Paragonimus is a digenetic, hermaphroditic trematode with a life-cycle that includes two intermediate hosts (snails and crustaceans) as well as the definitive (mammalian) host (Figure 17.1). The adult worm is reddish-brown, mobile and has a flattened ventral surface, with an average size of 10 x 5 mm and a thickness of 5 mm (Figure 17.4). The adult contains an oral and ventral sucker, reproductive organs, including ovaries and testes, an intestinal system and spines on its surface which are useful for species identification (Yokogawa et al., 1960). The mature adult resides in the mammalian lungs, where it can produce up to 40000 eggs/day. The eggs are yellow-brown, contain yolk cells and a flat operculum, and measure approximately 80 x 50 After developing for approximately 3 weeks, the eggs are passed into the environment from the lungs via the sputum or are swallowed and excreted in the feces. The eggs hatch into miracidia, which then infect fresh water snails, the first intermediate host. Several snails can serve this function, including the families Thiaridae, Pleuroceridae and Hydrobiidae (Malek, 1980). Semisulcospira libertina is one of the more common vectors for P. westermani. Within the snail, the miracidia develop into sporocysts, rediae and then cercariae (100 x 270 which are either secreted into the water or ingested by freshwater crabs or crayfish that are often found in streams. At least 21 species can serve as this second intermediate host including the genus Potamon (Nana and Bovornkitti, 1991). The encysted metacercariae (0.4 x 0.4 mm) mature in the gills and muscles over 42-106 days (Yokogawa et al., 1960). The metacercariae are then ingested by the definitive host when raw crabs or crayfish are consumed. Definitive hosts include dogs, cats, pigs, wild carnivores and humans.
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