensory cells, tactile in nature distributed abundantly on
ventral surface all over the body especially around the mouth, lateral margins and at the anterior
end. The sensory bristles of these cells project over the epidermis beyond the level of cilia and
give the sensation of touch.
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RHEORECEPTORS: Sensory cells sensitive to water currents. Dugesia is positively rheotactic
i.e. it travels against the water current with its head forward.
REPRODUCTIVE SYSTEM:
Dugesia reproduces both asexually and sexually. The reproductive organs develop temporarily
only during breeding season (during summer).
ASEXUAL REPRODUCTION: Asexual reproduction takes place by transverse binary fission.
Fission occurs when the animal has attained maximum size. The posterior end adheres firmly
while the anterior end moves forward so that a constriction appears behind the pharynx (Fig. 6).
The constriction gradually deepens, finally dividing the animal into two halves each of which
regenerates its missing parts.
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SEXUAL REPRODUCTION: Dugesia is hermaphrodite. Although hermaphrodite, cross
fertilization is a rule because of protandry ( male reproductive organs developing before the
female).
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MALE REPRODUCTIVE ORGANS:
Male reproductive system consists of testis, vasa efferentia, a pair of vas deferens and penis.
Numerous small and rounded testis are present on the right and left borders of the body. Each
testis opens into a small vasa efferentia which in turn opens into a vas deferens of its side. The
right and the left vas deferens unite at the middle of the body to form a median duct which
opens into a muscular penis. The median duct is swollen at the base of the penis to form a
seminal vesicle where the sperms are temporarily stored. The penis is thick walled, muscular,
copulatory organ consisting of two parts: A proximal part called penis bulb surrounded by
unicellular prostate glands and the distal part called penis papilla which is capable of
protruding through the gonopore (Fig. 8). The secretions of the prostrate gland serves to
stimulate the sperms into activity.
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FEMALE REPRODUCTIVE ORGANS:
It consists of ovaries, oviduct, vitelline glands and bursa copulatrix. A pair of small rounded
ovaries are situated at the anterior end of the body one behind each eye. Each ovary opens into
an oviduct. The proximal part of oviduct stores sperms which are received during cross
fertilization and is called vesicula seminis or seminal receptacle or bursa copulatrix. The two
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oviducts meet at the posterior and ventral part of the body to form a common oviduct which
opens into the genital atrium. Numerous small branching vitelline or yolk glands surround the
oviduct and open into it by fine vitelline ducts. The oviduct is thus actually an ovo-vitelline
duct. Yolk cells produced from the yolk glands pass into the oviduct. Numerous small cement
glands open into the common oviduct and genital atrium. The bursa copulatrix also opens into
the atrium. The genital atrium opens to the outside by a genital aperture situated on the ventral
side behind the mouth (Fig. 8).
COPULATION, FERTILIZATION AND DEVELOPMENT:
During copulation the two worms come together by their ventral surface facing in the same
direction (Fig. 9). The penis papilla of each worm dilates, protrudes through the gonopore into
the copulatory sac of the other. Copulation here means mutual exchange of sperms. Sperms
from the seminal vesicle of one worm are passed into the copulatory sac of the other where they
stay only for a short while. The Dugesia then separates and the sperms migrate to the oviduct
and reach seminal receptacle where they fertilize the ova as they descend from ovary. The
fertilization is thus internal. The fertilized egg passes down the oviduct and gets surrounded
by yolk cells secreted by yolk glands. Thus, a capsule or cocoon is secreted around the zygote
as it passes into the genital atrium. The cocoon is provided with an adhesive stalk secreted by
the cement glands. The egg capsule comes out of the body of the worm and gets attached to
stones, aquatic weeds etc. by their adhesive stalk. Development is direct without any larval
forms. Each zygote in the capsule develops into a young worm in about two weeks. Yolk cells
provide nourishment for the developing young ones within the cocoon. On maturation the
capsule wall ruptures and a young worm emerges out. The young one resembles the adult in all
respects except that it is smaller in size and is not sexually mature. They feed, grow and
develop into a mature adult.
Reproductive organs degenerate and disappear after each mating and the worm
reproduces asexually by fission. They again develop during the next breeding season.
During the breeding season, each worm may copulate many times and lay a succession of
capsules at intervals of few days.
FREGENERATION: Dugesia has a tremendous power of regeneration. Regeneration involves
two processes:
1. EPIMORPHOSIS: In this process, missing parts are formed.
2. MORPHOLLAXIS: It involves the adjustment and coordination between the old tissue and
regenerated tissues.
The process of regeneration occurs in the following manner:
If the animal body is cut into two or more pieces, each one will form the lost parts. A piece
from the middle will always regenerate a head towards its anterior end and tail towards its
posterior end. Each piece maintains its polarity. The metabolic activity is highest in head and
gradually decreases towards the tail end. Correspondingly, anterior end of each piece, having
greater metabolic activity regenerates the anterior part of the body and posterior end piece
having lesser metabolic activity regenerates the posterior part of the body (Fig. 7). This capacity
of regeneration from anterior to posterior end of the body due to a gradual difference in the
intensity of metabolic activity is called as the axial or metabolic gradient. Regeneration is
brought about by the free formative cells of the parenchyma. These cells migrate to the cut
surface and by repeated division produce new tissues. However, the regeneration is not always
perfect because at times the two halves fail to regenerate the sex organs and each grows into an
asexual individual. Also, if the anterior end of the animal is cut longitudinally into two or more
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parts, then each part would develop a new head, resulting in a heteromorph or many headed
monster.
Fasciola hepatica
L.,Fasciola means a small bandage; Gr., Hepar means liver
Commonly known as sheep liver fluke. It is also known as Distomum hepatica. It was the first
trematode to have been discovered by de Brie in 1379. Its life cycle was first worked out by
Thomas in 1883. It is the largest and commonest liver fluke that lives as an endoparasite in the
bile duct of sheep. It causes the economically important disease liver rot in sheep.
MORPHOLOGY:
Soft, oval, dorsoventrally flattened leaf like body about 15-30mm long and 10-15mm broad,
green or brown in color. Anterior end of the body has a conical projection known as the oral
cone or cephalic cone or the head lobe. The body widens in the middle and slowly tapers
towards the posterior end. There are two suckers: An anterior sucker or oral sucker situated
at the tip of the oral cone and a posterior sucker or acetabulum situated midventrally near the
broad portion of the body. Both the suckers are organ of attachment. They attach to the body
of the host by vaccum. The oral sucker along with the mouth also helps in the ingestion of the
food. Situated in the middle of the oral sucker is the mouth from the margin of which radiate
out muscles to the periphery of the oral sucker. A little in front of the acetabulum on the
midventral side is present a small genital aperture or gonopore. During breeding season, a
temporary opening known as opening of the laurers canal appears on the dorsal surface, in the
middle 1/3rd region of the body. A single excretory pore is situated ventrally at the posterior end
of the body (Fig. 10). The body surface is marked by the presence of a number of conical
projections- the spinules which are cuticular extensions of the body.
BODY WALL:
Body wall consists of three layers: Tegument, musculature and mesenchyme (Fig.11).
TEGUMENT: Body is bounded by an outer layer of non-ciliated, syncytial and anuclear (nonnucleated) tegument. It bears numerous microscopic backwardly directed spinules or spines or
scales. The spines help the animal to anchor to the bile passage of the host and provide
protection to the body. Its matrix contains mitochondria, endoplasmic reticulum, ribosomes,
golgi complex and various secretory bodies, vacuoles and pinocytic vesicle suggesting that it is
a highly synthesizing layer and metabolically active which is of physiological importance to
the parasite. Microvilli have also been identified on the outer surface of the tegument. The
microvilli increase the surface area for the absorption of food. Tegument is connected by
narrow cytoplasmic tubes to the cytoplasmic processes of certain tegumental secreting cells
lying in the parenchyma.
FUNCTIONS OF TEGUMENT:
1. Absorption of food from the host body.
2. Synthesis and secretion of materials.
3. Respiratory and sensory function.
4. Excretion and osmoregulation.
5. Outer layer is made up of mucopolysaccharide and phenol which makes the tegument
resistant to the host enzymes thereby protecting the worm from being digested by the host.
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BASEMENT MEMBRANE: Below the tegument lies a thin, delicate basement membrane. It
separates the tegument from the muscle layer.
MUSCLE LAYER: It consists of an outer layer of circular muscle fibres, middle longitudinal
muscle fibres and inner oblique or diagonal fibres. All the muscle fibres are of smooth type.
The muscles form stout bundles of radial fibres in the suckers.
MESENCHYME OR PARENCHYMA: Numerous unicellular gland cells are found sunken in
the parenchyma which open out on the surface of the tegument by protoplasmic projections
(Fig.11). They form the packing material between the muscle layer and the internal organs.
Parenchyma performs the function of skeleton and transporting system.
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DIGESTIVE SYSTEM:
Fasciola has an incomplete digestive tract with a single opening the mouth. Anus is absent.
Mouth is situated ventrally at the apex of the head lobe, surrounded by oral sucker. The mouth
leads into a muscular pharynx, having a narrow lumen and thick walls and provided with
pharyngeal glands. Pharynx leads into a short and narrow oesophagus which in turn opens into
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the intestine (Fig. 12). Intestine immediately divides into two branches which run on either side
of the body and terminates blindly at the posterior end. The two branches give off numerous
irregular side branches known as caecae or diverticulum all along its length filling up a greater
part of each side of the body (Fig. 12). The caeca of the inner side are short and simple while
those of the outer side are large and branched. Anus is absent.
FEEDING AND DIGESTION:
With the help of the oral sucker and pharynx, Fasciola sucks up the hosts bile and blood from
the walls of the bile duct. Glucose, Fructose and Amino acids directly diffuse into the body of
the fluke through the tegument. Digestion is extracellular taking place in the intestinal caecae.
Digested food diffuses out into the surrounding parenchyma and then distributed to different
parts of the body. The highly branched intestine helps in the distribution of food to all parts of
the worm. Undigested food is given out through the mouth. Food is stored in the mesenchyme
and muscles as glycogen and fats.
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RESPIRATORY SYSTEM:
Respiratory organs are lacking. As the animal is an endoparasite, respiration in them is
anaerobic. Glycogen stored in the body as reserve food undergoes anaerobic glycolysis to form
pyruvic acid which is further decarboxylated to form acetyl group and carbondioxide. Acetyl
group combines with CoA to form Acetyl CoA. Acetyl CoA then condenses to form volatile
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fatty acids- Acetic, Lactic and Propionic acids. Carbondioxide diffuses out through the general
body surface while the fatty acids are excreted out through the excretory system.
EXCRETORY SYSTEM:
The excretory system consists of protonephridia or flame cells scattered throughout the
parenchyma. Flame cells are connected to one another by a network of excretory ducts (Fig.
13A). Each flame cell is a modified mesenchymal cell and has structure similar to one
described in Dugesia (Fig. 13B). All the flame cells open by an excretory tubule into larger
excretory tubes. The excretory tubes of the anterior part of the body open into four ducts: two
dorsal and two ventral which in turn unite posteriorly to form a common median longitudinal
excretory duct. This median ductl runs up to the posterior ventral end of the body and opens to
the outside by a common pore called the nephridiopore. Excretory vessels of the posterior part
of the body open directly into the median longitudinal excretory duct.
Excretory products like ammonia, fatty acids and other waste metabolites diffuse from the
surrounding parenchyma into the flame cell. The excretory fluid is further kept moving through
the tubules by the ciliary movement and finally is excreted out through the excretory pore.
EXCRETORY WASTE
EXCRETORY PORE
MESENCHYME
MEDIAN EXCRETORY CANAL
FLAME CELL
VESSELS
NERVOUS SYSTEM:
It consists of a pair of cerebral ganglia situated one on either side of the oesophagus. Nerves are
given out to the head lobe and to the hind part of the body from the cerebral ganglia. A nerve
collar exists around the oesophagus and connects the cerebral ganglia. Three pairs of
longitudinal nerves, dorsal, ventral and lateral are given out posteriorly from these ganglia. Of
these longitudinal nerves, the lateral nerve cords are well developed and extend upto the
posterior end and on their way are connected by transverse commissures. As they run
throughout the length of the body they give out many small branches, some of which form
plexus (Fig. 14). Sense organs are lacking in them.
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REPRODUCTIVE SYSTEM:
Highly branched and complete genital organs lie between the two branches of the intestine.
Fasciola hepatica is a hermaphrodite.
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MALE REPRODUCTIVE SYSTEM:
A pair of testes is present, with one testis lying behind the other in the posterior half of the
body. From each testis arises a small duct called vas deferens which extends anteriorly towards
the ventral sucker and meet with its counter part to form a common duct. This dilates into a
muscular, sac-like seminal vesicle before opening into an ejaculatory duct. Seminal vesicle
stores the sperms. The ejaculatory duct is surrounded by numerous unicellular prostrate
glands. The alkaline secretions of the prostrate glands help in the free movement of sperms
during copulation. The ejaculatory duct in turn opens into the genital atrium. The end of the
gonoduct is modified into a male intromittent organ known as cirrus or penis (Fig. 15). Cirrus,
prostrate glands and seminal vesicle are enclosed in a pouch known as cirrus sac. The genital
atrium opens outside by a common gonopore situated mid-ventrally in the anterior half of the
worm.
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FEMALE REPRODUCTIVE SYSTEM:
A single, highly branched ovary lies in front of the testis. It occupies anterior one-third of the
body. All the branches of the ovary open into a short and narrow tube called the oviduct. The
oviduct extends backwards to meet the median vitelline duct. From the junction of the oviduct
and median vitelline duct arises the uterus. Uterus is a long, wide and highly convoluted tube
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that runs anteriorly upto the genital atrium near the base of the cirrus. Uterus contains a large
number of fertilized shelled eggs or capsules. Surrounding the junction of oviduct, median
vitelline duct and uterus are a cluster of unicellular glands called Mehlis gland. Terminal part
of the uterus has muscular walls and is called the metraterm. Metraterm ejects the eggs and
also sometimes receives the cirrus during copulation. Genital atrium opens to the outside by a
pore known as gonopore situated mid-ventrally close to the ventral sucker (Fig. 15). During the
breeding season, a temporary opening arises from the oviduct on the dorsal surface known as
Laurers canal which acts as the vagina and receives the penis during copulation.
VITELLINE GLANDS:
Associated with the female reproductive system are the vitelline glands. These glands occur as
cluster of follicles, and are scattered throughout the length on the lateral side of the body. The
follicles of each side are connected by minute interconnected ductules to a large longitudinal
vitelline duct. The longitudinal vitelline ducts of both sides are connected by a transverse
vitelline duct almost in the middle of the body. The transverse duct swells up in the centre to
form the yolk reservoir. A small median vitelline duct joins the yolk reservoir to the oviduct
(Fig. 15). The vitelline glands produce yolk cells or vitelline cells. These cells contain abundant
yolk for nourishing the embryo. They also contain granules called the shell globules or
vitelline globules. As the egg mass moves through the oviduct, a group of vitelline cells
surround it. Shell globules are extruded out from the vitelline cells which coalesce to form a
thin membrane that becomes the outer covering of the egg shell. More globules are released and
the shell is built up from within. Egg shell is transparent and is made up of protein.
FUNCTIONS OF MEHLIS GLAND:
1. Lubrication of uterus for the safe passage of the eggs.
2. Activation of spermatozoa by their secretion.
3. Gland also secretes a phospholipid that may effect the release of the egg shell
precursor from vitelline glands.
4. It also produces free phenols from mucopolysaccharides and activates phenolases
thereby allowing the oxidation of phenols to quinone, thus helping in the tanning of
egg shell.
5. It provides a membrane which serves as a template on which shell droplets accumulate
to form egg shell.
COPULATION AND FERTILIZATION:
Although hermaphrodite, cross fertilization is a rule because of protandry. Sometimes self
fertilization may also occur. Copulation occurs in the bile duct of sheep. The cirrus of one
worm is inserted into the laurers canal of the other worm and sperms are ejaculated. Sperms
travel up to the distal end of the oviduct where fertilization takes place. The eggs once released
from the ovary get fertilized in the oviduct. Yolk is provided by the yolk cells which also carry
the material for egg shell formation. Since the yolk occurs outside the egg shell the fluke eggs
are ectolecithal.
LIFE-CYCLE:
It is digenetic i.e. requires two host to complete its life cycle.
DEFINITIVE HOST: Herbivores especially sheep. Man is only an incidental host.
INTERMEDIATE HOST: Fresh water snail- Lymnaea truncata.
26
RESERVOIR: Sheep.
HABITAT: It lives in the biliary passage of liver, gall bladder or associated ducts of sheep and
other herbivores.
EGGS: The number of eggs produced are enormous,5,00,000 in its life time. The high rate of
egg production is a parasitic adaptation for its endoparasitic mode of life.
Eggs are large 140x75µm, ovoid and brownish yellow. The eggs when voided out of the host
body are operculated and unembryonated i.e. they contain a large unsegmented ovum
surrounded by yolk cells which provide yolk and shell globules. Each egg is enclosed in a
proteinaceous shell or capsule.
CLEAVAGE AND EARLY DEVELOPMENT:
Cleavage is holoblastic and unequal. The first division of the zygote results in a small
propagatory cell and a large somatic cell. The somatic cell divides repeatedly to form the
ectoderm of the larva. The division of propagatory cell results in the formation of two daughter
cells: a propagatory cell like itself and a somatic cell. The somatic cells, by further division
form the endoderm and mesoderm of the embryo while the propagatory cell pass into the
posterior end of the embryo and divide repeatedly to give rise to a number of germ cells.
Further development is not possible in the uterus of the liver fluke. A large number of
encapsulated embryos are given out from the gonopore of the fluke into the gall bladder of the
host, from where they enter the duodenum of the host. The encapsulated embryos are finally
voided out along with the faeces of the host.
Eggs pass from the uterus of the worm
into the gall bladder of the host
Bile or hepatic duct
Expelled with faeces
Duodenum
EMBRYONATION TAKES PLACE IN SOIL OR WATER:
FACTORS RESPONSIBLE FOR EMBRYONATION TO TAKE PLACE:
1. Moisture—60%
2.
Temperature---22-250c
3. pH --6.5
It normally requires 14-17 days for embryonation to take place but the process may be effected
by other environmental factors.
FACTORS RESPONSIBLE FOR HATCHING TO TAKE PLACE:
1. TEMPERATURE
2. OSMOTIC PRESSURE
3. LIGHT: It has been suggested that on exposure to light, a hatching substance is produced
by the miracidium larva within the egg shell which acts on the operculum binding material
from inside and the operculum flies open and the miracidium larva escapes out. The
27
hatching substance is probably a proteolytic enzyme. An alternate hypothesis putforth by
Wilson suggests that light stimulates the miracidium into activity which in turn results in
altering the permeability of the internal surface of the viscous cushion. Change in
permeability allows the egg contents to reach the cushion which then becomes hydrated
thereby increasing the internal pressure and finally rupturing the operculum and liberating
the miracidium.
In two weeks time a small ciliated larva known as miracidium larva emerges out of the egg
shell by forcing the operculum.
MIRACIDIUM LARVA: Miracidium means a little boy. It is a free swimming larva.
Body is more or less fusiform and ciliated. The body is covered by epidermal plates. Number of
plates is characteristic of a species. Its anterior end is broader than the posterior end and is
produced into an apical lobe or apical papilla. X-shaped eye spot are present at the anterior
end behind the apical lobe. At the anterior end is a median apical gland which is believed to
release a proteolytic enzyme that aides in the process of penetration. One to several bilaterally
arranged pair of penetration glands secrete a mucoid substance which assists in attachment to
the snails tissue. A pair of protonephridia are present, each opening to the outside by a
nephridiopore. Scattered in the posterior part of the body are germinal cells which are a mass of
undifferentiated cellular material (Fig. 16). Miracidium does not feed.
Miracidium freed from the egg shell can live only for a few hours and during this time it
swims about randomly in search of a suitable host. If it fails to encounter a suitable host
within 24 hours, it dies. Miracidium demonstrates a high degree of selectivity for their host.
Physical and chemical factors present in the hosts gut serve as the determinants of compatibility
or incompatibility of the host and the miracidium.
FACTORS AFFECTING THEIR SELECTIVITY:
1. Chemical attractants (Chemotactic factors).
2. Mucus: Miracidium are highly selective and can perceive the mucus secreted by a particular
species of snail.
However, many miracidium reach their intermediate host purely by chance, although tropism
(phototropism or geotropism), temperature, salinity, pH, may guide them to areas of host
concentration and thereafter chemotaxis would operate.
Age of mollusk is probably also a controlling factor in selection and entry of miracidium.
ENTRY OF MIRACIDIUM INTO THE BODY OF THE INTERMEDIATE HOST:
The miracidium penetrates through the body surface of the snail in the head-foot complex or the
tentacles. The miracidium probes with their anterior end and attempt to bore into almost any
object, including various species of unsuitable snails and other animals like planarians or even
unanimate objects. These larvae may keep trying to enter an unsuitable host or an unsuitable
part of the hosts body like the shell until they die of exhaustion. Once the miracidium comes in
contact with the head-foot complex of the suitable snail, Lymnaea truncata, it attaches itself to
the body of the snail by the apical papillae and perform boring action. Penetration glands
produce a mucoid substance that enables the miracidium to adhere to the snail. The substance
also functions in lubrication. A cytolytic substance is also produced by the apical glands which
break down the host tissue and make a perforation in the skin. Apical papillae elongates and
works its way through the snails epithelium like a drill. Miracidium sheds its cilia and thrusts
itself into the soft tissues of the snail and ultimately makes its way into the digestive gland of
the snail.
28
SPOROCYST: Sporos means seed and kystis means cell or bladder.
The miracidium within the snail elongates to become a motile, vermiform, sac like larva called
sporocyst. Apical glands, penetration glands, apical papillae, eye spot present in the miracidium
larva all degenerate and disappear. The sporocyst retains all the body wall layers of the
miracidium except the ciliated epidermis which is lost in the process of penetration and is soon
replaced by a thin cuticle. Protonephridia of each side soon divide and the two so formed open
through a common excretory duct. Sporocysts are essentially germinal sacs containing germinal
cells which multiply and form new germinal masses (Fig. 17). Sporocyst moves about in the
hosts body by muscular contractions, absorbing nutrition from it. Within the sporocyst, next
larval stage is developed from the germinal sac through a process which is a repeat of formation
of miracidium from egg. These are called rediae.
REDIA: Named after the Italian scientist Francesco Redi
Rediae emerge from the sporocyst by rupture of its bodywall. Each redia is elongate and
cylindrical. Bodywall consists of the usual layers viz. cuticle, musculature and the
mesenchyme. Rediae show the beginning of the adult characteristics each having developed an
oval sucker and embryonic gut. It bears a mouth at its anterior end surrounded by sucker.
Mouth leads into a short muscular pharynx followed by an elongated sac like intestine.
Numerous unicellular pharyngeal glands open into the pharynx. Redia feeds on the hosts
digestive juices. Near the anterior end of the body is a ring like muscular swelling called the
collar. A birth pore is present at the anterior end, a little behind the collar through which the
next generation larvae escape out. The protonephridia branches further and forms a much
elaborate system. All the flame cells of each side open out through a common excretory duct.
Present ventrolaterally at the posterior end are the Lappets or Procruscula which are the
conspicuous feature of rediae (Fig. 18). The body of the larva is packed with germ balls and
germ cells. The rediae moves about in the hosts tissue. The movement is brought about by the
muscular contractions of the body. Collar and lappets aid in the movement. Moving rediae
enters into various organs of snail but prefer to migrate to the digestive gland. The next larvae
i.e. the cercariae develop within the rediae. These emerge out through the birth pore. During the
summer months when sufficient nourishment is available, instead of cercariae a second
generation of rediae is formed. However, during the winter months cercariae are formed. Each
redia forms 14-20 cercariae.
29
30
CERCARIA: Kerkos means a tail.
The cercaria larva escapes out of the rediae through the birth pore and enters the digestive gland
of the snail. Cercaria larva is a tailed immature sexual form that resemble the adults in general
body form. Mature cercariae possesses two suckers (an oral sucker and ventral sucker), a bifid
gut and a tail. The tail is a secondary adaptation. The usual body layers are present (Fig. 19).
31
Excretion is by flame cells. The basic pattern of flame cells is same as that of the adults.
Cercariae may have eye spots or photoreceptors consisting of sensory and pigment containing
cells (Fig. 19). Numerous cystogenous glands are present below the body wall, the secretions of
which help to form the cyst wall to become the next larva-The metacercariae. Germ cells
represent the rudiments of adult genital system. Cercariae emerge from the mollusk into the
water and alternate the periods of swimming by lashing the tail violently with short periods of
resting. Life span of cercariae is limited.
A single miracidium can produce a large number of cercariae.
Life cycle in snail is completed from 60 days to 90 days i.e. from miracidial penetration into
snail to the emergence of cercaria from the snail.
Released cercariae show
thermotropism.
negative geotropism, positive phototropism, and positive
FACTORS THAT STIMULATE CERCARIAE EMERGENCE:
1. Temperature: Between 9-260C favours its emergence. Above 260C the snails cannot
survive.
2. Rainfall has a stimulating effect.
3. Light has a positive effect.
4. Circadial rhythm: Maximum production of cercariae is between midnight and 1a.m.
On emergence the cercariae attach to objects like vegetation, grass, blades of some aquatic
weeds etc. in water and shed off its tail. The cystogenous glands secrete a protective cyst wall
about themselves and soon disappear. Encysted cercariae are round in shape and are called as
metacercariae or Adolescercaria or juvenile fluke. Metacercariae are prototype of the adult
(Fig.20).
Metacercariae is the infective stage. When swallowed by the final host, cyst wall is dissolved
by the digestive enzymes of the host, and young fluke is released. Uncysted cercariae if
swallowed by the primary host are destroyed by the acidic juices of the stomach. Cyst wall
is resistant to the acidic juices of the stomach.
Further development of the metacercaria takes place only if swallowed by the final host, the
sheep. However, the metacercariae are not infective until 12 hours after encystment. The sheep
gets the infection while grazing on the aquatic weeds. Once the metacercaria enters the lumen
of the intestine, the cyst wall is digested by the action of the hosts digestive juices and the
young fluke emerges which bores its way through the wall of the intestine and enters the liver
through the hepatic portal system. The young flukes remain in the liver for 7-8 weeks feeding
mainly on blood. They then enter the bile duct and bile passages where they become sexually
mature.
The incubation period in sheep is 3-4 months. Adult fluke lives in the sheep for about 5 years.
In man it lives relatively longer, for about 9-13 years.
32
ADULT LIVER FLUKE
EGGS
(In the bile duct and biliary passage of sheep)
ENTERS THE DEFINITIVE HOST
VOIDED OUT ALONG WITH THE FAECES OF
THE HOST
SHEEP
METACERCARIA
(Encysted Larva----Infective Stage)
OUTSIDE
MIRACIDIUM LARVA
(Infective stage for intermediate host)
SNAIL
CERCARIA LARVA
REDIA LARVA
SPOROCYST LARVA
MODE OF INFECTION: Oral
PORT OF ENTRY: Alimentary canal
INFECTING AGENT: Metacercariae
SITE OF LOCALIZATION: Bile duct of herbivores
PATHOGENICITY: Causes fascioliasis. In traversing the liver tissue, it causes parenchymal
injury.
EFFECT ON HERBIVORES:
1. The adult causes biliary obstructive symptoms. In bile duct, it causes inflammation and
hepatitis, intermittent obstruction and dilation of the biliary tract with considerable
thickening of its wall, followed by calcification and formation of gall stones. The
parasite may cause fibrosis of bile duct and gall bladder.
2. Heavy infestation may upset the normal metabolism of liver. This is due to haemorrhage
caused and irritation inflicted by cuticular spines. This disease is called liver rot.
Symptoms of liver rot are more acute in lambs than in sheep. Frequently death may
result due to cerebral apoplexy or acute anaemia.
3. Apetite declines, rumination(chewing of cud) becomes irregular and at times there is
fever and an increase in the respiratory activity.
4. Conjuctiva becomes whitish yellow.
5. In sheep the wool becomes dry and brittle and falls off.
6. Lactation and breeding are reduced.
7. In general, growth, milk yield, body weight and wool of sheep may be reduced.
33
EFFECT IN MAN:
1. In man it causes more severe inflammatory response and hepatitis resulting in the
massive destruction of the liver tissues and inflammation of the bile duct.
2. Some larvae penetrate the liver and diaphragm to reach the lung, brain or other tissues.
3. Initially, the patients suffer from fever, eosinophilia and hepatomegaly. Later they
develop acute epigastric pain, anaemia and obstructive jaundice. Occasionally, ingestion
of raw liver of infected sheep results in a condition known as halzoun meaning
suffocation. Halzoun is more common in Lebanon, and other parts of Middle East and
North Africa.
DIAGNOSIS: 1. Eggs in feces.
2. Duodenal and bile intubation.
PROPHYLAXIS:
1. Erradication of the intermediate host, Lymnaea truncata from the water courses
inhabited by
2. sheep. Ducks feed on snails and can be used as biological control.
3. Breeding of snails can be checked by removing vegetation from ponds and streams
that they inhabit.
4. Snail population can be checked by adding copper sulphate solution in ponds and
ditches or by
5. draining their pastures as the snails cannot survive long dry periods.
6. Killing heavily infected sheep.
7. Destroying the eggs and manure of infected sheep.
8. Feeding infected sheep with salt and a little dry food.
9. Proper sanitary disposal and personal hygiene.
10. Proper disinfection of watercresses and vegetation before consumption.
SCHISTOSOMA OR SPLIT BODY
Gr.: Schistos means divided and soma means body
ONLY TREMATODE TO LIVE IN THE BLOOD STREAM OF WARM BLOODED
ANIMALS HENCE THE NAME BLOOD FLUKE.
Causes SCHISTOSOMIASIS OR BILHARZIASIS.
Schistosoma was discovered by Bilharz in 1851.
MORPHOLOGY:
Schistosoma has an elongated, slender body well adapted to live in the narrow blood vessels. It
is the only fluke which is dioecious i.e. sexes are separate. Males are shorter and stouter than
females. Lateral margin of the male body is folded ventrally into a groove for much of its length
forming a gynaecophoric canal in which the females are held. (It was this groove that gave the
34
name SCHISTOSOMAS OR SPLIT BODY). Gynaecophoric canal is not usually long enough
to enclose the female so loop of female body can be generally seen extending from the canal. At
the anterior end of the body is the mouth surrounded by oral sucker. Acetabulum or ventral
sucker is situated on a short projection from the body at the anterior end. The ventral sucker is
larger and more muscular in the male and serves to maintain the position of the worm within
the blood vessels. The suckers are armed with delicate spines. Unlike the liver fluke,
Schistosoma lack a muscular pharynx (Apharyngeate). Just behind the acetabulum is the genital
pore. Excretory pore is present ventrally at the terminal end of the body (Fig. 21). The intestinal
caecae reunite behind the ventral sucker to form a single caecum. The length of reunited
intestine varies in different species. The cuticle of males is provided with numerous minute
papillae. Presence or absence of male is one of the most significant features affecting the
functioning of female. If the female develops in the absence of sexually mature males, ovary is
small and ova lacks cortical granules. It is possible that the female needs contact with the male
to absorb amino acids from it which is required in turn for the production of normal cortical
granules. Another effect of the absence of male is the inability of vitelline cells to mature
because of the absence of metabolites required to trigger the maturation of these cells. Adult
lives in the lumen of portal veins and its radicles of warm blooded animals normally in pairs.
Three species of Schistosoma are known to inhabit man.
S. japonicum or oriental blood fluke inhabits the small veins of portal system and mesenteric
veins draining the ileo- cloacal region.
S. mansoni inhabits the small branches of mesenteric veins of the rectal area and branches of
portal veins.
S. haematobium inhabits the blood vessels of bladder and urinary tract.
All the three species of Schistosoma are morphologically similar and their life- cycle is also
the same. They differ in their habitat. Hence, only Schistosoma japonicum will be described
here.
Schistosoma japonicum
COMMON NAME
: ORIENTAL BLOOD FLUKE
GEOGRAPHICAL DISTRIBUTION
: Japan, Korea, Philippines, Thailand, Cambodia,
Celebes, China, parts of Burma etc.
DEFINITIVE HOST
: Man
INTERMEDIATE HOST
: Fresh water snail–Oncomelania sp.
RESERVOIR HOST
: Pigs, Dogs, Cats, Cattle, Goats, Horses and
Rodents
LIFE CYCLE:
MECHANISM OF EGG LAYING AND EXPULSION OF EGG:
Adult blood flukes live chiefly in the superior mesentric veins, capillaries of last part of ileum,
caecum, ascending colon and rectal plexus of vein. Copulation takes place while the female is
held in the gynaecophoric canal of the male. After copulation, the females held in the
gynaecophoric canal of males extend their anterior end far into the smallest venules or leave the
male to lay eggs in small venules of mesentries of intestinal wall. Eggs are deposited
longitudinally one at a time. Each time an egg is laid; worm withdraws a short distance and lays
another egg immediately behind the first. In this way venules are filled with eggs pointing
35
backwards. The worm then migrates to adjacent venules. Eggs are held in position by the lateral
knob and also by the contraction of vessels resulting from, the withdrawl of parent worm.
When the vessels are laden with eggs, they rupture – Masses of eggs cause pressure on the thin
venule walls which are further weakened by secretions of histolytic gland of the miracidium
larva within the eggs.
VENULE WALL RUPTURES
EGGS penetrate the intestinal lumen making their
URINARY BLADDER
way through the vessels and mucosa
ESCAPE WITH URINE OR FECES
In heavy infestation, thousands of worms may be present in blood vessels.
EGGS when voided are EMBRYONATED AND NON-OPERCULATED.
HATCHING TAKES PLACE IN H2O
The factors responsible for hatching, i.e. for the emergence of miracidium larva and its
penetration into the intermediate host are same as that for liver fluke.
Total life span of miracidium within an egg is 20 days after which miracidium degenerates.
The life-cycle resembles that of liver fluke.
DIMORPHIC ADULT
EGG
ENVIRONMENT
MIRACIDIUM
TAKES 4-8 WEEKS TO
TRANSFORM INTO
TUBULAR SPOROCYST
MAN
SNAIL
SPOROCYST
CERCARIA
DAUGHTER SPOROCYST
Development takes place in the digestive gland of SNAIL
REDIA AND METACERCARIA ARE ABSENT.
The structure of miracidium and sporocyst larva is also the same as that of liver fluke
miracidium and sporocyst larva.
CERCARIAE: Structure is same as that of cercaria larva of liver fluke except that the
Schistosoma cercariae have a bifid tail and instead of cystogenous glands they have a pair of
penetration glands at the anterior end. The cercaria of Schistosoma is known as furcocercaria
because of the presence of forked tail.
Infection results when human beings bathing or walking in water come in contact with cercaria
larva. The cercaria larva locates its prey purely by chance. When cercariae comes in contact
with the skin of an appropriate host, they loop for variable periods of time by attaching
themselves alternately with the oral and ventral suckers. When unattached, the oral end of the
body constantly probes into every irregularity encountered. Points of entry include wrinkled
areas, bases of follicular eminences, points of scale attachment, distal hair, entry site used by
previous cercariae. These cercariae burrow in the skin with the help of penetration glands and
36
body movements. Eventually, cercariae become closely attached by their oral sucker and
assume a vertical or oblique position in relation to the surface. Penetration glands release an
enzyme probably hyaluronidase (which hydrolyses hyaluronic acid – one of the principal
substrate of connective tissue); although the exact nature of the secretion is not known.
Ramming motion and partial evertion of sucker brings the ducts of penetration gland in contact
with the skin. Through these ducts, gland secretions are poured into the host tissue. Alternate
contraction and elongation of the body and energetic movement of tail, thrusts the oral sucker
deeper.
37
After penetrating the skin of the definitive host, cercariae cast off their tail to become a
schistosomule and gain access to peripheral venules. From here they are carried through the
right heart into the pulmonary capillaries. It takes some days for the larva to pass through the
capillary bed in the lungs. Then they are carried through the left heart into the systemic
circulation. Majority of them are shunted into the abdominal aorta and gain access to the
mesenteric artery, pass through capillary bed in the intestine and enter portal cirulation ~ taking
5 days to reach the liver. The schistosomule mature into adults in about 3 weeks in intrahepatic
portal blood stream. Mature adults then move out of the liver and enter the superior mesenteric
vein from where they finally settle down in the capillaries of the last part of the ileum, caecum
and colon.
SCHISTOSOMULE FROM THE PERIPHERALVENULES OF SKIN
LEFT HEART
RIGHT HEART
PULMONARY CAPILLARIES
It takes some days for the larva to pass through
the capillary bed in the lung
SYSTEMIC CIRCULATION
LIVER
Schistosomule mature into adults in about 3 weeks in intrahepatic portal blood stream
MATURE SCHISTOSOMAS
SUPERIOR MESENTERIC VEIN
MOVE OUT OF LIVER
AGAINST BLOOD CURRENT
A month or more elapses from
the time of penetration by the cercariae
to the schistosomas eggs in feces.
FEMALE ENTERS GYNAECOPHORIC
CANAL OF MALE
CAPILLARIES OF LAST PART
OF ILEUM, CAECUM AND COLON
LIFE SPAN OF ADULT WORM IS 5-30 YEARS
Several thousand individuals may exist in a single host.
Schistosoma infection occurs much frequently in :
• Laboureres working in irrigated fields.
• Fishermen working in fish culture ponds and river.
• Women who wash utensils or clothes along the banks of canal or river
PATHOGENICITY: Schistosoma causes SCHISTOSOMIASIS or BILHARZIASIS in man.
SEVERE FROM OF DISEASE MAY CAUSE
ENLARGED
LIVER
OR
CALCIFIED
BLADDER
DEFORMITY
IN URETER
OR
38
OR
MALFUNCTIONING
OF KIDNEY
Antigens from the parasite induces the host to produce antibodies which results in antigenantibody reaction which is responsible for early clinical symptoms.
PATHOGENICITY OF S. JAPONICUM
•
Causes intestinal and hepatic schistosomiasis of orient also known as katayama
disease. Lesions produced are pronounced because of large output of eggs. Ileocloacal region is affected causing dysentery.
KATAYAMA DISEASE SYMPTOMS – Fever, Abdominal Pain, Diarrhoea and Allergic
manifestations.
•
Gastroinstestinal bleeding, Ulceration and necrosis of intestinal tissue.
•
Enlargement of spleen.
•
Migrating worms cause little or no damage or symptoms but occasionally serious
reactions occurs, such as pneumonia resulting from invasion of the lungs.
•
Liver invading phase may be symptomless or it may be marked by enlargement and
tenderness together with some toxic reactions.
•
Some common symptoms are: Cough, Fever, Diarrhoea, Eosinophilia, and Anaemia.
The arms, body and legs become alarmingly thin. Abdomen becomes enlarged and is
filled with fluid.
DIAGNOSIS: 1. Stool/ Urine examination.
2. Biopsy of infected Area.
PROPHYLAXIS
•
Erradication of intermediate molluscan host.
•
Prevention of Environmental pollution with urine and feces. Developing proper sanitary
disposal method.
•
Effective treatment of infected persons
•
Avoidance of swimming, bathing, wading or washing in infected water.
•
Infected person should not be allowed to wade in H2o with open wounds.
•
Health education.
Taenia solium
Commonly called as pork tapeworm; the armed tapeworm of man.
The life cycle of the parasite was first described by van Beneden(1854). He demonstrated the
larval stage in the muscles of pig after feeding it with eggs from human faeces.
Kuchenmeister(1855) demonstrated the adult tapeworm in the intestine of man.
GEOGRAPHICAL DISTRIBUTION
: Worldwide in distribution.
DEFINITIVE HOST
: Man
HABITAT : Adult worm lives in the small intestine (upper jejunum) of man. Normally only a
single worm is present but rarely several worms may be present. They remain coiled up in the
intestine.
39
MORPHOLOGY : The adult worm has a long ribbon like body measuring about 2-3metres
and consists of a scolex(head), neck and strobila made up of many proglottids(sometimes,
though wrongly, referred to as segments) (Fig. 22).
SCOLEX : The scolex is small, quadrate about 1mm in diameter with four cup-like suckers at
equatorial position and a conspicuous rounded distal bulge, rostellum, armed with double
row of 25-30 alternating large and small hooklets (Fig.23 A, B). The hooks and suckers are the
holdfast organs and help the animal to firmly attach themselves to the intestine of the host.
NECK: The neck is short and half as thick as the head. Neck is the zone of proliferation and
new proglottids are budded off from the posterior part of the neck –a process known as
strobilization. As new proglottids are added on behind the scolex, the older ones are pushed
away farther and farther from it during growth, so that the younger ones are nearer the scolex
and older ones away from it, and oldest proglottid is the last one (Fig.22).
STROBILA: The strobila consists of 800-1000 proglottids-immature, mature and gravid in that
order from front backwards. The immature proglottids are the youngest, undifferentiated just
behind the neck and are devoid of reproductive organs. They are about 200 in number and are
broader than long. The mature proglottid is a complete reproductive unit. The anterior 100-150
mature proglottid contain only the male reproductive organs while the later 250 mature
proglottid have both the male and female reproductive organs. The lateral margin of each
proglottid bear alternately on the right and left side a small protuberance called genital papillae
through which opens a common genital pore. The gravid proglottids(last 150-350) are the
posteriormost and bear only a highly branched uterus full of fertilized eggs. All the organs
except the uterus get atrophied in the gravid proglottids and they become twice as long as
broad (Fig. 22). These gravid proglottids are regularly detached and expelled out passively in
short chains of 5 or 6 along with the host faeces-a process known as apolysis. Apolysis limits
the size of the tapeworm which may otherwise attain enormous length due to continuous
proliferation in the neck region. Also, apolysis serves to transfer the developing embryos to
the exterior, where they can be ingested by the intermediate host.
LIFE CYCLE:
DEFINITIVE HOST
:
Man
INTERMEDIATE HOST
:
Pig
Copulation takes place between segments of the same worm. Because of protandry, the male
reproductive organs mature before the female, and thus during copulation, there is folding up
of the strobila bringing the proglottids, where the male reproductive organs have matured in
contact with those in which the female organs have also matured, and the sperms are
transferred by the cirrus into the vagina from where they enter the fertilization duct. The ova
are also discharged from the ovary into the oviduct from where they pass into the fertilization
duct. Fertilization takes place and the fertilized egg then passes into the ootype where they are
surrounded by yolk cells secreted by vitelline glands. The yolk cells form a thin shell known
as chorionic membrane around the egg. This encapsulated egg then passes into the uterus.
When a large number of capsules are collected in the uterus, it enlarges and gives off branches
so that the gravid proglottids contains only the highly brached uterus full of eggs, all other
organs get atrophied.
The fertilized egg or zygote undergoes holoblastic and unequal cleavage and develops into
three types of cells viz. micromeres, mesomeres, and megameres. Micromeres form the
morula which develops three pairs of chitinous hooks from cells known as onchoblast. The
morula is surrounded by inner membrane or embryophore formed by the mesomeres. Beneath
the embryophore is the basement membrane (Fig. 24). The embryophore is in turn surrounded
40
by the outer envelope of megameres. The yolk cells and the envelope of megameres give their
yolk to the developing embryo and disappear. The six-hooked embryo, called hexacanth,
possesses a pair of large penetration gland and is surrounded by two hexacanth membranes.
The hexacanth with all the membranes surrounding it is known as the oncosphere.
41
INFECTION TO THE SECONDARY HOST:
Gravid proglottids containing eggs are passed out along with the feces where they degenerate
setting free the oncospheres. These are ingested by the pig. Further development takes place
in the body of the pig which acts as the intermediate host. In the alimentary canal of pig, the
oncosphere looses its embryophore and basement membrane by the action of the acidic juices
of the stomach and hexacanth passes into the intestine where the two persisting hexacanth
membranes are also lost by the action of alkaline juices. Activated by the bile salts, the
hexacanth penetrates the intestinal wall with the help of hooklets and penetration glands.
With the help of the hooks the hexacanth anchors itself to the intestinal wall while the
secretions of the penetration glands dissolves the intestinal tissues and the embryo gains
entry into the portal vessels or mesenteric lymphatics and are carried in systemic circulation
to different parts of the body. Finally they are filtered out in the striated muscles usually of
the tongue, shoulder, neck, thigh etc. where in 10-12 weeks they develop into a larva known
as cysticercus cellulosae.
However, they may develop in other organs such as liver,
lung, kidney or brain. The hooks are no longer of any use and are shed off. The cysticercus
cellulosae or bladder worm is an ovoid, opalescent (milky white) bladder surrounded by a
fibrous capsule. It has an invaginated scolex with four suckers and a rostellum with a double
row of alternating large and small hooklets (Fig. 25). It absorbs nourishment from the hosts
tissue. The bladder contains a thick fluid rich in protein and salt. In fact the fluid within the
bladder is largely composed of hosts blood plasma. The cysticercus usually measures about
5mm-10mm and lies parallel to the muscle fibres. It can be seen as a thick white spot. The
pigs flesh which is infested with cysticercus is known as measly pork. The bladder worm can
remain viable for several months. The development time of cysticercus cellulosae is 10-12
weeks in pig.
42
Cysticercus cellulosae can develop into an adult tapeworm only when ingested by man. The
cysticercus larva remains dormant in the muscles or connective tissue of pig without any
further development, till it happens to be eaten by man.
43
INFECTION TO THE PRIMARY HOST: Man acquires the infection by consuming
inadequately cooked pork (measly pork) containing cysticercus cellulosae. The larvae are
digested out of the meat in the duodenum. The scolex evaginates on coming in contact with
the bile and anchors to the mucosal surface by means of their suckers. The neck begins to
proliferate forming a chain of proglottid and an adult tapeworm is formed in 5-12 weeks
(Fig. 25).
44
A man harbouring the adult worm may autoinfect onself by reverse peristaltic
movement of the intestine whereby the gravid proglottids are thrown into the stomach
and oncospheres are liberated.
The total life span of the adult worm is very long- about 25 years or may be more.
PATHOGENESIS:
Adult worm in the small intestine donot cause any harm apart from vague abdominal
discomfort, chronic indigestion, persistent diarrhoea alternating with constipation, anaemia,
weight loss and nervous discomforts. Hooks and suckers may cause mechanical irritation in
the intestine, which may initiate reverse peristalsis leading to autoinfection. The symptoms
caused by the adults may be referred to as taeniasis.
It is the larval stage that cause serious trouble and the disease is known as cysticercosis.
Cysticercus cellulosae usually occur in large numbers but sometimes may occur singly and
affect any organ or tissue. It may also affect the eyes, brain, and less often the heart, liver,
lungs, abdominal cavity and spinal cord. The effect produced depends upon the site affected.
They usually develop in the subcutaneous tissues and muscles forming visible nodules. The
larva evokes a cellular reaction, with infiltration of neutrophils, eosinophils, lymphocytes,
and plasma cells. This is followed by fibrosis and death of the larva with eventual
calcification.
In cysticercosis of the brain (neurocysticercosis), symptoms are more often due to dead and
calcified larvae rather than the living larvae. Epilepsy is the commonest manifestation, but it
can also cause behavioural disorders. Ocular cysticercosis may cause blurring of vision and
ultimately blindness.
Diagnosis of cysticercosis can be carried out by:
1. Biopsy of subcutaneous nodule which may reveal cysticerci
2. X-ray of skull and soft tissue may reveal calcified cysticerci.
3. CT scan of brain can accurately locate the lesion in the brain.
4. Ocular cysticercosis can be made out by ophthalmoscopy.
DIAGNOSIS: Stool examination for the eggs.
PROPHYLAXIS:
1. Personal hygiene and proper sanitary disposal.
2. Strict inspection of slaughter houses with condemnation of measly pork(infected meat).
3. Thorough cooking of pork.
4. For control of cysticercosis, prevention of faecal contamination of soil, proper disposal
of sewage and avoiding eating of raw vegetables grown in polluted soil.
5. Persons harbouring adult worms should be isolated and treated as they can develop
cysticercosis due to autoinfection.
45
Diphyllobothrium latum
Commonly known as fish tapeworm or the broad tapeworm.
Greek: diphyllobothrium means having two grooves in the head; latus means broad
(di means two; phyllon means leaf and bothrium means sucking organs i.e. a leaf-shaped
structure having sucking organs).
Although the head of the worm was found as early as 1777 by Bonnet, the life cycle was
worked out by Janicke and Rosen in 1917.
GEOGRAPHICAL DISTRIBUTION: Endemic in central and northern Europe, Japan, Central
Africa, Russia and North America. So far only one case of human infection has been reported
from Vellore in India in 1998. Till then India was free of the infection by this parasite.
DEFINITIVE HOST: Man. Also found in dogs, cats and their other wild relatives feeding on
fishes.
HABITAT: Adult worm lies folded in several loops in the small intestine, usually in the ileum
of the definitive host.
MORPHOLOGY: It is the longest tapeworm found in man, measuring upto 10 metres or
more in length. A freshly expelled worm from human intestine is ivory colored and consists of
a scolex (head), neck and strobila.
SCOLEX: It is spoon shaped, about 2-3mm long and 1mm broad. It bears two slit –like
longitudinal sucking grooves (bothria), one dorsal and the other ventral (hence the name).
Rostellum and hooklets are absent.
NECK: Behind the scolex is an unsegmented thin neck, several times longer than the scolex.
Neck is the zone of proliferation i.e. new proglottids are budded off posteriorly from the neck
so that the youngest proglottids are immediately behind the neck.
STROBILA: The strobila may have 3000 or more proglottids (often incorrectly called as
segments) consisting of immature, mature and gravid proglottids. A mature proglottid is
broader than long and contains male and female reproductive organs. Three genital openings
are present ventrally along the midline- the openings of the vas deferens, vagina and uterus.
LIFE CYCLE:
DEFINITIVE HOST: Man, and other fresh- water fish eating animals like dogs, cats, foxes
and wolf etc.
INTERMEDIATE HOST: D. latum is unique among human tapeworms because they require
two intermediate host to complete its life cycle.
FIRST INTERMEDIATE HOST: Small copepods mainly Cyclops.
SECOND INTERMEDIATE HOST: Freshwater fishes like salmon, trout, perch etc.
D. latum is a prolific egg laying worm and a single worm can discharge about a million eggs in
a day. The terminal proglottids become shrunken because of the constant discharge of the eggs.
Later these proglottids get dried-up and finally break off from the body in chains and voided out
along with the faeces in water.
EGGS: Eggs when voided out are oval in shape, yellowish brown in color (bile stained), 70µm
long and 45µm broad with a thin smooth shell. The eggs bear an operculum at one end and a
small knob at the other end. The eggs are resistant to chemicals but are readily killed by drying.
46
Further development of the eggs takes place in fresh water lakes, rivers or reservoirs.
Freshly passed eggs are not infective to man.
Within 1-2 weeks a ciliated embryo containing three pairs of hooklets ( hexacanth embryo)
develop within each egg shell. It emerges out through the operculum and is known as the
coracidium. It is the first stage larva, spherical in shape and bears cilia. Coracidium larva
swims about in water but can survive for only 12 hours in water by which time it must be
ingested by an appropriate copepod- Cyclops (crustacean) which is the first intermediate host.
In the midgut of the cyclops, the coracidium sheds off its cilia and with the help of the six
hooklets penetrates the midgut wall and gains entry into the haemocoel (the body cavity). In
the haemocoel of cyclops, it gets transformed into a second stage larva known as procercoid
larva which is spherical, 550µm long with a caudal appendage known as cercomer that bears
the hooklets which are of no more use. If this infected copepod (cyclops) is now eaten by a
suitable fresh water fish (the second intermediate host), the procercoid larva penetrates the
intestine of fish and migrates to the muscles, liver and fat of the fish within a few hours and
grows. It looses its caudal appendage and develops into a third stage larva- the plerocercoid
larva or sparganum (Fig. 26). The plerocercoid is a flattened, unsegmented, glistening white
larva measuring 10-20mm in length with a rudimentary invaginated scolex. This is the
infective stage for man.
Humans become infected when they eat undercooked, or raw freshwater fish infected with
plerocercoid larva. In the intestine of man, the plerocercoid larva develops into an adult and
attains maturity in about 5-6 weeks and begins to lay eggs which are voided out along with the
faeces. The cycle is then repeated.
An adult worm can live for about 10 years or more.
PATHOGENICITY:
The pathogenic effects of diphyllobothriasis depend on the mass of the worm, absorption of its
by-products by the host and deprivation of the hosts metabolic intermediates. In some persons,
the infection may be asymptomatic while in others the worm may cause discomfort, diarrhoea,
nausea, weakness and numbness of the extremities. Some patients develop mechanical
obstruction of the bowel because of the presence of a large number of worms. In a few cases,
pernicious anaemia called bothriocephalus anaemia may develop due to manifest vitamin B12
deficiency. D. latum adult has a great affinity for vitamin B12. It can absorb as much as 80100% of a single dose of vitamin B12, thereby competing with the host for this important
vitamin.
DIAGNOSIS: Stool examination for eggs or proglottids.
PROPHYLAXIS:
1. Thorough cooking of freshwater fish.
2. Freezing of fish intended to be eaten raw at -18ºC for 24-48 hours kills plerocercoids.
3. Preventing the contamination of lakes, ponds and river water by human faeces.
4. Effective sanitary disposal of faeces.
5. Protection of water supplies from faecal pollution.
47
Chymenolepis nana
commonly known as the dwarf tapeworm.
The smallest and the commonest tapeworm found in the intestine of man. It derives its
name from the Greek word: hymen means a membrane and lepis means rind or covering; and
nana in Greek means a small size (nanus- dwarf). It was first discovered by Bilharz in 1857.
48
GEOGRAPHICAL DISTRIBUTION: A cosmopolitan worm but more common in warm
climates.
H. nana infection is prevalent throughout India.
DEFINITIVE HOST: Man and rodents like mice and rats. In rodents, the tapeworm is
regarded by a different strain- H. nana var. fraterna. The murine strain does not appear to infect
man but the human strain may infect rodents, and therefore, constitute a reservoir of infection
for the human parasite.
HABITAT: In man the adult worm is found in the upper two third of the ileum in large
numbers while in rodents they reside in the posterior part of the ileum.
MORPHOLOGY:
It is the small, thread like worm measuring only 5-45mm in length and 1mm in diameter. Like
other tapeworms, the body consists of scolex, neck and strobila.
SCOLEX: The scolex is globular with 4 cup- shaped suckers and a retractile rostellum armed
with a single row of hooklets.
NECK: A long and slender neck is situated posterior to the scolex.
STROBILA: It consists of 200 or more proglottids. They are much broader than long.
In an infected person as many as 1,000-8,000 worms may be present.
LIFE CYCLE: It is a unique tapeworm requiring only one host to complete its life cycle.
Eggs and proglottids are voided out along with the faeces. The eggs are spherical or ovoid,
measuring about 30-45µm in diameter with a smooth, thin colorless outer membrane and an
inner embryophore enclosing the hexacanth embryo (oncosphere) with 3 pairs of hooklets.
There is a clear space between the outer and inner membrane which is filled with yolk granules.
Man acquires the infection by ingesting the eggs via fecal-oral route( by consuming food and
water contaminated by H. nana eggs or by ingesting eggs from contaminated hands). Rarely,
man can acquire infection by ingestion of food contaminated with fleas harbouring the
cysticercoid larvae (Fig. 27). When the eggs are swallowed, they hatch in the lumen of the
small intestine(duodenum or jejunum) and a small hexacanth embryo is liberated. The
hexacanth now penetrates the jejunal villus and develops into a cysticercoid larva in about 4
days. The villus then ruptures and cysticercoid larva becomes free in the lumen of the small
intestine. The cysticercoid is a solid pyriform structure, with an invaginated scolex and a short
conical posterior end. In the lumen of the small intestine, the mature larva evaginates its scolex
and attaches itself to the mucosa by its suckers (Fig. 27). It starts strobilization to become a
mature worm and in about 30 days after the infection of eggs, the proglottids begin to appear in
faeces and the cycle is repeated. Internal autoinfection may also occur when the eggs released
in the intestine instead of being voided out along with the faeces hatch in the lumen of the
intestine itself.
Life span of adult worm is about two weeks.
H. nana also has an indirect life-cycle with insect as an intermediate host. The insects as
intermediate host include flour-eating beetles like the species of Tribolium and Tenebrio, fleas
such as Xenopsylla cheopis, Pulex irritans and Ctenocephalides canis. The adult insects or their
larvae eat the eggs of H. nana and reach the gut of the insect. In the lumen of the gut, the
enzymes stimulate the oncosphere to free itself from the enclosing membrane. The oncospheres
with the help of the six hooklets and the glandular secretions penetrate the gut wall and enter
the haemocoel of the insect where they transform into a cysticercoid larva, which is infective to
49
the final host. Man gets the infection by accidentally ingesting these insects. In the intestine of
man the cysticercoid larva develops into an adult worm.
Rodents get infected when they eat these insects.
PATHOGENICITY: Infection with Hymenolepis does not generally produce any illness.
Symptoms may sometimes occur due to allergic reactions. Some of the symptoms are:
abdominal discomfort, headache, diarrhoea, restlessness, dizziness, sleep disorder and anorexia.
DIAGNOSIS: Stool examination for the presence of eggs.
PROPHYLAXIS:
1. Proper personal hygiene.
2. Sanitary improvement.
3. Protection of water supplies from faecal contamination.
4. Avoid eating flour or rice infested with insects.
50
Echinococcus granulosus
Commonly known as dog tapeworm or the hydatid tapeworm. In man it causes the disease
unilocular echinococcosis or hydatid disease. Adult E. granulosus was first described by
Hartmann(1695) in the intestine of dog. The life cycle was demonstrated by Von
Siebold(1853). Naunyn (1863) demonstrated that the hydatid cyst in man is the bladder worm
stage of Echinococcus found in the intestine of man.
51
GEOGRAPHICAL DISTRIBUTION: World-wide but is more prevalent in sheep and cattleraising countries. In India, a large number of cases have been reported from Andhra Pradesh,
Gujarat, Tamilnadu, West Bengal, Orissa, Bihar, Punjab, Uttar Pradesh and Pondicherry.
DEFINITIVE HOST: Dog and other canine carnivores.
INTERMEDIATE HOST: Sheep, goat, cattle, pig, horse and man.
HABITAT: The adult worm lives in the jejunum and duodenum of dogs and other canine
carnivores with its scolex buried in the mucosa. A large number of them may be present in the
infected dogs.
MORPHOLOGY: It is a small tapeworm measuring 3-6mm in length. The body consists of a
scolex, a short neck and strobila composed of only 3 proglottids (occasionally four). The first
proglottid is immature, the second one is mature and contains both the male and female
reproductive organs while the third one is the gravid proglottid containing a highly branched
uterus full of eggs. The scolex is pyriform in shape with four suckers and a protrusible
rostellum with two circular rows of hooklets.
LIFE- CYCLE: E. granulosus completes its life cycle in two host. The adult lives attached to
the mucosa of the small intestine of dogs and other canine animals. The eggs are discharged
along with the faeces of the definitive host. Sheep and other intermediate host get infested by
consuming these eggs while grazing in the field. Man is an accidental host.
EGGS: The eggs are spherical, brown in color, measuring 31µm-43µm in diameter. The egg is
made up of two layers- an outer thin layer and an inner embryophore. Within the embryophore
lies a hexacanth embryo with 3 pairs of hooklet.
Within eight hours of ingestion, the hexacanth embryo hatches out in the duodenum of the
intermediate host and penetrates the intestinal wall to enter the radicles of the portal vein from
where they are carried to the liver. The liver acts as the first filter for the embryos where they
get arrested in the sinusoidal capillaries. Some of the embryos escape and gain entry in the
pulmonary circulation. Lungs now act as the second filter. However, some of the embryos
escape even the pulmonary circulation and enter the general circulation from where they lodge
in various organs like heart, brain, spleen, kidneys, bones, muscles, etc.
At the site of deposition, an active cellular reaction takes place around the parasite and a large
number of them are destroyed by the hosts defense mechanism. Some of the embryos that
escape destruction gets surrounded by fibrous tissue which is known as pericyst (Fig. 28). This
pericyst merges with the normal surrounding tissue. The parasite derives its nutrition through
this layer. Inside this pericyst, the embryo develops into a fluid filled bladder or cyst known as
hydatid cyst (Greek: hydatis means a drop of water). The hydatid cyst is composed of two
layers: an outer laminated ectocyst and an inner germinal endocyst. From the inner layer of the
cyst, brood capsules with a number of scolices develop. The inner layer also secretes the
hydatid fluid and gives rise to the outer layer. In a few cysts, brood capsules fail to develop or
even if they develop, they donot contain any scolices. Such cysts are called sterile or
acephalocyst. These acephalocyst if ingested by the definitive host donot cause any infection.
When sheep or cattle harbouring hydatid cyst die or are slaughtered, dogs may feed on the
carcass and get infected. When the hydatid cyst reaches the intestine of dogs, the cyst wall
ruptures, scolex evaginates and develops into an adult. The adult attains maturity in 6-7 weeks
and produce eggs to repeat the cycle. This is the natural cycle of the host. However, if man gets
the infection by direct contact with the dogs, or by consuming water and food contaminated
with the dogs faeces containing eggs of Echinococcus, then the life-cycle of the parasite comes
to a dead end as the human hydatid cyst are unlikely to be consumed by the dogs.
52
PATHOGENECITY:
Infection is normally acquired during childhood when intimate contact with pet dogs is more
likely. However, the clinical symptoms develop several years later.
In majority of cases, the primary hydatid occurs in the liver. Hence, hepatomegaly, pain and
obstructive jaundice are the usual manifestations.
When the cyst enters the lung, the symptoms are: cough, chest pain and dyspnoea.
In the kidney it causes pain and haematuria.
Erosion of bones may lead to pathological fractures and degeneration of bone structures.
Hydatid cyst of the spleen, heart and brain may present a tumour-like condition or abscess.
DIAGNOSIS: Ultrasonography and CT scan in most of the cases can reveal the diagnosis.
PROPHYLAXIS:
1. Strict personal hygiene.
2. Dogs should not be allowed to eat the carcasses of slaughtered animals in endemic
areas.
3. Reduction of stray dog population has been found to be helpful.
4. Periodical deworming of the pet dogs is useful.
5. Kissing of pet dogs should be discouraged.
53
EVOLUTION OF PARASITISM IN HELMINTHES
It is believed that the parasitic platyhelminthes have been derived from the free living
turbellarian progenitors which were ectoparasites on molluscan, crustacean and echinoderm
host. According to Hyman, mollusk could have been the original hosts for digenetic trematodes.
Some ancestral rhabdocoels (Turbellarians) with a suctorial pharynx could have entered the
mollusc body accidentally while feeding on the soft tissues. The adults may have left the
mollusk body to lay eggs. With the evolution of vertebrates, they invaded the new hosts but the
54
connection with the original host retained which then became the intermediate host. During the
course of evolution, the endoparasites acquired certain adaptations for their survival in the body
of the vertebrate host. These adaptations were both morphological and physiological. Thus, a
vertebrate or the second host became obligatory for these endoparasites. Even a third
intermediate host was incorporated in the life cycle of these endoparasites. The disadvantages
of two or three hosts may have been compensated for by the development of polyembryony and
larval multiplication in the molluscan host. This may have led to the evolution of the modern
digenean endoparasites with their complicated life-cycle.
The monogenean parasites of vertebrates probably arose from the free living rhabdocoels long
after the digenean parasites. The ancestral monogeneans probably fed on the skin of the
sluggishly moving early vertebrates just like the turbellarians, and developed a haptor which
allowed them to maintain a permanent association with their food supply. Generally the
monogeneans are ectoparasites but some of them may have invaded the bladder, intestine and
coelom of their host e.g. Gyrocotyle. Living in a medium of pre-digested food, the parasite lost
its gut while retaining its characteristic monogenean features of posterior haptor and a direct
single host life-cycle.
The Gyrocotylidean-like monogeneans may have been the ancestors of cestodes.
PARASITIC ADAPTATION IN HELMINTHES
Parasitic adaptation may be defined as morphological, physiological and behavioural
modifications developed by the organism to lead a parasitic mode of life.
MORPHOLOGICAL ADAPTATIONS: Morphological adaptations can either be:
¾ Loss of organs
OR
¾ Attainment of new organs
LOSS OF ORGANS:
¾ LOCOMOTION: Locomotion is generally needed by the animal to procure food and
shelter. As Fasciola is an endoparasite, it does not need to make excursion for food and safety.
Hence, the locomotory organs are absent. However, locomotory organs are present in the free
living larvae of the parasitic forms. For example, the miracidium larva of Fasciola has cilia
while the cercaria larva possesses a tail for locomotion.
¾ DIGESTIVE SYSTEM: As the endoparasites feed on the digested or semi-digested food of
the host, the digestive system is either absent as in class cestoda or is greatly simplified as in
Flukes. Trematodes eg. Liver fluke have an incomplete gut with a single opening, the mouth;
anus is absent in them. Since the food is already in the digested state, there is complete absence
of digestive glands. Cestodes like tapeworm directly absorb the hosts digestive juices through
the tegument and hence there is a complete absence of the alimentary canal.
¾ CIRCULATORY AND RESPIRATORY SYSTEM: Absent
¾ EXCRETORY SYSTEM: Excretory system is quite efficient to remove the excretory wastes
so that proper metabolic activities may be maintained.
¾ NERVOUS SYSTEM: Nervous system is poorly developed and sense organs are lacking.
Since Fasciola is an endoparasite it does not react with the external environment and the
internal environment of the host is more or less uniform, hence the sense organs are absent in
them.
ATTAINMENT OF ORGANS:
55
¾ Dorsoventrally flattened body. Flat and thin body enables helminthes to live in narrow
spaces.
¾ ORGANS FOR ADHESION: Adhesive organs like suckers in Fasciola and hooks and
suckers in Taenia help the parasite to attach itself to the hosts tissues.
¾ BODY COVERING: Tough and resistant tegument is an important parasitic adaptation. The
body covering is frequently provided with scales and spines which afford suitable protection
to the parasite. The tegument also provides protection against the action of digestive juices
of the host.
¾ Fasciola has a muscular and suctorial pharynx, adapted to suck the hosts digestive juices.
¾ REPRODUCTIVE SYSTEM: The internal parasites are characterized by a complicated
reproductive system designed and perfected to meet the need for the tremendous egg
production. Almost all parasitic helminthes are monoecious except Schistosoma.
Hermaphroditism is of distinct advantage to the parasite as:
•
It ensures copulation even when few individuals are present.
•
After copulation both individuals lay eggs, thus doubling the rate of production.
•
In the absence of another member of the same species, the parasite can reproduce
offspring by itself, like in Taenia.
¾ In event of the failure of cross fertilization, they resort to self fertilization in which close
proximity of the cirrus and vulva is of great help.
¾ In flatworms, the ovaries and testes are either greatly enlarged or show an increase in
number so that they are capable of producing a large number of gametes. In cestodes, the
reproductive system is much more elaborate and each mature proglottid possesses one or two
sets of male and female genetilia. In a gravid proglottid all other organs of the system
degenerate to make room for the highly branched uterus.
PHYSIOLOGICAL ADAPTATIONS:
¾ Tegument is highly permeable and the helminthes can absorb the hosts digestive juices by
the tegument by simple diffusion.
¾ These endoparasites are facultative anaerobes. They respire by the breakdown of glycogen.
¾ They are well adapted to the hosts osmotic concentration and since their osmotic
concentration is same as that of the host they don’t need to osmoregulate. However, in the
intestinal tapeworms the osmotic pressure is a little higher. This permits the ready absorption
of the hosts digested juices by the parasite.
¾ They have high pH tolerance.
¾ Parasites secrete antienzymes to neutralize the digestive enzymes of the host.
¾ These parasites have a complicated life cycle. They require two or more host to complete its
life cycle. A very large number of eggs are produced by helminthes-10,000 or even more. Such
an enormous number of egg production is needed because the chances of survival of the eggs
are less. Rate of reproduction of an organism is directly proportional to the chances of death it
faces at various phases of its life cycle. In flukes, a single egg produces several embryos, a
process called polyembryony. In them, the various larval forms are produced by the simple
asexual multiplication of germ cells. Thus, a single zygote gives rise to several adults.
¾ The eggs are covered by a resistant capsule or shell as a result of which they can remain
viable for a long time even in adverse conditions.
56
¾ Involvement of several larval forms in the life cycle ensures transmission of parasite from
one host to another. Intermediate host ensures wide dispersal of the species.
¾ Parasites have successfully solved the problem of extinction by finding out suitable
reservoirs in which they thrive but never cause any harm to them.
CLASSIFICATION:
Phylum Platyhelminthes is divided into four classes of which only class turbellaria include free
living forms while the remaining three classes include parasitic forms:
™
™
™
™
CLASS TURBELLARIA
CLASS MONOGENEA
CLASS TREMATODA
CLASS CESTODA
Classification has strictly been followed from Invertebrate Zoology by Robert D. Barnes(5th
edition)
CLASS TURBELLARIA: (L., turbella means a stirring)
1. Mostly free living either terrestrial or aquatic found in both fresh and marine water.
Most aquatic forms are bottom dwellers living in sand or mud, under stones and shells,
or on sea weed.
2. Broad, flat leaf-like soft body covered with a cellular or syncytial epidermis having
secretory cells and rhabdites. The epidermis is partly ciliated.
3. Reproduction is either asexual or sexual. Asexual reproduction is by fission or by
strobilization.
4. Mostly hermaphrodite. Simple life cycle. Development is direct in most turbellarians,
however some polyclads produce a free swimming larva.
5. They have a great power of regeneration.
Despite their similarity in appearance, turbellarians exhibit considerable internal complexity,
and the class is composed of a relatively large number of diverse groups.
Turbellarians are further grouped into two: 1. Archoophoran 2. Neoophoran
Archoophoran Turbellarians:
1. They reflect a more primitive level of organization.
2. Yolk glands are absent.
3. Entolecithal eggs.
4. Spiral cleavage.
ORDER : Acoela
1. Small marine flatworms usually less than 2mm in length.
2. Gut without pharynx and devoid of a cavity.
3. Protonephridia absent.
4. Gonads are often not bounded by a cellular wall.
5. A few species ate commensal living in the intestine of echinoderms.
Example: Anaperus, Convoluta, Afronta
ORDER : Nemertodermatida
1. A small group with digestive tract lined by epithelial cells.
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2. Small marine species similar to acoels but possessing uniflagellate sperm.
Example: Nemertoderma
ORDER : Macrostomida:
1. Small, freshwater and marine species with a simple pharynx and a simple saclike
ciliated intestine.
Example: Microstomum, Macrostomum
ORDER : Haplopharyngida
1. Small marine animals similar to macrostomida but possessing a proboscis and a
temporary anus.
Example: Haplopharynx
ORDER : Catenulida
1. Small freshwater species having a simple pharynx and a ciliated saclike intestine.
2. Gonads are unpaired.
3. Male gonopore is dorsal above the pharynx.
4. Female gonoduct absent.
Example: Stenostomum, Catenula
ORDER : Polycladida:
1. Marine flatworms of moderate size, with a flattened more or less oval body.
2. A pair of anterior marginal or dorsal tentacles may be present.
3. Muscular pharynx present.
4. Intestine is centrally located, elongate with many branched,
diverticula.
radially arranged
5. Numerous eyes.
6. Many of them are brightly colored.
Example: Stylochus, Notoplana, Leptoplana
Neoophoran Turbellarians:
1. They reflect an advanced level of organization.
2. Yolk glands present.
3. Ectolecithal eggs.
4. Development greatly modified from the spiral pattern.
ORDER : Prolecithophora:
1.
Usually small, freshwater and marine species having a bulbous pharynx and a simple
intestine.
2.
Ovary produces eggs and follicle-like egg cells.
Example: Plagiostomum
ORDER : Lecithoepitheliata
1. Freshwater and marine species with a simple intestine.
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2. Mouth and complex pharynx is situated at the anterior end.
3. Ovary produces eggs surrounded by follicle- like yolk cells.
Example: Prorhynchus
ORDER : Rhabdocoela
1. A large group of small freshwater and marine species having a bulbous pharynx and a
simple intestine and a pair of nerve cord.
Example: Gnathorhynchus
ORDER : Proseriata
1. Small, mostly marine with some interstitial species.
2. Tubular pharynx with a simple unbranched gut.
Example: Nemertoplana
ORDER : Tricladida
1. Relatively large, freshwater, marine and terrestrial turbellarians with a tubular pharynx
which is posteriorly directed. Gut has three branches.
Example: Freshwater species include Dugesia, Procotyla etc.
Marine species like Bdelloura is a commensal on the book gills of horse shoe crabs.
Land planarians include Bipalium, Orthodemus, Geoplana
GROUP FLUKES: They include both the classes Monogenea and Trematoda and contain over
8000 species of ectoparasites and endoparasites. Majority of them are parasites of vertebrates,
especially fish, but immature stages are harboured by invertebrates. Unlike the turbellarians, the
body of the flukes is covered by a non-ciliated cytoplasmic syncytium, the tegument. Adhesive
suckers are usually present around the mouth and may also be present midventrally. Rhabdites
are absent.
CLASS MONOGENEA:
1. They have a single host in their life-cycle with only one generation i.e. one egg
produces one adult.
2. Largely parasitic on marine and freshwater fishes, amphibians, reptiles, and cephalopod
mollusk.
3. Majority are ectoparasite, but some of them invade the body cavities like the mouth, gill
chambers and urinogenital tract.
4. Since they are attached to the skin of fast moving host, the monogeneans have a
dorsoflattened body.
5. These monogenetic flukes possess a large posterior attachment organ called opisthaptor
which bears hooks and suckers allowing the parasite to cling on to the host.
Example: Polystoma, Gyrodactylus
CLASS TREMATODA: (Gr., tremta means a hole, and eidos means form)
1. They are ecto- or endo- parasitic worms.
2. Unlike the monogenea, the trematodes require two to four host to complete its life cycle.
3. Unsegmented, dorsoventrally flattened leaf like body covered with a thick cuticle.
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4. Ciliated epidermis with rhabdites absent.
5. Bears suckers for attachment to the body of the host.
6. Hermaphrodite.
7. Life cycle is either simple or complex with one or more larval stages.
Example: Fasciola, Schistosoma
CLASS CESTODA: (Gr., kestos means girdle and eidos means form)
1. Endoparasites living in the intestine of vertebrates, commonly known as tapeworms.
2. Pseudosegmented , ribbon like body covered by a syncytial tegument.
3. Body is divided into a few to many proglottids(not true segments).
4. Ciliated epidermis with rhabdites absent.
5. Suckers and hooks are the organs of attachment.
6. Digestive system absent.
7. Hermaphrodite. Each segment is a hermaphrodite with one or two sets of male and
female organs.
8. Complicated life cycle usually involving two or more hosts.
SUBCLASS : Eucestoda
1. The great majority of cestodes belong to the subclass Eucestoda and are commonly
known as tapeworms.
2. They posses a long ribbon-like body divided into scolex, neck and strobila with many
proglottid(polyzoic).
Example: Taenia, Diphyllobothrium, Hymenolepis, Echinococcus
SUBCLASS : Cestodaria
1. A small group of cestodes that show some similarity to trematodes.
2. Unsegmented leaf-like body without a scolex and strobila(monozoic).
3. Body contains only one hermaphroditic reproductive system.
4. Trematode like suckers are sometimes present.
5. Digestive tract absent and the larvae resemble the larvae of tapeworms hence placed in
class cestoda.
6. They are intestinal and coelomic parasites of sharks, rays, and primitive bony fishes.
Example: Gyrocotyle that lives in fish.
BIBLIOGRAPHY
1. Adaptation: Fitness of an organism for its environment.
2. Apolysis: Shedding of gravid proglottids in taenia.
3. Axial gradient: The capacity of regeneration along the antero- posterior axis due to
difference in intensity of metabolic activity.
4. Bulbous pharynx: The pharynx is characterized by a sucking muscular bulb as found in
platyhelminthes.
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5. Cirrus sac: A pouch containing seminal vesicle, prostrate gland and cirrus of some
platyhelminthes.
6. Coracidium: A ciliated free swimming larva of Diphyllobothrium latum (a cestode).
7. Definitive host: A host which harbours the adult stage of the parasite and in whose body
the sexual mode of reproduction takes place.
8. Direct development: Without the occurrence of any larval stage.
9. Ectoparasite: A parasite that lives outside on the surface of the body of the host.
10. Endoparasite: A parasite living within the hosts body.
11. Gynaecophoric canal: The infolding of the lateral margins of the body of male behind
the ventral sucker for holding the female in Schistosoma
12. Host: An organism which harbours the parasite.
13. Intermediate host: A host that harbours the larval stages of the parasite and in whose
body the development of larva takes place.
14. Laurers canal: Represents the rudimentary vagina in platyhelminthes during the
breeding season.
15. Parasite: Organisms that live on the expense of other animals for food, shelter, and
dispersion.
16. Plerocercoid: The final stage in the life-cycle of Diphyllobothrium latum (a cestode).
17. Polyembryony: A developmental phenomenon in which the initial mass of embryonic
cells give rise to more than one embryo.
Suggested Reading :
1. Invertebrate Zoology , by Robert D. Barnes
Publisher: Saunders College International Edition (5th Edition)
2. Parker And HaswellText Book Of Zoology, Invertebrates, Volume 1
Edited by Marshall And Williams (7th Edition)
A.I.T.B.S. Publishers And Distributors
3. Parasitology, by K. D. Chatterjee
Publisher: 6, Amrita Banerjee Road, Kalighat, Calcutta
4. Biology Of Animals, by Ganguly, Sinha And Adhikari
Publisher: New Central Book Agency
5. Invertebrate Zoology, by Ruppert/ Barnes
Publisher: Harcourt Asia PTE Ltd.
6. Modern text book of Zoology, Invertebrates by R. L. Kotpal
Publisher : Rastogi Publications
7. Text Book of Medical Parasitology, by C. K. Jayaram Paniker
Publishers : Jaypee brothers
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