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


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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
Digenean 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.
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
Clonorchis liver fluke



Clonorchis is the most important liver fluke to infect
humans. Common in much of Asia (including China,
Japan and southern Asia).
Adult flukes live in the bile passages and shelled miricidia
pass out in feces. The miricidia enter snails eventually
leave the snails as cercariae and find a fish where they
encyst.
If fish is eaten raw or poorly cooked the person becomes
infected
Figure 14.12
8.8
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.
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.
Do trematode parasites favor sex in
hosts?


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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?

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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.
Class Monogenea
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
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The monogenetic flukes were previously classified as on
order of the Trematoda, but recent work suggests they
are more closely related to cestodes (tapeworms).
Monogeneans are small (usually < 2cm) typically
external parasites of fish that clamp onto the gills using
a hooked organ (often with suckers) called an
opisthaptor.
Some also parasitize the urinary bladder or rectum of
frogs and turtles; there is a species that parasitizes squid
and one that attaches to the eyeball of hippopotomuses.
Figure 14.16
8.11
Monogenean Fluke
Class Monogenea



Unlike the trematodes, Monogeneans have only a single
host (hence “Mono” in the name). Most feed on the
host’s epidermis using their protrusible pharynx, but
some are blood feeders.
Monogeneans are hermaphrodites (male organs develop
first) and can move around a host in search of a mate
(they will also self-fertilize).
The egg hatches into a ciliated larva (an
oncomiracidium) which seeks out its host in the water.
http://www.sci.sdsu.edu/salton/ssmudgyr14Lg.gif
Gyrodactylus olsoni on the gill filament of longjaw mudsucker, SEM. a - general view
Class Monogenea

Monogeneans typically occur at low levels
on fish and so do not inflict serious harm.
However, in fish farms, infestations may
become very heavy and lead to significant
mortality.
Class Cestoda (tapeworms)


Tapeworms are parasites of the vertebrate
digestive tract and about 4000 species are
known.
Almost all tapeworms require at least two
hosts with the definitive host being a
vertebrate, although intermediate hosts
can be invertebrates.
Class Cestoda
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Members of the Class Cestoda (tapeworms) are quite
different in appearance from the other members of the
Platyhelminthes.
They have long, flat, tape-like bodies composed of a
scolex for attaching to their host and a chain of many
reproductive units or proglottids called a strobila.
New proglottids form behind the scolex and the strobila
may become extremely long.
Figure 14.18
8.12
Tapeworm scolex
Hooks
Suckers
The scolex is equipped with suckers and hooks that
enable it to grip onto its host’s intestines.
Class Cestoda

Tapeworms live in the intestines and
because they are immersed in digested
food lack a digestive system of their own.
Instead they simply absorb food across
their tegument.
Class Cestoda


To facilitate the absorption of food a
tapeworm’s tegument has huge numbers
of tiny projections called microtriches,
which are broadly similar to the microvilli
of the vertebrate intestine.
They similarly increase the surface area of
the tegument for absorption.
Figure 14.17
8.13
Class Cestoda

Tapeworms are usually monoecious (have both
male and female reproductive organs).

A proglottid is fertilized by another proglottid in
the same or a different strobila.

Shell-encased embryos form in the uterus and
exit the proglottid via a uterine pore or the
entire proglottid may detatch and pass out of
the host.
Figure 14.20
8.14
Human tapeworms

Humans are definitive hosts to several
tapeworms including the beef tapeworm
Taenia saginata, pork tapeworm T. solium,
and fish tapeworm Diphyllobothrium
latum.
Human tapeworms

The lifecycles of these parasites are similar.

Shelled larvae are shed into the environment.

These are consumed by the intermediate host
and the larvae (oncospheres) hatch, bury into
blood vessels and make their way to skeletal
muscle or the body cavity where they encyst
becoming so called “bladder worms” or
cysticerci.
http://www.medirabbit.com/EN/GI_diseases/Parasitic_diseases/Tape_cyst.jpg
Tapeworm cysticerci in body cavity of a rabbit
Human tapeworms


The encysted larva develops an invaginated
scolex and waits, perhaps for years, for its host
to be eaten.
If the meat is uncooked the cysticercus extends
its scolex, attaches to the wall of the intestine
and within 2-3 weeks matures and begins
growing and producing eggs. A tapeworm may
be many meters long and live for years.
Figure 14.19
8.15
Adult cestodiasis



Cestodiasis is the term for being infected with a
cestode. Adult cestodiasis occurs when a human
is infected with an adult tapeworm.
Adult cestodiasis is the commonest form and is
generally not very harmful
However, heavy infestation can lead to physical
damage to the wall of the gut or perhaps
blockage of the intestines.
Larval cestodiasis


Under poor sanitary conditions, a person
with adult cestodiasis may infect
themselves with tapeworm eggs (or
someone else may infect them).
This infection is called larval cestodiasis
and is potentially very serious.
Humans as intermediate hosts
(larval cestodiasis)


Humans may become intermediate hosts for
tapeworms with potentially disastrous
consequences if they consume shelled larvae in
contaminated food.
In an evolutionarily unfamiliar environment,
cysticerci may encyst in inappropriate locations
such as the brain, which is frequently fatal.
Figure 14.21
Cysticerci in human brain
8.16
Tapeworm manipulations of hosts
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
Continuing the theme of parasite manipulations
we’ve seen this semester it’s not surprising that
some tapeworms also manipulate their hosts to
ensure they can complete their lifecycles.
A good example is the tape worm Hymenolepis
diminuta, which parasitizes rats and flour
beetles.
Tapeworm manipulations of hosts


Adults live in the bowels of rats (where
they can grow to 45 cm in length) and
produce eggs which pass out of the gut in
rat droppings.
Beetles are attracted to, and eat, rat
droppings that contain tapeworm eggs, by
an apparently highly attractive scent.
Tapeworm manipulations of hosts


Not clear whether the eggs, adult tapeworms, or
rat host produce this scent, but beetles strongly
prefer egg-containing feces to those that are
egg free.
Once in the beetle, the tapeworm produces
several chemicals that sterilize female beetles by
blocking the flow of nutrients that allows egg
formation.
Tapeworm manipulations of hosts


In order to reach its final host the
tapeworm needs to ensure that the beetle
gets eaten by the definitive host, a rat.
The tapeworm produces chemicals, which
make the beetle less likely to conceal itself
as well as sluggish and slow to escape if
attacked.
Tapeworm manipulations of hosts



As a final trick the tapeworm also inactivates the
beetle’s last line of defense.
Flour beetles have glands in the abdomen that
spray a foul-tasting liquid, which often will cause
a rat to spit out a beetle it has started to eat.
The tapeworm blocks the gland that makes this
chemical, so that the beetle doesn’t taste bad to
the rat and is consumed.