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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.


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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