ANIMAL PARASITISM 759 



adults pass from the liver to the intestine of the host by way of the 

 bile duet, then pass from the body in the feces. Snails probably be- 

 come infected by swallowing the eggs, while feeding on fecal matter 

 in the water. After hatching, the miracidia migrate into the lymph 

 spaces of the snail and develop into elongated sporocysts, each of 

 which gives birth to a number of redia (differing from sporocysts by 

 possessing a pharynx and a rudimentary gut). Each redia gives 

 birth to six or eight cercariae, which emerge from the snail and swim 

 around in the water by means of a very large, undivided tail. When 

 a cercaria comes in contact with a fish, it enters the skin and encysts 

 either in the skin or in the muscles just below the skin. It is now 

 called a metacercaria or agamodistomum. Man becomes infected by 

 eating these metacercariae in poorly cooked fish. When swallowed, 

 the cysts are dissolved by digestive juices of the host, tlie larva escapes 

 into the duodenum, migrates up the bile duct to the liver, and there 

 develops into an adult. There is evidence that the adult Clonorchis 

 may live as long as twenty years in the liver of man. 



Treatment of clonorchiasis is not very satisfactory. Prevention 

 is simple : avoid eating fish which are not thoroughly cooked. 



It will be noted that the Clonorchis life cycle involves three hosts: 

 a mammal as the final host, a snail as the first intermediate host, 

 and a fish as the second intermediate host. Infection of the fish is 

 by active invasion of the cercariae, and infection of the final host is 

 passive. 



Other Trematodes. — One of the best known parasites of domestic 

 animals is the sheep liver fluke, Fasciola hepatica, which occurs in 

 all sheep-raising countries in which wet pastures are common. It is 

 also a common parasite of goats and cattle. Like Clonorchis, Fasciola 

 lives in the bile passages, and its eggs pass out Avith the feces of 

 the host, but unlike Clonorchis, the eggs hatch in water and the 

 free-swimming miracidium actively seeks and penetrates the snail 

 host. Sporocijsts in the snail give rise to rediae which produce 

 cercariae, but the cercariae encyst on any surface, including grass | 



blades and even the surface film of the water. Sheep become infected I 



by eating grass bearing encysted larvae or by swallowing floating !, 



cysts while drinking water. Fascioloides magna, the large liver fluke >} 



of cattle and sheep in Louisiana, Arkansas, and Texas, is very similar i 



in structure and life history. 



