Introduction to Parasite diversity labs
Overview
The objective of this segment of the course is to introduce you to the diverse life histories
and extensive adaptive radiation of animal parasites. This is accomplished through the
combined use of a standard parasitology slide box, Internet material and specific
demonstration materials. You are encouraged to work at your own pace with the slide
boxes, using this lab manual, plus supplementary texts and diagrams as a guide. The
latter will be available to you in the lab. For those of you who wish to be more
independent, I urge you to visit at least one of these excellent web sites:
http://asp.unl.edu/photo.html
http://www.biosci.ohio-state.edu/~parasite/home.html
Both are award-winning sites that detail virtually all of the parasites that we will cover in
this section of the course.
The slide box is a valuable tool for appreciating the functional morphology and diverse
life cycles of common groups of animal parasites. It is here where you will be exposed to
parasite adaptations and diversity. I have only included the bare minimum of slide
material. However, thorough familiarity of the slides in your slide box will be an
essential part of the course. You are responsible for understanding and properly using the
terminology found within this manual. Most terms are defined in the text descriptions, in
my lectures, or in the material available on lab day. Study the slides carefully and
compare specimens to the diagrams and descriptions provided in this manual and in the
texts available. Make sure that you make labeled drawings for your future reference for
each of the specimens. Your book will be an invaluable reference source as you go
through your slide box.
You will get the greatest benefit from the slide box if you try to integrate the morphology
of each species with its life-cycle and by identifying similarities and differences among
related species. Where does the parasite live in the host ? How does it attach ? How does
it feed? How does it reproduce and transmit between hosts ? Morphology can provide
many clues to the life cycle of a parasite and life cycles can suggest the types of
morphological adaptations that might be necessary.
Classification scheme for species referred to in this manual
The classification scheme used in this manual is greatly simplified. However, I do ask
that you become familiar with the names of the few species I have included (to genus).
All of the following species are in your slide box or will be available on demonstration.
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Other species may also be presented as demonstration material, depending on their
availability.
Microparasites:
The Protozoa (note: the phylogeny of this group is extremely contentious. Each of the
texts in the lab treats the subgroups differently. I use a fairly traditional scheme here. You
are not responsible for the detailed phylogeny outlined here – I provide it for convenience
only).
Phylum Sarcomastogophora
Class Zoomastigophora
Order Kinetoplastida
Trypanosoma rhodiense and T. brucei
Order Trichomonadida
Trichomonas vaginalis
Order Diplomonida
Giardia lamblia
Phylum Apicomplexa
Plasmodium falciparum
Cryptosporidium parvum
Toxoplasma gondi
Macroparasites:
Phylum Platyhelminthes
Class Digenea
Order Strigeata
Schistosoma mansoni
Ornithodiplostomum ptychocheilus
Posthodiplostomum minimum
Order Echinostomata
Echinostoma revolutum
Fasciola hepatica
Class Cestoidea
Diphyllobothrium dendriticum
Triaenophorus crassus
Echinococcus granulosis
Phylum Acanthocephala
Neochinorhynchus sp.
Polymorphus paradoxus
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Phylum Nematoda
Trichinella spiralis
Trichuris trichura
Necator americanus (or Ancylostoma caninum)
Wucheria spp.
Phylum Arthropoda
Class Crustacea
Argulus
Sacculina
Hemobaphes
Class Arachnida
Argas persius
Dermacentor andersoni
Class Insecta
Diamanus
Pediculus humanis
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Parasite diversity I. Microparasites
The microparasites include the protozoans, bacteria, fungi and viruses. They are
characterized by their small size (almost always microscopic and often only 1-2
micrometers in size), their modes of reproduction, and their ability to illicit strong and
specific host immunity. They are fascinating to study, partly because they include the
majority of the major parasitic diseases of humans, but also because they demonstrate
brilliant adaptations for the parasitic way of life. Unfortunately, their small size makes
them exceptionally difficult to study. This difficulty leads to the fact that we know so
little about the general biology of even our most pathogenic microparasites. In this lab,
living material is very difficult to acquire. We will focus on the protozoan parasites via
demo material, slide material and text material.
The parasitic Protists
The study of protozoans requires considerable patience and skill as a microscopist. In
order to find the diagnostic characters on your slides, you will probably require the use of
oil immersion techniques (if you are shaky on the use of this technique, ask me for a
refresher!).
The parasitic Protists are an extremely diverse assemblage of species. Your slide box
contains only a very small sample of this diversity. As you go through the protozoans,
pay particular attention to the many morphological differences between species, as well
as the diversity of life-cycles. You will see that there are direct and indirect modes of
transmission, sexual and asexual multiplicative phases and various levels of response by
the host.
Rather than provide detailed line figures of the various types of parasitic protists, you
should use the material available to you (texts, websites) to sketch your own! In a
nutshell, you should be able to sketch the life-cycles of each of the groups listed below,
and you should be able to make a rough sketch of the generalized body plan.
Phylum Mastigophora
Trypanosoma rhodiense and T. brucei
T. rhodesiense is the causative agent of the dangerous human disease, African sleeping
sickness. Over 10,000 cases are diagnosed annually; 50% of these die and the other 50%
suffer permanent damage. T. brucei is responsible for the immensely important disease
of livestock known as nagana. Both species use tsetse flies as vectors. African sleeping
sickness has plagued humans ever since they first encroached on the domain of the tsetse
fly. The disease has kept over 4 million sq miles of grazing land in Africa out of
agricultural production. Consider the political, ecological, and socioeconomic
implications if a vaccine for sleeping sickness were ever found (note: a third species, T.
cruzi, is known as American trypanosomiasis and infects over 10 million people; it uses a
reduvid bug as vector).
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This group is characterized by a spindle-shaped body containing a central nucleus (or
kinetosome). The flagellum arises from this structure. A kinetoplast is also situated near
the base of the flagellum. It is a mass of DNA within a single mitochondrion and is
easily seen with the light microscope. Your slide is a smear from an infected human’s
blood. You will see numerous trypomastigotes. This stage is the final developmental
stage of the trypansomes (see diagram in text or website). Under oil immersion, you
should be able to see the undulating membrane, flagellum, nucleus and kinetoplast.
Trypomastigotes undergo asexual reproduction (binary fission) in the vertebrate host
(human, domestical mammal, native ungulates). The fact that the parasite can survive in
so many reservior hosts is a major factor in the epidemiology of the disease. During a
blood meal, the tsetse flies ingest infective trypomastigotes. Once ingested by the fly,
these multiply by binary fission and then metamorphose to a long and thin form within
the insect’s forgut. These migrate to the salivary gland and transform into a further form,
which can then be inoculated into another mammalian host during a blood meal. This
species is fascinating for its ability to evade the host’s immune system by continuously
changing the antigenic structure of their external surface (the phenomenon of antigenic
variation).
Trichomonas vaginalis
This is a cosmopolitan species, found in the urogenital tracts of men and women. You
are not responsible for the functional morphology of this group. I include it for two
reasons. First, this species is one of several parasitic protists that are transmitted via direct
physical contact (T. vaginalis is primarily an STD). It therefore represents a third type of
transmission strategy (in addition to using a vector and via cysts). Members of this family
span the range of symbiotic life-styles, from purely free-living, to commensal (one group
is found only in the gut of termites; another is found between the teeth, and on the tonsils,
of humans) to parasitic. Refer back to your notes on the relationships between
Rhizobium and Agrobacterium, and between the pathogenic vs. mycorrhizal fungi. Can
you imagine any parallels ? Members of this group are easily recognized by having an
anterior tuft of flagella, a stout median rod and an undulating membrane. Can you
imagine the selection pressures occurring on the Trichomonads that may have led to their
different ways of exploiting hosts ?
T. vaginalis is transmitted primarily through sexual intercourse. Many strains are of very
low pathogenicity, especially in men. Some strains, particularly when infecting women,
can cause an intense inflammation, with itching and copious white discharge that is
swarming with trichomonads. They feed on bacteria, leukocytes and cellular debris.
Giardia lamblia
Your slides contain the feeding form (trophozoite) and cysts of this, the most common
intestinal parasite of humans. It is the causative agent of giardiasis or ‘beaver-fever’.
This is the disease most associated with wilderness campers and is typically associated
with severe diarrhea. The parasite matures in many reservoir hosts (such as beavers and
cattle) which is an important factor in the epidemiology of this disease.
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Giardia has 4 pairs of flagella arising from the centre of the cell; they are typically lost
during staining. Using the oil immersion lens you should be able to see two nuclei and a
central pair of median bodies. The trophozoites are cup-shaped and the surface of the
ventral side is concave and thickened to form a large adhesive disk, used for attachment
to the host’s intestinal villus (see diagram in text or website). Locate a cyst using the
high power objective and advance to oil immersion power and note the thick cyst wall,
enclosed flagella, nuclei and median bodies.
Mammalian definitive hosts ingest infective cysts in drinking water. Giardia excysts in
the intestine, releasing trophozoites. These attach to villi and multiply by binary fission.
In victims of giardiasis massive infection is typical; the presence of several billion cysts
in a single stool is not unusual! Cysts are the infective, resistant stage. They can
withstand freezing and stomach acidity. Unlike the trypansomes, only one host is
required to complete the life cycle.
Phylum Apicomplexa
All members of this phylum are parasitic (recall that it is quite unusual for any phyla to
be exclusively parasitic) and they infect members of all animal phyla. All are
characterized by an apical complex that is only revealed under the electron microscope.
This structure is important for recognition and penetration of host cells. Members of the
phylum include some of the most serious diseases of humans and domestic animals. The
life cycles of apicomplexans are complex; with direct or indirect pathways. Species with
direct life cycles have resistant spores or oocysts that bridge the gap in the external
environment between hosts. Those with an indirect life cycle involving vectors remain
within a host and thus have no need for a protective cyst.
Monocystis lumbrici is an Apicomplexan parasite that lives in the seminal vesicles of
terrestrial earthworms. The worm becomes infected when it ingests a spore containing
several sporozoites. These hatch in the gizzard, where the released sporozoites penetrate
the intestinal wall, enter the dorsal blood vessel, and then make their way to one of the
host’s 5 or so ‘hearts’. From there they penetrate the seminal vesicle, where they enter the
sperm-forming cells in the wall. At this point they ingest and destroy the developing
spermocytes. Then they move into the lumen of the vesicle where they become mature
trophozoites. After a period of feeding, two of these will come together, flatten against
each other, and secrete a common cyst around each other. This is the gametocyst, usually
containing 2 gamonts. Each now undergoes extensive division of their nuclei, pinches off
a small portion of cell cytoplasm, which together then bud off to become the gametes.
The fusion of a pair of gametes forms a zygote, each ultimately becoming a spore. Three
cell divisions later forms 8 sporozoites. Thus, each gametocyst now contains many
oocysts. New hosts become infected by ingesting gametocysts, or more commonly, by
ingesting individual oocysts. Thus, meiosis is zygotic. Only the zygote is diploid, and
reductional division in sporogony returns the sporozoites to the haploid condition.
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Proceedure:
Dissect the anterior end of a freshly anesthetized worm. Remove the seminal vesicles and
place in a drop of water. Take small pieces of seminal vesicle, squash under a cover slip
and look for the different stages of Monocystis (see figure above). Make drawings of
each stage and construct an annotated life-cycle. Record as many different stages of
infection as you can.
Plasmodium falciparum
This intracellular parasite is the most dangerous of the 4 species that cause malaria in
humans. This species will be the focus of our discussions in lecture. It is the most
common and debilitating of human parasitic diseases. Over two million people each year
die from the disease. Moreover, the disease has played a major role in shaping our
history and civilizations. It is impossible for you to understand this disease without a
thorough understanding of its life cycle.
Your slides only show a fraction of the various stages of the malaria life-cycle. One slide
shows gametocytes inside host red-blood cells as deeper-staining structures (often
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crescent or bean-shaped). Using the oil immersion lens you should see that
microgametocytes have a large nucleus and irregularly distributed granules.
Macrogametocytes have a small compact nucleus with a dark red nucleolus. Further
devleopment of these forms only continues inside a mosquito’s stomach. Sometimes, the
gametocyte-infected cells have been distorted to such an extent that it ruptures during the
fixing process. Thus, you may see some gametocytes which are not enclosed within a red
blood cell membrane.
You also have slides which show trophozoites. They can be distinguished by the large
food vacuole surrounded by a thin layer of cytoplasm and including a peripheral nucleus.
This gives the characteristic ‘ring’ which forms the well-known ‘ring-stage’. Again, the
infected RBC may appear distended or abnormal in shape. Use the demonstration slides
of the various life-cycle stages and life-cycle diagrams to help you understand the
biology of this important parasite.
Cryptosporidium parvum
This waterborn parasite, together with Giardia, represents one of the two major waterborn
parasites of humans. Although the genus was first discovered many years ago (as a
parasite of turkeys), it has only recently received attention from biologists and disease
specialists. In 1982 the American Centre for Disease Control, found 21 males from large
cities to be suffering from severe diarrhea; all had AIDS. Then, in 1993, there was a
severe outbreak of cryptosporidiosis in Milwaukee, affecting over 400,000 people. There
has since been an outbreak in Kelowna (1995), in Shaughnessy (near Picture Butte),
Alberta in 1997 and in Winnipeg in 2001. Not surprizingly, it is one of the main
parasites under study by parasitologists, agriculural agencies, water quality technicians
and environmental groups. The Lethbridge area is considered one of the most high-risk
areas in Canada for an outbreak of Cryptosporidiosis, due mostly to the numbers of cattle
feed-lots and our climatic conditions.
It is generally a non-lethal disease for humans but does cause severe diarrhea, vomiting,
weight loss and cramping. People with normal immune systems expel the parasites in
about 2 weeks, and are then immune to further infection. However, it is usually lethal in
patients with compromised immune systems (especially the elderly and HIV-infected
hosts). The main concern is that cysts are highly resistant to standard disinfectants such
as chlorine and chlorine dioxide. In the US, the main method of control is the conversion
of water-treatment plants to use ozone rather than chlorine as a disinfectant. Costs run
into the billions of dollars.
This parasite is a recent problem, so that biology supply houses do not have specimens.
Concentrate on the various life-cycle stages. Note the similarities and differences in the
life cycle stages between this coccidian parasite and Plasmodium.
Toxoplasma gondi
This is a cosmopolitan parasite, found in humans and a wide variety of other carnivores.
It is intracellular within the cells of muscle tissue and the intestinal epithelium. The lifecycle always includes an intestinal and extra-intestinal stage in cats and other felines, but
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extraintestinal stages only occur in other hosts. Sexual reproduction occurs in cats,
asexual reproduction occurs in the other hosts. Cats release oocysts in their feces and
then these hatch out in the guts of intermediate hosts, including humans. The normal lifecycle involves cats and either mice or rats.
Many of us carry antibodies to Toxoplasma, so either we have come into contact with
oocyst-laden cat feces, or we have eaten infected meat. Yet most of us don’t get sick
from this infection and never know we have it. Thus, context-dependency is a common
feature of this interaction. Factors such as stress, age, diet etc. can lead to reduced
immunocompetence and increased pathogencity. Martina Navratilova lost the US Open
in 1982 after withdrawing due to complications with this parasite.
Certain stages of the parasite proliferate in almost any tissue, often killing the cells faster
than they are replaced. If they end up in the brain, and many do so, problems can result.
One tragic form of the disease is congenital toxoplasmosis. If a mother is infected, then
Toxoplasma often will infect the developing fetus. Most infections will be nonpathogenic but many will cause significant morbidity and death. Stillbirths and
spontaneous abortions are usually the cause. This is especially true in sheep, where
spontaneous abortions can reach epidemic propotions. Severe damage is usually caused
by CNS disorders due to the rapidly dividing parasites. It is for this reason that pregnant
mothers and cats are a bad mix.
Myxobolus cerebralis
This bizarre species is a member of an enigmatic group called the Myxozoans. Some
view these oddballs as highly adapted protozoans, others view them as previously
multicellular cnidarians (jellyfish?) that lost their cellular structure when they became
parasitic. All members of this diverse group are parasitic. We consider them (albeit
briefly) for three reasons. First, it is another group which is locally common in most of
our fresh-water fish (although most are completely unknown and not yet identified).
Specimens are usually easy to obtain, depending on the time of year. Second, one of the
species, Myxobolus cerebralis is a disease agent of fish, particularly important as agents
of mortality among salmonids. A third reason is that this obscure group provides an
excellent example of how controversial and acromonius protozoan taxonomy can
become.
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Lab 3. Parasite diversity II. Macroparasites - the worms
As for the previous lab on microparasites, we can only hope to scratch the surface of
macroparasite diversity. There is a great deal of information here, especially when you
combine this with the information in the available texts and in your slide boxes. Don’t
get overwhelmed! You are not expected to ‘memorize’ this entire component of the lab
manual. You are expected, in a nutshell, to know the life-cycles and functional
morphology of a representative trematode, cestode, nematode and several parasitic
arthroprods such as the ticks and copepods. You are also expected to understand and
appreciate the diversity of types of life-cycles and morphology. If you can sketch the
general life-cycle and morphology of a representative specimen, then you will satisfy
what is required for the lab exam.
The Platyhelminths
Digenea
This group is so-named for the two generations characteristic of their complex-life
cycles. The first consists of repetitive cycles of asexual reproduction in the first
intermediate host that is almost always a gastropod snail. The second generation
involves transmission, sometimes by means of a second or third intermediate host, to a
definitive host where sexual reproduction occurs. The basic anatomy is uniform and
includes several features that have been modified from their Turbellarian ancestors to
permit parasitism. Features such as a large ventral sucker (for attachment) and an oral
sucker (for feeding) are usually distinctive. The majority of the rest of the body is
devoted to reproduction. They are almost always hermaphroditic, with both male and
female components to their reproductive systems.
The life-cycle is notoriously complex. Spend as much time as you need in order to follow
the steps between an egg and the reproductive adult. Be sure you can sketch a standard
life cycle. You should be familiar with the following stages: miracidia, sporocysts,
cercaria, metacercariae, and adult. Be sure you know their functional morphology and
whether they are produced sexually or asexually. Use the figures in any of the material in
the lab to fully understand this complex life-cycle. We will see many of these stages live
in the lab as the term proceeds.
Schistosoma mansoni
These are among the best known of the digenetic trematodes and schistosomiasis is one
of the most debilitating of human diseases. Human schistosomiasis is not endemic to
Canada, but schistosomes of waterfowl and muskrats are common. Swimmers Itch, a
common phenomenon in our local waters, results when cercariae of these other species
attempt unsuccessfully to penetrate our skin. Our immune systems kill the cercariae but
in doing so it triggers an intense, hypersensitive reaction and a long-lasting ‘itchy’ rash.
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Unfortunately, the most important group of trematodes to humans is the one that is highly
atypical of the group. Oddly, they are one of the few ‘worms’ that is dioecous. Also,
adults don’t reside in the GI tract of their hosts, but in the blood stream. Lastly, they do
not have a metacercarial stage. Instead, cercariae penetrate the skin directly. Take careful
note of these exceptions when you study the functional morphology of the males in
females in your slide box.
Fasciola hepatica
This giant lives in the livers of mammals, especially ungulates such as cows, sheep and
deer. It is cosmopolitan in distribution and it is of great economic significance to the
livestock industry. It too is a highly modified trematode. Use the material on the lab
benches and the two InterNet sites to help understand its life-cycle and functional
anatomy.
Echinostoma sp.
I include this group because they represent a typical life-cycle and because they have a
straightforward anatomy that usually shows up well when stained. They commonly infect
our local mallards and geese. In fact, it is notorious generalist, even reported from
humans. The metacercariae are found in pond snails. The obvious spines at the anterior
end are distinctive to the group and aid in attachment. Note the spined tegument
(epidermis), also used to assist in attachment. There is an obvous, muscular pharynx,
esophagus and branched cecae. The male reproductive system is comprised of 2 oval
testes in the posterior body and the roundish seminal receptacle located posterior to the
ovary. The vitelline ducts drain the lateral field of vitellaria (yolk glands) which overlap
the cecae. The uterus is located anterior to the ovary and should contain 100’s of eggs.
Ornithodiplostomum and Posthodiplostomum
I include these two species because they are readily available as live specimens in local
populations of fathead minnows. The important aspect of these two rather obscure
species is to examine the various stages of their life-cycles in live preparations. Don’t
worry about adult morphology. Both species are extremely small as adults.
Cestoda
All adult cestodes, or tapeworms, are flatworms that are parasitic in the intestine of
vertebrates. Typically, they have bodies divided into three sections: the head, neck and
strobila. The head (or scolex) contains some combination of hooks, suckers, tentacles or
grooves, each of which is used for attachment. The neck is the zone of proliferation,
where repeated asexual reproduction (budding) of body segments occurs. This section of
the worm is always very metabolically active.
The strobila contains the segments (or proglottids) of the tapeworm. There can be
thousands on each individual. The youngest ones occur just behind the neck; the oldest
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are usually the longest and widest ones at the posterior end. The latter are usually
nothing more than a sac filled with eggs. Most cestodes are hermaphroditic, with each
proglottid containing one or more complete sets of male and female organs. The overall
structure of the reproductive systems (within a proglottid) is similar to that described
earlier for trematodes. Cestodes have no digestive system; nutrients are absorbed
directly across the tegument.
As for trematodes, the life-cycles of cestodes are complex. All cestodes produce a larval
stage called an oncosphere that is housed within the eggs. It always has 6 hooks. Stages
after the oncosphere are diverse, depending on the species. In many cases, particularly
for aquatic cestodes, the oncosphere penetrates an intermediate host (often a copepod or
other crustacean), developes within the body cavity and is then further ingested by
another intermediate host (often a fish). After the infected fish is eaten by the vertebrate
final host, it developes into a sexual adult.
Diphylobothrium ditremum
Members of this group of aquatic cestodes are known as broad fish tapeworms. The
species D. latum is a pathogen of humans in northern communities wherever the
ingestion of undercooked fish is common. Some small coastal towns in Scandinavia can
be 100% infected. We will look at the larval stages of these distinctive cestodes in
whitefish collected from Northern Alberta.
D. dendriticum is widespread throughout Canada’s north. They are found as white cysts
on the viscera in numbers that can exceed 900 per host. They are most common in Lake
Whitefish, Cisco and arctic grayling. The larvae reside within the cyst and then
metamorphose into adults when eaten by loons or grebes. The species can infect humans
although the extent they do so is not known. Presumably infection in humans is rare
because the cysts are associated with viscera, which we tend not to eat!
Triaenophorus crassus
We should also have this species on display. It is another ‘northern’ problem, mostly
because the larvae that encyst in whitefish and trout make them unmarketable. The lifecycle is similar to D. dendriticum, except the final host is northern pike. We will spend
some time on the details of this local problem during the lab.
Echinococcus granulosis
This cestode is among the most serious and debilitating parasites of human in many parts
of the world. It has a terrestrial life-cycle. This feature alone should indicate that it’s
functional morphology and ecology will depart from the standard cestode plan. The
larval cyst (hydatid) contains germinative tissue that gives rise asexually to thousands of
‘heads’ per cyst. It is this unique cyst that is associated with pathology in humans and
other intermediate hosts. There are two slides in your slide box; one with the hydatid
larval stage and the other with the small adults. On your slide of the adult, look for the
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head that is armed with a double row of non-retractable hooks and four suckers. There
are only 2-3 strobila per individual.
Adults occur in the intestine of dogs, wolves and other canids. Eggs pass in the feces, are
ingested, and hatch within the intermediate host. This host is usually a large, herbivorous
mammal, such as moose, sheep, deer, or kangaroo. Oncospheres enter the hepatic portal
system and then eventually lodge in the liver, lungs or brain where they develop into a
hytadid. The cyst is complex. The external wall is of host origin and is usually tough
and fibrous. Lining the internal cavity of the cyst is a layer of germinative tissue. Brood
capsules line the tissue, each producing 1000’s of scolicies. These scolices are referred to
as hytadid sand (see your slide box). When eaten by a canid, these hatch in the gut and
develop into adults.
The danger of this parasite to humans occurs when we act as intermediate hosts by
ingesting eggs of the parasite. Hydatid cysts then develop in our organs causing serious
pathology (= hydatid disease). Of course the risk to humans increases when we are in
close proximity to canids, especially in the north where house pets and sled dogs are
often fed on infected big game.
The Acanthocephala
This is an obscure group. However, we will cover various aspects of these worms in
class, so it is best to include them here. They are also locally common (in waterfowl) and
we will have examples of certain species in the lab. They are best known for their ability
to alter the behaviours of their hosts in order to facilitate transmission to the next host.
They are commonly called ‘thorny-headed’ worms because of their unique attachment
organs at the anterior end. It is a retractible proboscis armed with many hooks. Adults
are always parasitic in the guts of vertebrates. Larval stages almost always use
crustaceans or insects as intermediate hosts.
The two sections, head and body, should be obvious on the prepared material. The body
is distinctive, with no digestive system and no true circulatory system. The unique body
wall is used to transport nutrients directly from the host’s gut to the unique pseudocoelom
that comprises most of the interior of the trunk. Not surprisingly, the rest of the trunk is
devoted to reproduction. All acanths are dioecous, with females usually larger than
males. There are a pair of tandem testes and a common sperm duct leading to the
posterior gonopore and penis. There are usually a group of cement glands that produce a
substance that is stored in a reservoir. This substance is used to seal the female
reproductive tract after copulation. It can also be used to plug neighbouring males! The
female tract is typically comprised of lose ovarian balls that lie free in the pseudocoelom.
Life cycles are similar for almost all species. Eggs are typically ingested by a crustacean,
penetrate the gut wall and then lie free in the body cavity (this is the stage you will see in
our freshwater shrimp). Once it encysts and becomes infective, it is called a cystacanth.
This stage is ingested and then becomes a reproductive adult after ingestion.
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Neochinorhynchus sp.
These are common acanthocephalans of fish and turtles. I include them here mostly
because their internal anatomy is usually fairly straightforward. Try to discern as much
of their functional morphology as you can on the slides available. They use aquatic
copepods and snails as intermediate hosts.
Phylum Nematoda
Roundworms are among the most common and abundant organisms on earth. In addition
to numerous species of nematodes that are parasitic on plants and animals, many are freeliving. They are incredibly conservative in form, some would say ‘boring’! They are
typically elongate, cylindrical and tapered at both ends. They are covered with a
resistant, impermiable cuticle and possess a complete digestive system with a mouth and
anus. The cuticle is an immensely important feature of these animals, allowing them to
colonize terrestrial habitats, and yet also evade the host immune system. The esophagus
pumps in food and forces it into a thin-walled non-muscular intestine. Waste products
are forcefully excreted from the anus.
Most nematodes are dioecous, with females usually larger than males. Males have one
testis and a pair of scleritized, copulatory spicules. These are inserted into the female
during copulation in order to transfer the ameboid sperm.
All nematodes have a similar development pattern. Eggs are produced which hatch into
small L1 larvae. These metamorphose into L2 larvae, which tend to be fatter and shorter.
These typically infect an intermediate host. These metamorophose into long, thin L3
larvae which are infective to the final host. The latter typically act as the infective stage
for parasitic species.
Trichinella spiralis
This nematode causes trichinosis. It has an unusual life-cycle where the L1 is the
infective larvae, not the L3. Your slide box contains 2 slides of this species. One has the
adults collected from the small intestine of a mouse. Note the male and female anatomy.
The other slide shows striated muscle infected with L1 larvae. When the larvae become
encysted in muscle cells, they are referred to as ‘nurse cells’. Once in the muscle fiber,
the larvae increases in size and becomes coiled.
Adults live in the small intestine of carnivores and omnivores (pigs, humans, bears etc.).
They mate, the males die and then the females release L1 larvae. These enter the
circulation system and are carried to the heart and then to the rest of the body. They
encyst in specific locations in the body, especially in muscles associated with the tongue,
the back of the eyes and the diaphragm. When eaten by a carnivore the muscle larvae are
freed from their cyst and metamorphose into adults.
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Necator americanus and Ancylostoma caninum
These two species represent the two most common worm parasites of humans. Most
populations in the tropics are > 50% infected. They are commonly known as
‘hookworms’. They are the leading cause of childhood anemia in the tropics, often not
leading to death, but to decreased cognition and reduced neural function. Hookworms
are also common in the southern US.
There are two distinctive anatomical features to note on your slides. In male adults, the
posterior end contains a copulatory bursae that is used to anchor the male to the female
during copulation. They also have chitenous specializations at the oral end that are used
as cutting plates to pierce the intestinal lining.
All hookworms have similar and fairly simple life cycles, involving the direct penetration
of L3’s, usually through the soles of the feet.
Take special note of A. caninum. This parasite can be a problem for puppies, but also
adult dogs. Infection occurs when dogs ingest L3’s. Alternatively, L3’s can invade the
skin and then penetrate into the lungs. If they take this route, the larvae travel up the
trachea and then are coughed into the mouth where they are swallowed. Some however,
don’t make it into the gut and end up travelling throughout the body. In humans, this
leads to the condition known as larval migrans. It is the main reason that we are
encouraged not to get too close to our dogs! Alternatively, some of the larvae also end up
penetrating the mammary glands of bitches. These enter the milk and are highly infective
to newborn puppies. The larvae can also penetrate fetuses directly. This is the reason
why we have to treat almost all newborn puppies.
Wuchereria
These include the notorious worms that cause elephantiasis in humans. Other members
of the filarid worms infect amphibians, reptiles, birds and mammals. Most have no
economic influence at all on humans. However, there are about 4 species that cause
some of the most horrifying and debilitating diseases of humans in the world.
Wuchereria bancrofiti along with two other species, can lead to the condition known as
elephantiasis. Ancient Greek and Roman writers write horrific accounts of this disease as
being extremely common. Today, it has a trans-equitorial distribution and like
hookworms and other parasites, almost certainly arrived in the New World via the slave
trade.
The long, thin adults live in the lymphatic vessels, tightly coiled into nodular masses.
They are especially common in the lower lymph vessels near the legs and scrotum. The
females produce thousands of live larvae, known as microfilariae. These get picked up
by a mosquito where they undergo further development and then transmission to another
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host. Transmission is typically mechanical – they enter the wound made by the biting
insect.
Pathology is almost solely due to a hyperinflated immune response directed towards
gravid females in the nodules. In almost all cases of acute elephantiasis, it occurs in
people that are repeatedly exposed over long time periods.
The Parasitic Arthropods
The arthropods constitute an enormously diverse phylum, in numbers of species
and in the range of life styles adopted. Species parasitic on plants can be found
throughout this phylum. Some groups are entirely parasitic, whereas others have only a
few parasitic representatives among a large number of primarily free-living species. In
your slide box are representatives of a few of the arthropod taxa commonly thought of as
parasites. Many parasitic arthropods serve additionally as intermediate hosts for helminth
parasites (e.g. fleas), or as vectors for protozoan (e.g. malaria), bacterial or viral diseases
(e.g. Lyme’s disease) of humans and livestock. Groups of parasitological importance are
the Crustacea (copepods, barnacles, isopods), Arachnida (ticks and mites) and Insecta
(fleas and lice).
Morphologically, arthropods have segmented bodies, including jointed appendages, and
are covered with a chitinous body that serves as an exoskeleton. Arthropods must
undergo periodic molts as growth and development proceeds. In some cases, particularly
in the larval stages, some parasitic arthropods may look no different than their free-living
counterparts. In other cases, such as with some parasitic copepods and barnacles, they
have become so modified in adopting a parasitic mode of life, that it is often difficult to
determine that they are even arthropods! Sexes are separate; morphological differences
among them are few in some species, marked in others.
Developmentally, arthropods proceed from an egg, through larval and/or nymphal stages,
to the adult. Generally, the term ‘larvae’ applies to stages in which major morphological
changes occur; these stages are often fixed in nature. The term ‘nymph’ is applied to
stages which change in little other than size between molts, and are usually indeterminant
in number. Of course, given the great diversity of arthropods, there has developed a
diversity of nomenclature for the different larval or nymphal stages that rivals that of the
Platyhelminthes.
Class Crustacea
Argulus
These superficially resemble copepods, but differ in their possession of compound eyes, a
dorsoventrally flattened body with a carapace-like bilobed dorsal shield and a sucking
proboscis. This is a common ‘branchiuran’ of freshwater fish (also common in the
Lethbridge area). Note the above features as well as the pair of ventral compound eyes
and the dorsal median eye on the carapace (turn your slide over). On the ventral side
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focus carefully (40X) on the first pair of antennae and note the holdfast claws. Between
the antennae is stylet (stinger) and posterior to this is the proboscis and mouth. They
bury this proboscis into a blood vessel and feed on host blood. Mechanical injury and
anemia are associated with heavy infections of this parasite.
The life cycle involves free-swimming males and females which leave their hosts
regularly during the host breeding season. The nauplius stage is passed in the egg and
they hatch as free-swimming larvae. Upon finding a host, the larvae attach and feed for
several weeks before becoming sexually mature.
Lepeoptheirus salmonis
There are many representatives of parasitic copepods, with a bewildering array of body
forms. Most members exhibit marked modifications for a parasitic life style. As you
observe your slide material and the specimens on display, recall that they are related to
the familiar Daphnia and Cyclops. Consider some of the adaptations these bizarre
species have undergone in their exploitation of the parasitic lifestyle.
If you have caught salmon or steelhead on the west coast, you have seen Lepeoptheirus,
typically around the anus. They are commonly referred to as sea lice and they can be
devastating to aquaculture operations on both the west and east coasts. Take a specimen
from the display counter, make a wet-mount and examine the ventral side under a
dissecting microscope. Note the oval cephalothorax, the broad and flattened genetal
segment and the much recued abdomen. There are four pairs of swimming legs. Note
the red median eye on the dorsal surface (turn specimen over). On the sternum, posterior
to the mouthparts, is a two-pronged structure used for attachment.
The reproductive system in your female specimen consists of a coiled ovary in the lateral
lobes of the last thoracic segment and the beginnings of the long ovisacs at the posterior
end of the genital segment (see diagram). Adult females attach to the body surface of
marine fish. Eggs are laid and stored in the ovisacs. Nauplius larvae hatch and leave the
ovisacs. In water, larvae undergo a series of molts; the last stage molts to the adult stage
and attaches to a fish. Males are similar in size to females (this is actually rare; males are
typically much reduced) but they live only briefly. The adults may move readily from
fish to fish, an important feature in their epidemiology in fish farms.
Be sure to look at some of the other specimens of parasitic copepods on display.
Haemobaphes is a common parasite in intertidal fish on the west coast. The ‘head’ and
‘antennae’ regions of this species are highly modified to form an appendage which is
perfectly adapted to anchor within the host’s heart (!!!). Needless to say, it is a
permanent parasite of these fish. We will view a live video of this species and observe
the greatly altered blood flow which this parasite induces in its host.
Cecrops is another bizarre species which is found on the gills of the sunfish, Mola mola.
Look also at Chondocanthus zei, also from the West Coast. This group of parasites
contains species which are among the most highly specialized, and among the most
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pathogenic. Typically, the males are dwarfs, living permanently attached to the posterior
of females. The mouth is a gaping orifice; it contains a blade-like mandible which is
characteristic of the family. The biology of this group is very poorly known. We do
know that digestion occurs partly outside the parasites body, and this digestion can cause
a great deal of host pathology. Attachment to the host often leads to a host response
which acts to cover the head with host tissue, permanently anchoring the parasite in
place.
Sacculina
This is surely amongst the most bizarre of animals. These are known as the
Rhizocephalan barnacles. Considering the common occurrence of commensal barnacles,
it should not surprize you that parasitism has evolved within this group. Members of this
group live mostly in crabs. Their larvae are identical to those found in the free-living
barnacles. Adult Rhizocephalans possess no appendages and are anchored to the host by
a stalk from which roots proceed into host tissues. The life cycle is remarkable. The
parasite larvae attaches to its crab host via modified antennae. It metamorphosies into a
different larvae which is carried via the blood; these then form root-like branches
throughout the body of the crab. Eventually the entire body is penetrated, eventually
leading to molt inhibition and castration. A large cellular mass then forms, containing the
genital system. Your slide shows a cross-section of an infected shore crab from the West
coast. This barnacle is currently undergoing tests as a potential bio-control agent of an
introduced European crab which is devastating local shellfish operations.
Class Arachnida
Argas persius
This is a representative of the Family Argasidae, which comprises the so-called soft ticks.
Species within this genus are parasites of birds and bats, although most will bite humans.
This is the fowl tick and is an important parasite of poultry and other birds. Their
populations can increase in a henhouse to such an extent that they can kill chickens.
Soft ticks hide during the day and emerge at night to feed on sleeping hosts. Most
species are found in dry habitats where transmission typically occurs in bird nests.
Females lay eggs in dark hiding places such as nests. The emerging larvae and several
subsequent nymph stages have the same feeding habits as their parents. Most soft ticks
can go months or even years without feeding.
Dermacentor andersoni
This species, commonly called the Rocky Mountain Wood tick, is one of the most
famous transmitters of human disease in North America. It is a vector of Rocky
Mountain spotted fever, colorado tick fever and produces tick paralysis in man and other
animals. It is also the vector for tularemia; the causative agent is a plague-like bacterium.
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Other species of Dermacentor throughout the world are vectors of several diseases caused
by viruses and rickettsias, of humans and domestic animals.
This is an example of a ‘hard tick’ (Family Ixodidae), named for the chitinous dorsal
plate. Other hard ticks are known to transmit other important diseases (e.g. Lyme disease
is transmitted by deer ticks of the genus Ixodes and is the most frequently diagnosed ticktransmitted disease in North America). Dermacentor albipictus is a very common parasite
of moose and elk in Alberta and causes the pathogenic condition known as Ghost Moose.
Intensities of up to 100,000 individuals/host are not uncommon in Elk Island National
Park, and elsewhere. The expression ‘Ghost Moose’ comes from the premature loss of
hair in winter, indirectly caused by engorged female ticks. This loss of hair leads to the
ghostly pale colour and leads to severe problems with thermoregulation.
The body of hard ticks is in two parts; the anterior Gnathostoma and the posterior body.
Adults have four pairs of walking legs. Note the anterior chelicera, which end in pincers.
Ventral to the the chelicera is an exposed hypostome armed with teeth or hooks. One
part of the complex mouth region grasps a fold of skin, another part (the chelicerae) cut
through it and then the hypostome is anchored within the wound.
The specimen in your slide box is a female. Note the small dorsal plate (in males this
plate is much more extensive). The small dorsal plate allows the body to become grossly
bloated (=engorged; = disgusting) when the female feeds, but not the male. Copulation
occurs after feeding. Engorged females drop off the host and lay eggs on vegation and
soil. Eggs hatch to release 6-legged larvae, which then attach to small mammals and
feed. These ticks drop off and metamorphose into 8-legged nymphs. These may
hibernate or else attach to another, larger host, such as a rabbit. Nymphs then drop off,
and then molt to the adult stage which is infective to large mammals. For such a
disgusting animal, this is a pretty impressive 3-host life-cycle ! The ability to utilize three
hosts permits the transfer of pathogenic viruses and bacteria from one group of animals
(e.g. mice) to another (e.g. humans). Life cycles are typically long; hard ticks frequently
require 2-3 yr for completion of a full generation.
Class Insecta
Diamanus
This is an example of a typical flea, in this case one that is commonly found in species of
squirrel. All fleas are wingless, laterally compressed insects. They are armed with
backwardly directed spines, long spined legs and short antennae (these lie in grooves
along the side of the head). The mouthparts are adapted for piercing the skin and sucking
blood. Find these characters on your specimen. Locate the spiracles and note the
chitinous exoskeleton (makes them tough to kill !). The spines (or combs) are holdfast
structures which are important in the taxonomy of fleas. Note also the large, deeply
pigmented eyes on the head. Consider the functional morphology of fleas by comparing
your specimen to the one below.
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Fleas pass through a complete metamorophosis from egg, to larvae, to pupa to adult.
Females fleas lay their eggs in the hosts nest, or directly on the hosts fur or feathers.
Most fleas lay large numbers of eggs in the spring which drop off into nests (or onto
carpets !). Here, they transform into maggot-like larvae which feed on organic debris,
mature and transform into pupae. Emergence is dependent on temperature and moisture
and may be as short as 2 weeks or may last for many months.
Fleas are often intensely irritating to their hosts, partly due to the bite itself, and partly
due to salivary secretions. Fleas are also important as transmitters of disease. The
common dog flea, for example, is the intermediate host of a common dog tapeworm. The
most dangerous of all human diseases transmitted by fleas is bubonic plague (Black
death). It is caused by a bacillus bacterium whose vector is the rat flea.
Pediculus
This is an example of an Anopluran sucking louce (i.e. the familiar lice). Most are
ectoparasites of birds and mammals. Most are small, with a fused thorax and fivesegmented antennae. Their mouthparts are specialized for piercing the skin and sucking
blood. They too, are of economic importance due to their 1) attacks on humans and
domestic animals resulting in irritation and loss of blood and 2) involvement in
transmission of microparasites. This parasite (called the human body louse) acts as a
vector for trench fever and typhus. The latter has certainly influenced the history of
mankind. Epidemic typhus has also been recently isolated from flying squirrels in the
US. It is currently maintained in flying squirrel populations by lice of the flying squirrel
and can be transmitted to humans.
Note the dorso-ventrally flattened body and locate the eyes, antennae and mouthparts on
your specimen. The thorax bears three legs; each of which is armed terminally with a
claw on the tarsus and an apposing spine on the distal end of the tibia. Locate these
structures on diagram below.
Adults live on humans and feed on blood. Eggs (=nits) are laid and are attached to hairs.
Eggs hatch as nymphs and undergo three nymphal instars before reaching adulthood.
These disperse to other human hosts if the opportunity arises, especially under crowded,
unsanitary conditions.
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