PHYLUM PIAl YHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 



145 



SOME COMMON TREMATODES OF MAN 



the trematodes especially in Africa, Asia 

 Minor, Arabia, West Indies, Brazil, and 

 Venezuela; the cestodes throughout the 

 world. The scientific names, location of de- 

 velopmental stages, geographic distribution 

 and hosts of some of the species that live 

 in man are presented in the accompanying 

 tables. Hydatid cysts represent a larval stage 

 of the tapeworm, Echinococcus granulosus. 

 They occur in the liver and other organs of 

 man, cattle, horses, sheep, etc., and may at- 

 tain the size of a child's head. One abdom- 

 inal cyst has been reported which contained 

 42 liters of fluid. Within each cyst the 

 germinal layer may give rise to thousands of 

 brood capsules or daughter cysts in which 

 scolices develop. Operative measures only 

 are effective in treatment. 



Certain schistosome cercariae incapable of 



infecting man cause a severe swimmer's itch 

 (dermatitis) when they penetrate the skin 

 of bathers who have become sensitized by 

 repeated exposures. This condition is com- 

 mon in the north central states, in southern 

 Canada, as well as in some other parts of 

 the United States, Europe, and India. Swim- 

 mer's itch can be avoided by swimming in 

 deep water. The first symptom of swimmer's 

 itch is a prickly sensation followed by the 

 development of ver\' itchy pimples, which 

 sometimes become pustular. The itch or- 

 ganism can be controlled in small bodies of 

 water by the use of copper sulfate to kill the 

 snail in which the cercariae spend part of 

 their life cycle. One midwest state used over 

 25 tons of copper sulfate in a recent summer 

 to destroy infected snails to free the beaches 

 from swimmer's itch. 



