The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
• Trematodes are parasites of vertebrates. They have
complex life cycles requiring one or more intermediate
hosts. Most are hermaphroditic and capable of selffertilization.
• Eggs shed by the adult worm pass outside to the
environment, and a larva (called a miracidium) hatches
and swims away to infect the intermediate host.
• Each species requires a certain species of mollusk
(snail, clam, etc) as an intermediate host. A series of
generations occurs in the mollusk, resulting in the
liberation of larvae known as cercariae.
The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
To reach the vertebrate host, depending upon
species, cercariae :
• Penetrate directly through skin and develop
into adults.
• Enter a second intermediate host, and wait to
be ingested (they are now called
metacercariae).
• Attach to vegetation, secrete a resistant cyst
wall, and wait to be eaten (now called
metacercariae).
The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
Intestinal:
• Fasciolopsis buski
• Heterophyes heterophyes
• Metagonimus yokogawai
Liver / Lung
• Fasciola hepatica
• Clonorchis sinensis
• Paragonimus westermani
Blood
• Schistosoma mansoni
• Schistosoma haematobium
• Schistosoma japonicum
The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
General appearance of adult flukes:
• Body is non-segmented, flattened dorsal-ventrally,
and covered with a cuticle which may be smooth or
spiny.
• Attachment organs are two cup-shaped suckers,
oral and ventral. Tegument is metabolically active.
• Oral cavity leads to muscular esophagus; intestine
branches into 2 cecae, which end blindly near the
posterior end of the worm.
• Simple digestive system, no anus - waste products
are regurgitated.
The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
Reproductive system:
• Uterus is the largest organ; a single ovary, two
testes & a series of glandular structures that
produce egg shell materials.
• Uterus may be filled with thousands of eggs.
The Platyhelminthes (Flatworms): Flukes
Class Digenea (Trematoda) - the flukes.
Trematode eggs:
• Trematode eggs usually have a smooth, hard,
transparent, yellow-brown shell and may have
"shoulders" and/or spines.
• Size varies greatly, ranging from less than 30 microns
to 175 microns or more.
• Most have an operculum (escape hatch for the
miracidium).
• Larger trematode eggs cannot be concentrated
successfully using flotation techniques (due to size and
weight). Sedimentation procedures are recommended.
The Platyhelminthes (Flatworms): Flukes
Fasciolopsis buski - The Giant Intestinal Fluke
• Epidemiology - Found in China, Taiwan,
Vietnam, Thailand, Indonesia, Malaysia,
and the Indian subcontinent. Infection rate
may be as high as 40%. It is the most
common intestinal fluke in the orient.
• Life cycle - Humans infected through
ingestion of metacercaria on uncooked
water plants. Encysted metacercariae
hatch & attach to intestinal wall, develop
into adult worms; eggs pass in feces miracidium hatches, penetrates snail cercariae emerge and encyst on aquatic
plants.
The Platyhelminthes (Flatworms): Flukes
Fasciolopsis buski - The Giant Intestinal Fluke
Diagnosis - Demonstrating eggs in feces
specimens. Eggs are difficult to differentiate
from those of Fasciola hepatica.
Morphology of eggs:
• Oval, yellow-brown eggs measure 150 x 90
microns.
• Operculum - the transparent shell has an
operculum (sometimes seen “popped open”).
Major pathology - adult worms attach to the
intestinal wall, producing local inflammation
possibly leading to hemorrhage, ulceration and
abscess formation. Absorption of worm
metabolites can produce toxic or allergic
reactions (edema, ascites).
The Platyhelminthes (Flatworms): Flukes
Fasciola hepatica - The Sheep Liver Fluke
Epidemiology - rare in the USA, probably due
to our diet not including the plants (wild
grown) containing metacercariae. Common
in sheep, cattle, & goats around the world.
Life cycle - similar to F. buski; the difference
being that hatching metacercariae do not
attach to wall of intestine, rather, they
penetrate the intestinal wall and migrate
through peritoneal cavity until coming in
contact with liver. They penetrate the liver &
migrate to bile ducts where they grow to
adult worms.
The Platyhelminthes (Flatworms): Flukes
Fasciola hepatica - The Sheep Liver Fluke
Pathology and Clinical Manifestations - Migration
through liver causes damage and symptoms
proportionate with worm burden. Fever,
abdominal pain, diarrhea, and eosinophilia
may be experienced. Heavy worm burdens
later result in extensive inflammation, epithelial
hyperplasia, and fibrosis around the adult
worms. This can lead to obstruction of the bile
ducts.
Distribution - Cosmopolitan distribution in sheepand cattle-raising countries.
The Platyhelminthes (Flatworms): Flukes
Clonorchis sinensis - The Chinese Liver Fluke
Life cycle:
• Humans eat undercooked fish containing
the metacercaria.
• Immature worms migrate to liver and bile
duct, mature and produce eggs.
• Eggs in fresh water are consumed by 1st
intermediate host (snail). Cercariae develop
in snail.
• Released cercariae penetrate the second
intermediate host (fish) and migrate to
muscles to transform into the infective
metacercariae.
The Platyhelminthes (Flatworms): Flukes
Clonorchis sinensis The Chinese Liver Fluke
• Diagnosis - detection of eggs in feces.
• Morphology of eggs - Eggs are small
(15 x 35 microns), flask-shaped, with a
small operculum on the narrow end. A
small “comma-shaped” extension can
often be seen on the abopercular end.
They cannot be easily distinguished
from eggs of Heterophys or
Metagonimus.
The Platyhelminthes (Flatworms): Flukes
Clonorchis sinensis - The Chinese Liver Fluke
Pathology and Clinical Significance:
• Light infections are usually asymptomatic.
• Heavy infections can result in jaundice, hepatomegaly,
abdominal pain, diarrhea and anorexia.
• Patients may have severe hepatic complications in
chronic cases with heavy worm burdens from repeated
infections.
• Distribution - Far East, especially China.
The Platyhelminthes (Flatworms): Flukes
Heterophyes heterophyes &
Metagonimus yokogawai
Life cycle:
• Humans eat undercooked fish containing the
metacercaria.
• Adults develop in the intestine, mature and
lay eggs.
• Eggs in water are consumed by first
intermediate host (snail). Cercariae develop
in the snail.
• Released cercariae penetrate the second
intermediate host (fish), migrate to muscles
and transform into infective metacercariae.
The Platyhelminthes (Flatworms): Flukes
Heterophyes heterophyes & Metagonimus yokogawai
Diagnosis:
• Recovery and identification of eggs in feces.
• Difficult to differentiate from each other and from C. sinensis
unless adult worms are recovered.
• A patient history is helpful since the infections are more
common in one country or another. These are intestinal flukes,
therefore, no liver involvement and symptoms would be
expected. Treatment is similar for all.
Pathology - Asymptomatic in light infections. Chronic mucous
diarrhea and abdominal pain in heavy infections. Eggs may
travel into tissue causing granulomas and other tissue
disorders.
The Platyhelminthes (Flatworms): Flukes
Paragonimus westermani - The Oriental Lung Fluke
Life cycle –
• Eggs in sputum or feces get into fresh water.
• The miracidium hatches from the egg within 2 weeks, penetrates a
snail. Cercariae develop and escape the snail.
• Cercariae penetrate freshwater crabs or crayfish and encyst,
becoming metacercaria.
• Man eats metacercaria present in undercooked crab or crayfish.
• Immature fluke penetrates intestine, migrates through peritoneal
cavity, penetrates diaphragm and burrows into the lung.
• Worms gather around bronchioles, discharge eggs into bronchial
secretions. Eggs are found in sputum or, if sputum is swallowed, in
feces.
• Infections may persist for years.
The Platyhelminthes (Flatworms): Flukes
Paragonimus westermani The Oriental Lung Fluke
Morphology:
• Adults are 6 x 10 mm, plump,
usually reddish-brown.
• Eggs are 80 to 125 x 45 to 65
microns, opercular end is broader
than the other end and has
“shoulders,” abopercular end often
has marked thickening of egg shell.
The Platyhelminthes (Flatworms): Flukes
Paragonimus westermani - The Oriental Lung Fluke
Pathology • Chronic chest pain, cough, blood tinged sputum, lung
infiltration, nodules, abscesses.
• X-ray may resemble tuberculosis.
• Abdominal paragonimiasis is usually asymptomatic but
common.
The Platyhelminthes (Flatworms): Flukes
Schistosoma spp. - The Blood Flukes
General • Separate male and female worms exist in this group of flukes.
• The female is long and slender, the male is shorter, cylindrical and
characteristically in-curved to form a gynecophoral groove in
which the female reposes. Adult males are 7 - 20 mm long; females
are a bit longer.
• The female leaves the male to lay eggs in the small veins close to
lumen of the intestine or bladder.
• Eggs are firmly wedged in, and spines present on the eggs
facilitate their retention.
• The release of enzymes along with necrosis of tissue results in
release of eggs into the intestine or bladder. Expansion and
contraction of affected organs aid eggs in crossing the tissue
barrier into the lumen for discharge.
The Platyhelminthes (Flatworms): Flukes
Schistosoma spp. - The Blood Flukes
Life cycle • Eggs are released into fresh water; miracidia
hatch.
• Miracidia penetrate snails and undergo
essential developmental stages, leaving the
snail as free-swimming cercariae.
• Free-swimming cercariae penetrate the skin of
humans while they are swimming or bathing.
• Cercariae lose their tails upon penetration,
travel through body via the bloodstream and
develop to adults in blood vessels of the host.
The Platyhelminthes (Flatworms): Flukes
Schistosoma mansoni - Manson's blood fluke
Life cycle (follows general life cycle)
• After penetration of the skin immature worm
(schistosomule) travels through the blood
vessels.
• Most adults will develop in smaller branches
of the inferior mesenteric vein in the region of
the lower colon (but can be found elsewhere).
• S. mansoni lives on ingested blood. The
female can ingest up to 330,000 rbc’s/hour,
the male 39,000/hour.
• Eggs are non-operculate; measure 114 to 180
x 45 to 75 microns, and exhibit a prominent
lateral spine. 100 - 300 eggs per worm per
day are normally shed.
The Platyhelminthes (Flatworms): Flukes
Schistosoma mansoni - Manson's blood fluke
Diagnosis:
• Recovery and identification of eggs in stool or rectal biopsy (best
to get 4 rectal snips in different areas of the bowel).
• Most of the eggs are swept into the liver where granulomas form.
Periportal pipestem fibrosis may result in cirrhosis with little
hepatocellular regeneration.
• Travels history as well as clinical signs and symptoms are
important aids in determining possibility of infection.
• Distribution - Africa, South and Central America, with foci in the
Caribbean and West Indies.
The Platyhelminthes (Flatworms): Flukes
Schistosoma mansoni - Manson's blood fluke
Major pathology and symptoms • Mostly due to the reaction to eggs in the liver.
• Dermatitis at the site of cercarial penetration.
• Acute phase of first infection resembles typhoid fever.
• Cirrhosis of the liver, bloody diarrhea, bowel
obstruction, hypertension, and toxic reactions due to
granulomas around eggs in the liver. The worm
secretes metabolites, resulting in a hepatitis
syndrome; acute cases experience a serum sicknesslike syndrome (Katayama syndrome).
• Many chronic cases are asymptomatic.
The Platyhelminthes (Flatworms): Flukes
Schistosoma japonicum – The Oriental Blood Fluke
Life cycle - follows general life cycle.
• Found in virtually all mammals exposed to infested
water in endemic areas.
• After penetration, the parasite inhabits the branches of
the superior mesenteric vein adjacent to the small
intestine.
• The inferior mesenteries and caval system may also be
invaded. Over time, worms will migrate farther and
farther from the liver.
• Produces more & smaller eggs than the other
schistosomes; eggs free in the circulation are filtered
out in the liver, lungs or other organs.
The Platyhelminthes (Flatworms): Flukes
Schistosoma japonicum – The Oriental
Blood Fluke
• Morphology of eggs - measure 55 to 95
x 40 to 60 microns; exhibit a very small
lateral spine (may not be apparent).
• Diagnosis - based upon recovery and
identification of eggs in stool samples
or rectal biopsies.
The Platyhelminthes (Flatworms): Flukes
Schistosoma japonicum – The Oriental
Blood Fluke
Major Pathology and Symptoms • Infection with even a few worms may
be very serious. Cerebral atrophy
can occur due to eggs becoming
trapped in the brain.
• Hepatic and pulmonary cirrhosis are
seen in the chronic stage of this
infection; CNS symptoms occur due
to lodgment of eggs in nerve tissue.
Distribution - the Far East.
The Platyhelminthes (Flatworms): Flukes
Schistosoma haematobium - The Bladder Fluke
Life cycle – (see general life cycle)
• Adult worms mature in the sinusoids of the
liver, and migrate from that organ reaching
vesicle, prostatic and uterine plexuses by
way of the hemorrhoidal veins.
• Eggs are deposited in the walls of the
bladder, uterus, vagina & prostate.
• Eggs in the bladder break into the lumen
and escape with urine. The spine and
contraction & expansion of the bladder aids
movement of eggs through tissue.
The Platyhelminthes (Flatworms): Flukes
Schistosoma haematobium - The Bladder
Fluke
Morphology of the eggs • Measure 112 to 170 x 40 to 70 microns,
& exhibit a prominent terminal spine.
Diagnosis • Eggs in urine.
• Eggs may be in feces due to worm
rarely inhabiting intestinal mesenteric
vessels.
The Platyhelminthes (Flatworms): Flukes
Schistosoma haematobium - The Bladder Fluke
Major pathology and symptoms • The bladder wall becomes increasingly infiltrated with eggs.
• Bleeding occurs from the bladder wall as it contracts. As it
becomes more involved ulceration occurs.
• Hematuria becomes more severe and calcification of the bladder
and obstruction of ureters and the neck of the bladder lead to
bacterial infections.
• Death from renal failure may occur but damage is reversible if
treated early enough.
• Clinical correlation exists between S. haematobium infection and
bladder cancer.
Distribution - epidemic in the Nile Valley. Present across Africa and the
Middle East.