Trematodes

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The most significant trematodes from a clinical point of view are blood flukes, Schistosoma mansoni, S. japonicum and S. hematobium. Other trematodes of significance are intestinal fluke, Fasciolopsis buski, liver fluke, Clonorchis sinensis and lung fluke, Paragonimus westermani.

Schistosomiasis (Bilharziasis)

The three species of Schistosoma have different geographic distributions. S. hematobium is prevalent in Africa, S. mansoni is found in Africa and America and S. japonicum is common in the far east.

Epidemiology

Approximately 250 million people are infected with schistosomes and 600 million are at risk.

Morphology

Adult worms are 10 to 20 mm long; the male has an unusual lamelliform shape with marginal folds forming a canal in which the slender female worm resides. Unlike other trematodes, schistosomes have separate sexes.

Life cycle

Man is infected by cercaria in fresh water by skin penetration. The cercaria travel through the venous circulation to the heart, lungs and portal circulation. In about 3 weeks, they mature and reach the mesenteric (S. japonicum and S. mansoni) or the bladder (S. hematobium) vessels where they live and ovulate for the duration of the host's life. Eggs germinate as they pass through the vessel wall into the intestine or bladder and are excreted in feces (S. japonicum and S. mansoni) or urine (S. hematobium). In fresh water, the larval miracidium hatches out of the egg and swims about until it finds an appropriate snail. After two generations of multiplication in the snail, the fork-tailed cercariae emerge into the water and infect another human.

Symptoms

Penetration of cercariae causes transient dermatitis (swimmers' itch). The symptoms of schistosomiasis are primarily due to a reaction against the eggs and include splenomegaly, lymphadenopathy and diarrhea. In the bladder, they produce granulomatous lesions, hematuria and sometimes urethral occlusion. Most bladder cancers in endemic areas are associated with chronic infection. In the intestine, they cause polyp formation which, in severe cases, may result in life threatening dysentery. In the liver, the eggs cause periportal fibrosis and portal hypertension resulting in hepatomegaly, splenomegaly and ascites. A gross enlargement of the esophageal and gastric veins may result in their rupture. S. japonicum eggs are sometimes carried to the central nervous system and cause headache, disorientation, amnesia and coma. Eggs carried to the heart produce arteriolitis and fibrosis resulting in enlargement and failure of the right ventricle.

Pathology and Immunology

The 'swimmers' itch is due to physical damage to the skin by proteases and other toxic substances secreted by the cercaria. The host develops both type I and type IV hypersensitivity reactions to schistomal secretions and egg constituents. Embryonated eggs cause collagenase-mediated damage to the vascular endothelium. Host immune responses, both humoral and cell mediated, have been shown to be of some protective value. IgE and eosinophil mediated cytotoxicity has been suggested as a mechanism of killing the adult worm.

Diagnosis

Diagnosis is based on a history of residence in an endemic area, swimmers' itch and other symptoms. The eggs are very characteristic and confirm diagnosis. S. hematobium eggs in urine (55 to 65 by 110 to 170 micrometers) have an apical spine or knob. S. mansoni eggs in feces (45 to 70 by 115-175 micrometers) have a spine on the side. S. japonicum eggs (55 to 65 by 70 to 100 micrometers) are more round with a vague spine on the side.

Treatment and control

Praziquantel is effective against all species. Contaminated water should be avoided. Control measures include sanitary disposal of sewage and destruction of snails. No vaccine is available.

Fasciolopsis buski (Giant intestinal fluke)

Epidemiology

This is a parasite of central and southeast Asia.

Morphology

The elongate oval fluke is 2 to 7 cm long and lives in the small intestine of man.

Life cycle

Man is infected by ingesting water chestnuts contaminated with metacercaria which find access to the small intestine, attach themselves to the mucosa and mature in 25 to 30 days. The fluke eggs are passed in the feces and hatch in fresh water producing miracidia which must penetrate a suitable snail within hours. The miracidia in the snail develop into cercaria and enter fresh water where they attach themselves to water plants (water chestnut) and encyst to become metacercaria.

Symptoms

Epigastric pain, nausea and diarrhea are experienced, especially in the morning. In heavier infections, generalized edema and ascites occur.

Pathology

The fluke attaches itself to the intestinal mucosa where inflammation, ulceration and abscesses occur.

Diagnosis

Diagnosis is based on clinical symptoms in endemic areas. Eggs in feces (75 to 100 by 130 to 150 micrometers) provide the final diagnosis.

Treatment and control

Praziquantel has proven effective. Water chestnuts from contaminated waters should be avoided. Sewage should be treated before disposal.

Clonorchis sinensis (Chinese Liver Fluke)

Epidemiology

This is a widespread parasite of man, dogs and cats in the southeast of Asia. It is extraordinarily common in China and is also found in Korea and Japan. Related flukes parasitizing European cats (Opisthorchis felinus) and dogs (O. viverini) infect humans in the endemic areas.

Morphology

These are spindloid flukes measuring about 16 by 4 mm. The eggs measure 29 by 16 micrometers.

Life cycle

Man is infected by eating raw or improperly cooked fish which carries the infective metacercaria in a cyst. The cyst is digested and the larval worm migrates up the bile duct to the liver where it matures into an adult. The eggs, deposited in the biliary duct, pass in the feces and find their way to fresh water. Upon ingestion by a suitable fresh water operculate snail, the egg hatches to produce a miracidium. The miracidium in the snail develops into cercaria which breaks out in water to penetrate under the scales of fish. In fish, the cercaria encysts in the muscle and forms metacercaria infectious to man.

Symptoms

The worm causes irritation of the bile ducts which become dilated and deviated. The liver may enlarge, become necrotic and tender and its function may be impaired. Modest infections results in indigestion, epigastric discomfort, weakness and loss of weight. Heavier infections produce anemia, liver enlargement, slight jaundice, edema, ascites and diarrhea.

Diagnosis

Diagnosis is based on symptoms and the presence of endemic infection in the area. Definitive diagnosis is dependent on finding the characteristic eggs in the feces or biliary drainage.

Treatment and control

Praziquantel has proven of value. Fish should be cooked well before consumption. Sewage must be treated before disposal.

Paragonimus westermani (Lung Fluke)

Epidemiology

Lung fluke is most commonly encountered in parts of Asia, Africa and South America.

Morphology

It is a plump reddish brown oval worm measuring 10 by 4 mm. The ovum measures 85 by 55 micrometers.

Life cycle

Lung fluke infects man (and domestic carnivores) when crabmeat infested with encysted metacercaria is consumed. The metacercaria reach the small intestine, exit their shell and bore their way, as young flukes, through the intestinal wall, through the thoracic diaphragm and penetrate the lung. There, they become enclosed in 1 to 2 cm cysts and reach maturity. The eggs are found in the sputum or, if swallowed, in the feces, 2 to 3 months after infection. The eggs, when introduced in fresh water produce a miracidia which penetrates the suitable snail. In the snail they develop into cercaria which break out in water and penetrate gills, muscle or viscera of fresh water crabs and become encysted in flesh as metacercaria.

Symptoms

The fluke provokes the development of a fibrous tissue capsule with bloody purulent material containing eggs. There is inflammatory infiltrate around the capsule. The symptoms include a dry cough, followed by production of blood stained rusty brown sputum. Pulmonary pain and pleurisy may develop. Worms may migrate to the brain where they lay eggs and cause a granulomatous abscess resulting in symptoms similar to epilepsy.

Diagnosis

Diagnosis is based on history and symptoms. Eggs are found in rust colored sputum, often being examined for tuberculosis.

Treatment and control

Praziquantel taken orally is quite effective. Adequate cooking of crustaceans is a preventive measure. Improved sanitary conditions have lowered the infection rate in endemic areas.