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 cercarise can 

 be seen (Fig. 73 A) . While still in the cysts the cercari 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 about 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 consists either in the destruction of the 

 snails which act as the first intermediate host by the use of copper 

 sulphate, as outlined on p. 211, or by abstinence from the use of 

 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 Para- 

 gonimus 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 endemic human parasite in 

 this country suggests that the intermediate host may be an anima : 



