Main » Infectious diseases » Snail-mediated Helminthiases » Paragonimus westermani
Four species of Paragonimus have been identified in man. Paragonimus westermani commonly causes human infection in the Far East where it is widely distributed among other mammals. Several other species are probably involved. Human infections with Paragonimus africanus and Paragonimus uterobilateralis have been recognised in West Africa, and Paragonimus heterotremus in Thailand. Unidentified species occasionally produce infection in man in parts of Central and South America.
Distribution of lung flukes.
Photo 1. Adult Paragonimus westermani and Paragonimus hetrotremus. The adult fluke normally lives in the lungs. It is rather lemon-shaped and about one cm long when alive. The preserved specimens shown here have become flattened during preparation. (x6)
Photo 2. Adult Paragonimus westermani and Paragonimus hetrotremus (2).
Photo 3. Eggs in human sputum. Eggs are passed in the sputum or swallowed to be passed later in the faeces. The miracidia hatching from the eggs penetrate snails of various genera including Semisulcospira and Thiara (see Table). After the usual cycle of development in the snail, microcercous cercariae emerge and encyst inside fresh water crayflsh and crabs. Various crab-eating carnivores thus become the natural reservoirs of Paragonimus species.
Photo 4. Potamon rathbuni, a host of lung fluke. Crabs are commonly eaten raw, or in the form of an uncooked paste with which the metacercariae are ingested.
Photo 5. Metacercaria in crab gills. Compare with the smaller metacercaria of C. sinensis (421). fx 10)
Photo 6. Lung of dog with adult Paragonimus westermani. Metacercariae excyst in the duodenum from which they migrate through the peritoneal cavity and diaphragm to the lungs, where they mature inside capsules formed by the host tissue.
Photo 7. Section of lung. The figure shows a section of lung containing encapsulated adult Paragonimus westermani. (x20)
Photo 8. Chest X-ray. Human infection is manifested by cough, haemoptysis, and other signs and symptoms which are commonly confused with those of tuberculosis. Typical shadows caused by the encysted adult trematodes may be seen on X-ray. This patient was infected with Paragonimus uterobilateralis in Eastern Nigeria. This species and Paragonimus africanus (which occurs in the nearby Cameroon Republic) have a similar life cycle and, like Paragonimus westermani, man is infected by eating crabs or crayfish. The animal reservoirs are not yet known.
Photo 9. Paragonimus cyst in brain. Paragonimus westermani cyst found at post mortem in the brain of a 21-year-old Japanese girl.
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