Parasit Trematode s Summer 2012..

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 self-fertilization.

• 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.

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).



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


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.


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.


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.


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.


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).


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.


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 sheep-

and cattle-raising countries.


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.


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.


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.


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.

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.



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.


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.


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.


Schistosoma spp. - The Blood FlukesGeneral -• 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.


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.


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.

Schistosoma mansoni - Manson's blood flukeDiagnosis:• 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.



Schistosoma mansoni - Manson's blood flukeMajor 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 sickness-like syndrome (Katayama syndrome).

• Many chronic cases are asymptomatic.


Schistosoma japonicum – The Oriental Blood FlukeLife 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.


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.


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.


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.


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.


Schistosoma haematobium - The Bladder FlukeMajor 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.

Parasit Trematode s Summer 2012..

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