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Major characteristics
Dorsoventrally flattened
Digestive tract greatly reduced or missing
Excretory system consists of flame cells or
• Great deal of work being done in
Platyhelminthes systematics.
• One of the most current classification
schemes is given on page 189-190 of text.
• The older, more traditional classification
scheme is given on page 192. We will use
the older, much simpler classification.
• Class Turbellaria - mostly free-living
flatworms (commensals)
• Class Monogenea - parasitic on skin and
gills of fish and amphibians. (ectoparasites)
• Class Trematoda - all parasitic, called flukes
• Class Cestoidea (formerly Cestoda) all
parasitic, the tapeworms. Adult tapeworms
found in intestine of definitive host.
Class Monogenea (chapter 19)
• Most are hermaphroditic ectoparasites of
fish, amphibians, turtles
• Some are found in urinary bladder, lower
intestine of frogs and turtles.
• Have a direct life cycle
• In nature they cause little economic damage
but can be very destructive in fish farming.
• Most distinguishing
feature is the
presence of a large
attachment organ
called an
opistohaptor (see fig
• Have a single host.
Some Interesting Examples
• Dactylogyrus sp - cause large kill-off in fish
in commercial fish ponds.
• Gyrodactylus sp. can cause large fish-kills
in fishponds. These organisms are
viviparous and 4 generations may be seen
within an adult worm. (see fig. 19-12).
• Diplozoon paradoxum - two larval forms
fuse together (fig. 19-13)
Digenetic Trematodes
• Require two or more hosts
• Some very important parasites of man
• Most are hermaphroditic, Schistosomes are
exception being dioecious.
• Some can reproduce parthenogenetically
Digenetic Trematode Structure
Trematode Reproduction
Male reproductive system
• Usually contains two testes (number may vary)
• Vas efferent leaves each testis and forms the vas
• The vas deferens gives rise to a seminal vesicle
which stores sperm
• the muscular cirrus pouch stores the male
copulatory organ called the cirrus.
Trematode Reproduction
• Usually a single rounded ovary but in some species may be
lobated or branched.
• A short oviduct leads from the ovary and has a sphincter called
the ovicapt which regulates the passage of ova.
• Vitellaria or vitelline glands are important because they
contribute yolk to the eggs.
• Where the oviduct and the vitelline duct join there is a
expansion which forms the ootype.
• Mehlis glands surround the ootype and in combination with the
ootype and vitelline glands forms the oogenotop or egg
forming apparatus.
• Beyond this, the duct expands to form the uterus and
on to the genital pore.
Fasciola hepatica: Trematode
Life Cycle Representative
Ova or egg
(Daughter sporocyst, or redia)
Stages of Trematode Life Cycle
• Ova or egg - maybe a better term is shelled
– Contains miracidium inside shell
– Under appropriate conditions, the operculum
(cap on shell) opens to allow miracidium to
– Many of flukes have very distinctive eggs.
Stages of Trematode Life Cycle
• Miracidium
– Ciliated organism that can be mistaken for a
ciliated protozoan.
– In species that hatch in water, it contains
penetration glands that release histolytic or
proteolytic enzymes to help penetrate snail
– Some species do not hatch until eaten by snail
host. In these the miracidium is not as
Stages of Trematode Life Cycle
• Sporocyst
– The miracidium develops into sporocyst often
in the digestive gland of the snail.
– The sporocyst is an embryonic bag or germinal
sac that has asexual reproduction occurring.
– The sporocyst will produce many daughter
stages called rediae or in some cases daughter
sporocysts. The term daughter has nothing to
do with gender.
Stages of Trematode Life Cycle
• Rediae or Daughter Sporocyst
In function they are very similar to sporocysts.
Contain digestive tract and are more active
Asexually reproduce to yield many cercariae.
Some species they can live for many years.
Stages of Trematode Life Cycle
• Cercariae
– Usually escape snail and often swim by some
means of tail structure. (see fig 15.22)
– Responsible for transmission from snail to the
next host.
– Differences in this mechanism will be discussed
Stages of Trematode Life Cycle
• Metacercaria
– Resistant stage that is formed in many species
– Cercaria that have this stage contain cystogenic
glands that helps the organism encyst on
– Cercaria that form metacercaria in second
intermediate hosts, often have penetration
glands that enable them to penetrate the second
intermediate host.
Stages of Trematode Life Cycle
• Adult
– Always found in the definitive host
– Responsible for sexual reproduction
– Often restricted to specific region of host.
Often very host specific.
Stages of Trematode Life Cycle
• .
Digenetic Trematodes: Order
• Superfamily Strigeoidea
– Strange looking (see fig 16-1, page 234)
Anterior end large spoon-shaped oral sucker
with pseudosuckers on the sides
– Alaria americana • may require 4 hosts: snail, tadpole or frog,, water
snake, and frog or snake eating vertebrate.
• Man can become infected eating infected frogs
Digenetic Trematodes: Order
• Superfamily Schistosomatoidae - the blood
• Differ from other trematodes
Do not have second intermediate host.
Penetrate integument of definitive host
Not really flat but rounded.
Schistosoma sp.
• Adult worms are found in blood vessels of
digestive tract or urinary bladder thus called
blood flukes
• male worm has a split body called the
gynecophoral canal. The female is usually
found within this canal “safe in the arms of
her lover.” She leaves only during the egg
laying period.
• Migratory phase - 4-10 weeks after
infection. Is characterized by fever and
toxic or allergic reactions resulting from
migration of immature organisms. Often
results in bronchitis, hepatomegaly,
splenomegaly, and diarrhea.
• Acute phase - 10 weeks to years. Eggs can
become trapped and produce granulomas
and scar tissue.
– Form fibrous nodules called pseudotubules.
– Eggs may lodge in gastrointestinal, renal,
neural, and other systems.
• A person infected with 50 mating pairs would be
exposed to about 15,000 eggs per day for several
years. ½ of eggs might remain trapped in tissues.
• Chronic phase - persons living in endemic
regions are often asymptotic. May have
mild, chronic bloody stools or urine. Often
have formation of granulomas.
Hepatomegaly, Spleenomegaly, Ascites
(accumulation of fluid in abdominal cavity
fig 16.17, page 245.
Human waste into water.
Moslem religious practice of ablution
Agricultural projects
Irrigation extends snail habitats
Use of night soil (human feces)
Planting and harvesting of water crops
Dependence of people on rivers.
• Before the Aswan Dam was built, the region
between Cairo and Aswan was subject to
annual floods. The prevalence of
Schistosomiasis was only about 5%. Four
• years after completion of the dam the
prevalence ranged from 19%-75% (average
35%) or a 7 fold increase.
Schistosoma sp.
• Important parasites of man and some
domesticated animals
• Three species infect man
• Schistosoma mansoni
• Schistosoma japonicum
• Schistosoma haematobium
• similar see table 16.1 pg 237 for differences
Schistosoma japonicum.
• Common in parts of Japan, China, Taiwan,
Philippines, Thailand, and other parts of
Southeast Asia.
• Most pathogenic and most difficult to
• Located in blood vessels of small intestine.
• Eggs may lodge in brain causing CNS
damage, coma, and paralysis.
• Low host specificity
Schistosoma mansoni
• Common in Egypt, the Middle East, parts
of Africa, and parts of South and Central
• Found in portal veins draining large
• The sharp lateral spine is distinctive
• Primary pathological effects come from the
damage done by eggs.
Schistosoma mansoni
• In heavy infections eggs become trapped in
the mucous and submucosa of the gut and
cause granuloma formation
• If extensive, they can cause colon blockage
and significant blood loss.
• In liver can cause hepatomegaly.
• Destruction of lungs and heart tissue.
• Reservoir hosts are of limited or no
Schistosoma haematobium
• often referred to as Bilharzia after Theodore
Bilharz who discovered it.
• found in parts of Africa, and parts of the
Middle East, southern Europe and some
parts of Asia.
• Found primarily in the veins of the urinary
bladder. Eggs released in urine.
• They are least pathogenic
Diagnosis, Treatment, and Control
Finding eggs in feces or urine
Biopsy - in chronic cases if eggs not passed
Treatment - very difficult - page 245-246.
Control is very difficult
– Customs and traditions
– Agricultural practices
– Socioeconomics
Breakthroughs in Control
• new molluscicides
• Biological controls
– Compeditory and Predatory snails in Puerto
Rico compete with intermediate snail host
– Crayfish from North America introduced into
drainages of Kenya and have significantly
reduced prevalence of snail hosts.
– fresh water shrimp that eat snail hosts. Natives
eat shrimp
Schistosome cercarial dermatitis
or swimmers itch
• Schistosomes of animals other than man
(usually rodents and birds) try to penetrate
the skin of man, they can not establish
themselves in the blood vascular system of
• Often cause a dermatitis which can be
severe and in some cases life threatening.
• Allergic reaction
Swimmer’s Itch
Order Echinostomata
• Most parasites of wild animals, but a few
cause diseases in man or domestic animals
• have spines or scales near anterior end
• Some common examples
Echinostoma revolutum
Fasciola hepatica
Fasciolopsis buski
Fasciola gigantica
Echinostoma revolutum
• Cosmopolitan
• Shows little host specificity and is common
in any bird or mammal that eats molluscs,
planaria, fish, or tadpoles found infected
with the metacercarial stage.
• Man usually becomes infected by ingesting
metacercaria found in raw snails
Fasciola hepatica
• Commonly known as the sheep liver fluke
• Important parasite of sheep and cattle (other
grazers) can be found in humans.
• Morphology
– Large size, frequently over 30 mm long
– Characteristic cone-shaped projection at
anterior end followed by wide shoulders
Fasciola hepatica adult
Fasciola hepatica
• Adult in bile duct of definitive host passes
eggs in feces.
• If eggs land in water, they hatch into
miracidium that actively swims until it finds
an appropriate snail.
• Penetrates snail, develops into germinal sac
(sporocyst), asexual stages of rediae and
cercariae formed.
Fasciola hepatica
• Cercariae leave snail, encyst on vegetation,
and form metacercaria.
• Herbivore infected when it ingests
vegetation with metacercaria.
• Metacercaria develop into adult penetrates
gut wall, moves to the liver.
• Humans infected by eating watercress that
has metacercaria on it.
Fasciola hepatica Pathology
• Necrosis of the liver by wandering fluke
• Anemia common, inflammation and
damage of the bile ducts, abscesses formed
• May damage other organs (eye, brain, lung)
• halzoun - Ingestion of raw liver - adult
worms migrate to the lungs and cause a
respiratory blockage.
Fasciola hepatica Epidemiology
• liver blockage and watercress consumption
• Prevention - Eschewing (shunning or
avoiding) watercress.
• Rabbits are probably important in spreading
• In some parts of southeastern United States,
it is important parasite of domestic animals
Fasciolopsis buski Intestinal fluke of man
• large fluke infects man when he ingests
metacercaria found on vegetation including
water chestnuts, bamboo, and water caltrop.
• eat these raw or peel or crack with teeth.
• elimination of feces (human and animal)
into water and use of night soil for farming
Fasciolopsis buski • Pathology
– Blockage of food passage or interference with
normal digestive processes
– Destruction of intestinal tissue resulting in
ulcers, hemorrhages, and abscesses formation
– Chronic diarrhea
– Verminous intoxication - result of absorption
of parasite metabolites (waste products), can
lead to death.
Order Plagiorchiata
• Adults show much variation but there are
many similarities in larval stages
• Some common examples
– Dicrocoelium dendriticum - lancet fluke
– Paragonimus westermani
Dicrocoelium dendriticum
• Common parasite of herbivores, rare in man
• Not dependent upon aquatic environment
• Eggs eaten by land snail (asexual
reproduction occurs producing cercariae.
• Cercariae surrounded forming slime balls.
• Ant eats slime ball and metacercaria
• Herbivore infected by ingesting ant.
Dicrocoelium dendriticum
Dicrocoelium dendriticum
• Modification of host behavior
• Causes ant to climb to tips of grasses early
morning and late evenings.
• Increases odds of ant being eaten.
Paragonimus westermani
• the human lung fluke, it is found in the
Orient including India and Philippines
• Definitive host becomes infected by eating
improperly cooked crustacean. Adult
infections become established in lungs but
larval forms may wander into brain, pleura,
mesentery, etc.(ectopic infection).
Paragonimus westermani
Paragonimus westermani
• Reservoir hosts include - dogs, cats, pigs,
rodents, and other animals
• Man becomes infected by eating improperly
cooked crabs, ingestion of metacercaria
from cutting boards where salads are fixed,
medicinal use of crab juices)
• Smoked or pickled crab do not kill
Paragonimus westermani
• Early invasive stages usually asymptomatic.
• In the lung or ectopic site, connective tissue
forms pseudotubertules. In the CNS, they
can cause paralysis and in rare cases can be
fatal. In the heart they can cause severe
damage and can be fatal.
• Lung infections cause chronic cough,
bloody sputum, pneumonia -like conditions.
Paragonimus westermani
• Prevention includes
– Cooking of crabs, crayfish
– Care when eating salads, no crab juice.
– Proper disposal of feces and sputum.
Nanophyetus salmincola
• Small fluke commonly found in salmon
• Transmits rickettsial organism, Neonckeitsia
helminthoeca (known as Salmon poisoning)
to dogs, other canines, raccoons, and other
fish eating mammals
• This rickettsial organism is highly toxic to
dogs with up to 90% mortality if not treated.
• People become infected with the trematode
but do not get salmon poisoning.
Order Opisthorchiata
Small to medium sized flukes
Testes located at posterior end
Lack cirrus pouch and cirrus
Eggs hatch only after ingestion by snail
Live in intestine of bile ducts of fishes,
reptiles, birds, and mammals.
• Second intermediate host are fishes
Clonorchis sinensis (=
Opistochonorchis sinensis)
• Commonly called the Chinese liver fluke,
Asian liver fluke, Oriental liver fluke, and
Human liver fluke
• Common in Orient including Japan, China,
Korea, Taiwan, Viet Nam.
• Many immigrants from Orient living in the
United States are infected.
Clonorchis sinensis
• Man becomes infected by eating improperly
cooked fish containing metacercaria.
• Metacercaria may contaminate other foods
such as salads, etc.
• Smoking, drying, pickling fish often fails to
kill the metacercaria.
Clonorchis sinensis
• .
Biology of Clonorchis sinensis
• Adults in bile ducts of man, eggs
(embryonated embryos) passed out in feces.
• Do not hatch until eaten by proper snail host
• Miracidium develops into sporocyst.
• Asexual reproduction occurs with sporocyst
producing many rediae and each rediae
producing many cercariae.
Biology of Clonorchis sinensis
• Cercariae leave snail, find and enter fish
and forms metacercaria
• Definitive host (man, dogs pigs cats rats,
camels, and other mammals) become
infected when they ingest this fish.
• Dogs and cats serve as important reservoir
• Disease of the wealthy who can afford
fancy cuts of raw fish (yeu-shan chuk is
• Disease of the poor whose only source of
protein is fish.
• 14% of Hong Kong, 80% of some rural
areas, and 100% of some villages are
•Why So Many?
•How To Break
the Life Cycle
• Fish farming often use night soil to fertilize
• Frequently dry, salt, pickle, smoke, and
freeze fish. This does not kill the organism.
• Be careful about eating dried or smoked fish
that you can buy in China Towns and other
Oriental food stores.
• Erosion of lining of the bile ducts.
• Severity of disease is dependent upon the
intensity of the infection.
• Most infected people have 20-200 worms
but as many as 21,000 worms have been
recovered from a person at autopsy.
• Chronic destruction of the epithelial lining
of the bile ducts leads to thickening of the
walls and a condition known as "pipestem
• Ascites (accumulation of large amounts of
fluid in the abdominal area) almost always
occurs in fatal cases.
• Eggs and sometimes worms have gallstones
form around them.
• Liver cancer is higher than normal in Japan
and Clonorchis infections are suspected but
the relationship has not been proven.
• Proper cooking of fish
• Do not contaminate other foods with juices
from infected fish. Careful with salads!
• Care must be taken not to eat fish that are
dried, smoked, frozen, pickled etc. without
proper cooking.
• Proper and controlled disposal of human