Organism Unique amongst all the helminths, the trematode Paragonimus wester-mani selectively inhabits the lung.
Clinical features Foreign body reaction to the parasite in the lung results in fibrosis, compensatory dilation and abscess formation. Haemoptysis is often an important feature, mimicking tuberculosis. Symptoms include cough and chest pain. If the parasite migrates to a site other than the lung, it can cause CNS, liver, intestinal, genitourinary or subcutaneous disease.
Diagnosis is by finding eggs in the sputum, or if swallowed, in the faeces (Fig. 9.1). Any case of haemoptysis without other signs of tuberculosis should have a sputum examination, on which an acid-fast bacilli (AFB) stain has not been used, as this destroys the eggs.
Transmission The egg on reaching water softens and a miracidium frees itself from the egg capsule and searches for a snail of the genus Semisulcospira. Passing through the sporocyst and redia stages, the cercaria encysts in the gills and muscles of freshwater crabs and crayfish. Humans are infected by eating uncooked, salted or pickled freshwater crab (Eriocheir and Pota-mon) or crayfish (Cambaroides), while an animal reservoir (mainly cats and dogs) helps to maintain the disease. The liberated metacercaria pass through the intestinal wall and penetrate the diaphragm to enter the lung. Adults develop in the lungs to produce eggs, which are liberated into the sputum. Occasionally they find their way to unusual sites, the brain being particularly serious (Fig. 9.4).
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