Trematodes
By Godwin E. Ibe
ML 609
Class Trematoda
There are about 9000 species of
trematodes all of which are parasitic.
Most parasitize vertebrates.
Adaptations for parasitism include suckers
and hooks for attachment, glands to
produce cyst material and increased
reproductive capacity.
Sheep liver fluke
Class Trematoda
Structurally, trematodes are similar to
turbellarians having a well developed
digestive system and similar nervous,
excretory, and reproductive systems.
However, a major difference is the
tegument.
Tegument
The tegument (found in all parasitic
Platyhelminthes) is a nonciliated, cytoplasmic
syncytium that overlays layers of muscle.
The syncytium represents extensions of cells
that are located below the muscle in the
parenchyma.
The tegument protects the parasite against its
host (e.g. against digestive enzymes).
Figure 14.05
8.5
Types of Trematodes
There are three subclasses of
Trematodes, but two are small, poorly
studied groups.
The third group, the Digenea, however is
a large group of major medical and
economic importance.
Flat worms, a rough
classification
Platyhelminthes
Trematoda
Planaria
Aspidobothrea
Digenea
Monogenea
Cestoda
Pretty planaria, not all
flatworms are ugly parasites
Flat worms, a rough
classification
Platyhelminthes
Trematoda
Planaria
Aspidobothrea
Digenea
Monogenea
Cestoda
trematodes or flukes - when they
say ‘flat’ worms they mean it
 All digenea are parasitic
 Small dorso-ventrally
flattened worms with
simple anatomy and
without segmentation
 No coelom (secondary
body cavity lined by
mesoderm), but animals
are filled with mesodermal
parenchyma
 No blood vessels, simple
ladder nervous system
trematodes or flukes know your worm
 Digenea posses two
suckers (oral and ventral
acetabulum) which they
use to attach within the
host
 Oral sucker contains the
mouth
 Muscular pharynx permits
the worm to pump food
into the blind ending gut
 Most trematodes are
hermaphrodites (they are
male and female, and
cross as well as selffertilization occurs)
trematodes or flukes know your worm
 The gut of trematodes is
blind ending but can be
quite extensive and
highly branched (here
shown in living Fasciola
(liver) flukes, the dark
staining is due to bile)
 Smooth muscle fibers
(longitudinal and cross)
run under the tegument
and around all the
organs (the gut is shown
in this picture)
trematodes or flukes know your worm
 The gut is not the only organ
these worms use for food
uptake
 The tegument (“skin”) is highly
active in nutrient uptake
 The epidermis is essentially a
single cell (a syncytium formed
by fusion of multiple cells)
 The tegument’s cell bodies and
nuclei underlie the two muscle
layers
 Actin spines are found in many
species and help the worms to
anchor themselves
trematodes or flukes know your worm
 Platyhelminths have a simple
protonephridial excretion
(kidney) system
 A protonephridium consists of
flame cell and tubule cell
 Both cells interdigitate forming
a micro filter and cilia beating
within this cage act as the
mechanical pump
 Excretes are collected from
protonephridia through small
ducts which merge to a central
duct which opens into the
excretion porus (usually
towards the end of the animal)
There is a nice little animation of this process a http://www.biology.ualberta.ca/facilities/multimedia/uploads/zoology/Excretion.swf
trematodes are massively fertile –
but their love life is complex
 To enhance the chances
to complete the complex
life cycle trematodes
produce massive number
of offspring
 The adults are
hermaphrodites
 The reproductive systems
takes up a large portion of
the body of the animal
 In particular the female
system is complex and
different physiological
functions are distributed
onto different organs
trematodes or flukes know your worm
 Trematodes form complex
ectolecithal “eggs”
 Oocytes (developing in the
ovary) meet with vitelline or
yolk cells (from the vitelary,
which carry the bulk of the
nutrients for the embryo) in
the ootype which is
surrounded by the Mehlis
gland
 An egg shell forms from
secretions of the vitelline
cells (the contribution of the
gland to the shell is unclear)
trematodes or flukes know your worm
 Worm eggs travel through the
uterus to the genital pore (tens
of thousands a day)
 On this way the proteinacous
egg shell is hardened by
quinone tanning
 The amino acid tyrosine is
modified into a highly reactive
quinone in several enzymatic
steps
 The quinone then cross links
free amino groups of adjacent
proteins generating a very
stable shell
 The tanning process can be
visualized by following the
progressive darkening of the
egg shells along the uterus
Trematode life cycles
 Trematodes produce an
enormous number of offspring
by combining sexual and asexual
reproduction cycles
 Asexual reproduction occurs in
germinal balls. These areas are
home to omnipotent (stem celllike) progentior cells that can
initiate the development of
embryos without fertilization
 All have at least two hosts of
which one is a snail
 Not all stages are found in the life
cycle of all species
 Miracidia and cercariae are
infective (invasive) stages
Trematode life cycles -the egg
 The egg contains an embryo
rather than an oocyte
 Eggs are shed at different
degrees of maturity by
different flukes
 Eggs have to leave the body of
the final host to continue
development
 The mature miracidium within
the egg uses light, osmolarity
and temperature as clues to
when hatching is appropriate
 Hatching proceeds in most
species through a preformed
“door” the operculum
Trematode life cycles -the miracidium
 The miracidium is highly motile
due to the cilia on its surface
 Miracidia have simple eyes
(they avoid light) and several
chemical and mechanical
receptors which they use to
find the intermediate snail host
 Penetration glands secrete
proteases and other lytic
enzymes on contact with
appropriate host
 Miracidia of flukes with land
snails as intermediate host will
hatch upon ingestion by the
snail and penetrate the gut
epithelium
Trematode life cycles -the miracidium
Dept. Biology, Univ. of Alberta, Canada
Trematode life cycles -the sporocyst
 After penetration the
miracidum undergoes
metamorphosis into the
sporocysts
 This stage has most organ
systems reduced to the bare
minimum and acts as a
germinal sac
 The sporocyst takes up
nutrients only over its
tegument and the germinal
mass expands and develops
into daughter sporocysts,
redia or cercaria
Trematode life cycles -the redia
 Sporocyst can produce
cercaria or a next
amplification generation the
redia
 Redia have features of the
adult fluke like oral and
ventral sucker, a gut and
“birth pore” to release
cercaria
 Redia are mobile in the snail
and can prey on sporocysts
and redia of the same or
other species (competition)
Trematode life cycles -the cercaria
 Cercaria are the stages that leave
the intermediate host and infect
the final host
 There can be many consecutive
waves of “shedding” from the snail
 Cercaria already show many
anatomical features of the adult
fluke
Trematode life cycles -the cercaria
 Reflecting the ecology of
their hosts cercaria have
developed an array of
adaptations to achieve
successful infection
 Direct penetration of host
skin upon water contact
(Schistosoma),
 Encystation within the
muscle of intermediate
hosts (e.g. metacercaria in
fishClonorchis)
 Encystation on plants
(Fasciola)
Trematode life cycles -enhance transmission
 Dicrocoelium dendriticum
the lancet fluke
 One metacercaria becomes
the ‘brain worm’ and lodges
into the central ganglia of
the end
 The brain worm manipulates
the behavior of the ant. In
the evening when the
temperature drops they
experience spasms of their
manidibles
Trematode life cycles -enhance transmission
 Leucochloridium sp. is a
tiny digenic trematode living
in the gut of small song
birds
 Worm eggs are passed with
the feces and are taken up
by amber snails.
 Miracidia hatch, penetrate
the gut epithelium and
develop into sporocysts
within the hepatopankreas.
 Within the sporocyst
cercaria develop which
infect birds that eat infected
snails.
Trematode life cycles -enhance transmission
Amber snails (uninfected, upper
panel and infected, lower panel) and
Leucochloridium sp. sporocyst
dissected from a snail (lower right)
Trematode life cycles -enhance transmission
Dr. Oldrich Nedved, Univ. South Bohemia
Monogenean Trematodes



Monogeneans possess the simplest life cycle among the parasitic
platyhelminths. They have no intermediate hosts and are
ectoparasitic on fish (seldom in the urinary bladder and rectum of
cold-blooded vertebrates).
Although they are hermaphrodites, the male reproductive system
becomes functional before the female part. The eggs hatch
releasing a heavily ciliated larval stage known as an
oncomiracidium. The oncomiracidium has numerous posterior hooks
and is generally the life stage responsible for transmission from host
to host.
No known monogeneans infect birds, but one (Oculotrema
hippopotami) infects mammals, parasitizing the eye of the
hippopotamus.
Aspidogastrea Trematodes



Their life cycle is much simpler than that of digenean trematodes, including
a mollusc and a facultative or compulsory vertebrate host. There are no
multiplicative larval stages in the mollusc host, as known from all
digeneans.
Host specificity of most aspidogastreans is very low, i.e., a single species of
aspidogastrean can infect a wide range of host species, whereas a typical
digenean trematode is restricted to few species (at least of molluscs). For
example, Aspidogaster conchicola infects many species of freshwater
bivalves belonging to several families, as well as snails, many species of
freshwater fishes of several families, and freshwater tortoises.
Trachinotus blochi (Teleostei, Carangidae), on the Great Barrier Reef. They
produce large numbers of eggs which are shed in the faeces. If eaten by
various prosobranch snails, larvae hatch in the stomach, and—depending
on the species of snail—stay there or migrate to the digestive gland where
they grow up to the preadult stage which has all the characteristics of the
adult including a testis and ovary.
Digenean Trematodes


The flukes have a complex life cycle in
which a snail is the first (or intermediate)
host and a vertebrate the final (or
definitive host).
The definitive host is one in which the
fluke reproduces sexually.
Digenean Trematodes


In some species there may be 2 or 3
intermediate hosts before the definitive
host is reached.
Trematodes inhabit a variety of sites in
their hosts including the digestive tract,
respiratory tract, circulatory system,
urinary tract, and reproductive tract.
Digenean Trematodes

Digenean life cycles are very complex and
the fluke passes through numerous
stages.
Digenean Trematodes

A typical example would include the
following stages:



Adult
Egg (or shelled embryo) shed into water
Miracidium: a free swimming, ciliated larva
that finds and penetrates a snail intermediate
host
Sheep liver fluke egg (top) and miracidium (bottom).
http://cal.vet.upenn.edu/projects/paraav/images/lab6-227.jpg
Digenean Trematodes


Sporocyst: reproduces asexually in
intermediate host producing more sporocysts
or another asexually reproducing stage called
a redia.
Redia produce more redia or cercariae.
Cercariae leave the intermediate host and
swim. Then they penetrate the skin of
another intermediate host or the definitive
host.
Adult Fasciola hepatica
Sheep liver fluke (above)
Redia (right)
http://cal.vet.upenn.edu/projects/paraav/images/lab6-227.jpg
Digenean Trematodes


Cercariae that enter an intermediate host may
encyst in muscle and wait to be consumed by the
definitive host or may leave the intermediate host to
actively search for the definitive host.
Cercariae that enter the definitive host make their
way to their desired home and develop into an adult
fluke which reproduces sexually and produces eggs.
http://www.biology-blog.com/images/blogs/trematode-cercaria-482810.jpg
Trematodes of medical
importance
Schistosoma, blood flukes
Clonorchis & Opistorchis, liver flukes with
metacercaria in fish
Paragonimus, lung flukes with
metacercaria in crabs
Fasciolopsis, Fasciola, Dicrocoelium,
intestinal and liver flukes with
metacercaria on plants
Human liver fluke disease
 Caused by
Clonorchis sinensis
and Opistorchis
felinus and viverini
 All locally common in
East Asia and Eurasia
 ~20 million people
infected
Human liver fluke disease
 Clonorchis and
Opistorchis are quite
similar causing similar
disease
Human liver fluke disease
 Metacercaria are found in
many fish especially various
carp related species
 Raw or undercooked fish
dishes are a source of
human infection
 Fertilization of ponds with
untreated night soil boost
infection in fish
 Cats, dogs and other
carnivores can be additional
hosts and reservoirs
Human liver fluke disease
 Pathology depends on worm
burden, generally infections
are light and free of major
symptoms
 Heavy infections Flukes
residing in the bilary ducts
can chronically iritate the
epithelium resulting in
hyperplasia of the
epithelium and fibrosis
around the ducts (pipe stem
fibrosis)
 Blockage of bile ducts and
impairment of liver function,
liver swelling
Human liver fluke disease
Diagnosis occurs by
microscopic
demonstrations of
fluke eggs in the
feces (~30x15 mm)
Prepatency is a
month
Readily treated with
Praziquantel
Human lung fluke disease
 Paragonimus westermanii is
best known but a number of
other species infect humans
around the world
 Several carnivores serve as
reservoir
 Upon eating crabs by the final
host metacercariae excyst in
the duodenum and penetrate
the gut, penetrate the
diaphragm and pleura and
enter the bronchioles, mature
in 12 weeks
 May end up in ectopic
locations like brain, skin and
mesentery
Human lung fluke disease
Human lung fluke disease
 Adults are encapsulated in a
granuloma (often two at a time)
 Cyst rupture can result in cough
and increase sputum, and chest
pain
 Chronic high worm burden can
result in chronic bronchitis and
dyspnea and increasing fibrosis -symptoms can be very similar to
pulmonary tuberculosis
 Cerebral paragonimiasis produces
headaches, fever, nausea, visual
disturbances and convulsive
seizures
Fasciola & Fasciolopsis
 Important parasite of
livestock, ocasionally
infects humans
 Symptoms similar to
Clonorchis but
Fasciola is much
bigger
 Fasciolopsis buski the
human intestinal fluke
has similar ecology
 Usually asymptomatic
if not heavy burden
Fasciola
 Ecology of fasciolosis,
ponds and creeks in
direct vicinity of
pasture
Schistosomiasis


Schistosomiasis (also known as bilharzia)
is an infection with blood flukes and is a
major infectious diseases.
More then 200 million people are infected
worldwide with these flukes, which they
acquire swimming or walking in water in
which the intermediate snail host lives
Schistosomiasis


Schistosome eggs enter the water when infected
people urinate or defecate in or near water.
Eggs hatch and the miracidium seeks out a snail.
Inside the snail the parasite develops into a
sprocyst and asexual reproduction takes place.
Cercaria eventually are released into the water.
Schistosome
life cycle.
Schistosomiasis


When a schistosome cercaria swims it takes care
to avoid UV light which can damage it, but is
very sensitive to the scent of humans.
When it senses molecules from human skin it
swims rapidly and jerks around looking for the
person. When it makes contact it releases
chemicals that soften the skin and it burrows in
shedding its tail at the same time.
Schistosomiasis


The fluke searches until it finds a capillary and
enters it.
The capillary is only barely wide enough for the
fluke and it moves along using its pair of
suckers. Eventually, it reaches a larger blood
vessel in which it can float until it reaches the
lungs and enters an artery and eventually makes
its way to the liver.
Schistosomiasis


Once in the liver, the fluke feeds on blood
and begins to mature and develops
ovaries or testes depending on its sex.
The fluke grows dozens of times larger in
the course of a few weeks and then
begins to search for a mate.
Schistosomiasis


The fluke produces chemicals to attract
members of the opposite sex.
Females are slender and delicate, whereas
males are much bigger and have a spiny
trough or groove into which the female
fits and locks in.
Figure 14.13
8.9a and b
Schistosomiasis


Once paired up, the pair mature sexually
and travel from the liver to a permanent
home that is species-specific.
In Schistosoma mansoni it is near the
large intestine, in S. haemotobium it is the
bladder, and in S. nasale, a blood fluke of
cows, it is the nose.
Schistosomiasis


Once established the pair remain in situ
for the rest of their lives.
The male consumes blood and feeds the
female most of it, which she turns into
eggs, which pass out of the host and can
begin the life cycle again.
Schistosomiasis


Schsitosomiasis has a low mortality rate, but it is
a chronic illness that debilitates the infected
person.
Symptoms can include anemia, diarrhea, fever,
fatigue and van result in organ damage. In
children infection can result in reduced growth
and mental development.
Schistosomiasis


Schistosomiasis occurs in tropical
countries worldwide.
It can be treated with oral drugs, but a lot
of attention has focused on developing a
vaccine and on controlling the snails which
are the disease reservoir.
How do flukes manipulate their
hosts?


Many parasites infect an intermediate host that
needs to be eaten by the definitive host for the
parasite to complete its lifecycle.
There are many instances of parasites altering
their intermediate host’s behavior to make it
more vulnerable to a predator (the definitive
host). Such behavior is widespread in flukes.
How do flukes manipulate their
hosts?

In the Carpenteria Salt Marshes in
southern California lives the fluke
Euhaplorchis californiensis.

It has a life cycle that includes two
intermediate hosts, first the California
Horn Snail and then the California killifish
and a final host which can be any of a
variety of fish eating birds.
How do flukes manipulate their
hosts?



The fluke leaves its definitive host as an egg in
bird droppings which are eaten by the fluke’s
first intermediate host, the snail.
The fluke then castrates the snail (preventing it
from diverting energy into eggs and away from
the parasite).
The fluke then reproduces asexually and sheds
cercariae into the water.
How do flukes manipulate their
hosts?


The cercariae seek out the next
intermediate host the killifish and latch
onto the fish’s gills.
Each cercaria works its way into a blood
vessel and explores until it finds a nerve
which it then follows until it reaches the
fish’s brain.
How do flukes manipulate their
hosts?


The cercariae don’t penetrate the brain
but sit on top of it. Then they wait for the
fish to be eaten by a bird.
Once eaten by a bird they break out of the
fish’s head and move into the bird’s gut
where they produce eggs that continue
the cycle
How do flukes manipulate their
hosts?


The cercariae sitting on the bird’s brain
apparently don’t sit passively waiting.
Killifish when swimming occasionally
shimmy and jerk around flashing their
bellies. Those infected with cercariae are
four times more likely to do so than noninfected fish.
How do flukes manipulate their
hosts?

In field experiments in which penned fish
were made available to foraging birds
infected fish were 30 times (!) more likely
to be eaten than uninfected fish.
How do flukes manipulate their
hosts?


Research on how the flukes alter the fish’s
behavior has shown that the flukes
produce powerful molecular signals called
fibroblast growth factors.
These interfere with the growth of nerves
and may be the mechanism the flukes
use.
Do trematode parasites favor sex in
hosts?



Lively (1992) studied New Zealand
freshwater snail. Host to parasitic
trematodes.
Trematodes eat host’s gonads and
castrate it which imposes strong selection
pressure.
Snail populations contain both obligate
sexually and asexually reproducing
females.
Do trematode parasites favor sex in
hosts?


Proportion of sexual vs asexual females
varies from population to population.
Frequency of trematode infections varies
also.
Do trematode parasites favor sex in
hosts?


If evolutionary arms race favors sex, then
sexually reproducing snails should be
commoner in populations with high rates
of trematode infections.
Results match predictions.
White slice indicates
frequency of males
and thus sexual
reproduction
Frequency of males increases with increasing rates of
trematode infection.
The End!
Thank You!