OTHER SPECIES OF SCHISTOSOMA 217 



rendered non-infective by the addition of one part of cresol in 

 10,000 parts of water. 



Leiper 's work shows that transient collections of water are not 

 sources of infection after recent contamination, whereas all per- 

 manent collections of water, as in rivers, canals and marshes, 

 are dangerous if inhabited by a suitable intermediate host. 

 The removal of infected persons from a given body of water 

 would have no immediate effect in reducing its infectiveness, 

 since the snails discharge cercariae into the water for a prolonged 

 period. The preventive measures briefly outlined above were 

 worked out by Leiper for the special conditions existing in the 

 infected parts of Egypt but in a broad way they are applicable 

 wherever Schistosoma hcematobium occurs. 



Other Species. There are two other species of Schistosoma 

 which are pathogenic to man. One, S. mansoni, was long con- 

 fused with S. hcematobium, the only apparent differences having 

 been observed in the eggs. The eggs of S. mansoni (Fig. 65B) 

 are provided with a lateral instead of a terminal spine, and are 

 voided in the digestive tract and its appendages, whence they are 

 liberated with the faeces, instead of making an exit from the body 

 by way of the urinary organs. By experimental infections of 

 mice Leiper showed that cercariae from the snail Planorbis boissyi 

 (Fig. 66B) developed into worms somewhat smaller than those 

 from the species of Bullinus, with certain distinct differences in 

 anatomy. The cercariae from Planorbis produced only lateral- 

 spined eggs which were voided with the faeces, thus showing that 

 S. mansoni was really a distinct species and not merely an abnor- 

 mal type of S. hcematobium. This parasite occurs in common with 

 the foregoing species in many parts of Africa and is also common 

 in the West Indies, Venezuela and perhaps other parts of tropi- 

 cal America. Like the hookworm, it was probably introduced 

 from Africa in the slave days. The intermediate host in Ven- 

 ezuela has recently been shown by Iturbe and Gonzales to be 

 the snail Planorbis guadelupensis, whereas in Brazil Lutz has 

 shown. P. olivaceus to be the intermediate host. 



The anemia and debility caused by S. mansoni is similar to that 

 caused by S. hcematobium. The irritation and inflammation of 

 the urinary organs is replaced, however, by similar symptoms of 

 the intestine, and a kind of dysentery often results. 



The manner of transmission of the parasite is similar to that 



