SCHISTOSOMA H^MATOBIUM 215 



Examination of molluscs which were collected in the El Marg 

 canal resulted in finding 17 species of cercariae, among them the 

 cercariae with forked tails arid no bulb in the oesophagus, the 

 typical form of Schistosoma cercariae (Fig. 67C). Infected 

 molluscs may continue to liberate cercariae for several weeks. 

 Leiper later found that S. hcematobium developed only in the 

 species of Bullinus, the cercarise from Planorbis belonging to 

 another species, S. mansoni. In Natal and the Transvaal a 

 small dark-colored snail, Physopsis africana, acts as an inter- 

 mediate host. 



When fully developed the cercarise escape from the snails and 

 swim about in water in search of a final host. They do not live 

 at best as long as 48 hours, so a vast majority of the larvae must 

 perish from failure to find a suitable host. It has been shown 

 that not only man but also various species of monkeys and 

 rodents may be infected by the cercariae. 



Infection may occur in two different ways: by drinking water 

 containing cercariae, or by bathing in it, since the cercariae are 

 able to penetrate either the mucous membranes or the sound 

 skin, migrating through the body until they reach their desti- 

 nation in the abdominal veins. The natives of some parts of 

 Africa realize that infection may result from bathing, but from 

 the nature of the disease they believe that infection takes place 

 by way of the urinary passages and therefore employ various 

 mechanical devices to prevent infection in this manner. There 

 is little doubt, when infection occurs from drinking water, that 

 the cercariae adhere to the walls of the mouth and throat and bore 

 through them, since passage through the acid juices of the 

 stomach is apparently fatal for them. 



The disease usually develops in from two to four months after 

 infection. 



Treatment and Prevention. Once infection has occurred 

 there is no known means of eliminating the worms from the 

 veins in which they live, or of destroying them. Several drugs 

 injected into the veins, especially salvarsan and thymobenzene, 

 have been recommended, but their use has not met with uni- 

 formly successful results. X-ray treatment has recently been 

 tried but with little success. 



None of the injuries done by the worms can be readily allevi- 

 ated. Surgical treatment of the bladder to relieve growths or 



'v 



