LUNG FLUKES 223 



thick walls. The enclosed cercaria lies straight, unlike most 

 encysted cercarise, and the body is entirely covered by short 

 spines. In fully-developed specimens the suckers, digestive 

 tract and other parts of the anatomy of the enclosed cercariae can 

 be seen (Fig. 73 A) . While still in the cysts the cercarisB are fairly 

 resistant to unfavorable environmental influences, but are easily 

 destroyed after hatching. 



When an encysted cercaria is swallowed by a susceptible ani- 

 mal the cyst wall is dissolved off in the intestine, the active 

 liberated larva (Fig. 73B) bores through the intestinal wall, 

 wanders about in the abdominal cavity for some time, then 

 bores through the diaphragm into the pleural cavity, whence it 

 eventually penetrates the lungs from the outer surface. It 

 becomes mature in aliout 90 days. Occasionally the worms 

 apparently get lost and bore through the abdominal wall and 

 muscular connective tissues. It is probably in this way that 

 other organs than the lungs are penetrated by the flukes. 



There are two ways in which man may become infected, namely, 

 by eating infected crabs which are not thoroughly cooked, or 

 by drinking water containing cysts discharged from infected 

 crabs. As already remarked, the mature cysts make their way 

 to the gills, whence they can easily be removed, and whence 

 they probably escape readily under natural conditions, thus 

 becoming free in the water. Here they may remain alive for 

 some time, probably 30 days or more. Yoshida states that the 

 cysts sink to the bottom, in which case human infection could 

 occur only rarely if ever from infected water. Nakagawa, how- 

 ever, observed that 20 per cent of the larvae when freed float 

 on the surface of the water. 



Prevention of infection, in Japan at least, obviously consists in 

 abstinence from raw crabs as food and in avoidance of water for 

 drinking which may possibly be infected. Whether or not other 

 animals may serve as hosts for the cercarise is unknown, but if 

 the allied Paragonimus kellicotti is truly endemic in the United 

 States, where no fresh- water crabs are found, some other animal 

 must serve as an intermediate host, possibly certain species of 

 crayfish. The fact that the lung fluke is not known as an en- 

 demic human parasite in this country suggests that the inter- 

 mediate host may be an animal which is not used as food and the 

 habits of which give little opportunity for the parasites to gain 



