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Epidemiology and Pathology

The eggs of Paragonimus westermani are expectorated in sputum or, if swallowed, are passed in feces (Fig. 22.2). They are brown, oval, and measure approximately 90 x 55 Ám. The eggs are operculated with shoulders and have abopercular thickening. They mature in water and hatch after 2 to 3 weeks, depending on the temperature. Ciliated miracidia are released and swim freely to penetrate a suitable snail host. The species of snail varies in Asia, Africa, and the Americas (Fig.22.3). Maturation takes place after asexual reproduction and 2 to 3 months later, after passing through a sporocyst and two redial stages, 20-30 to hundreds of cercariae are released from the infected snail and are again free-swimming. Within 24 hours these cercariae must penetrate through the articulations in the shells of fresh-water crabs or crayfish, which are the second intermediate hosts; the crabs may also become infected by eating the snails. Within these secondary hosts, cysts containing metacercariae form.

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Fig. 22.2A,B Life cycle of Paragonimus westermani. (Drawing by Phyllis Smith, reproduced from Ash and Spitz: Pathology of Tropical Diseases, Saunders, Philadelphia).

Fig. 22.3 Snail host for Paragonimus. This species, Semisulcospira libertina from Taiwan, may serve as an intermediate host for P. westermani in appropriate areas.

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