Prevention and Eradication 79 



water be brought to bear upon the bilharzia-carrying molluscs so 

 as to ensure their permanent eradication from lands now heavily 

 infected and their exclusion from new areas about to be reclaimed. 



Protection of Troops and Personal Prophylaxis. 



Having dealt with the mode of transmission and suggested the 

 lines upon which eradication might be effected in the course of a 

 few years if undertaken by the State, we come now to consider the 

 preventive measures that should be adopted by the individual, or 

 collections of individuals, working in districts where the disease is 

 still rife. It is obvious from what has been said on p. 47 (Part I), 

 that in large towns where filtered water is supplied both for drink- 

 ing and bathing there is practically no risk to the European. A 

 study of the bionomics of the cercaria gives the data wherewith 

 unfiltered water can be rendered safe where filtered water is 

 unavailable or insufficient for all personal purposes. 



Bionomics of Bilharzia Cercari^e. 



The bilharzia cercarige move by louping and by 



Activity . . ^ l <r> j 



swimming. They crawl rapidly over any surface 



by alternate use of the oral and ventral suckers, the tail being 

 dragged behind passively. When swimming the tail and the whole 

 body gyrates and the cercaria progresses with the pronged tail 

 foremost. Swimming is not continuous. Brief periods of activity 

 are regularly alternated with periods of rest. During these latter 

 the cercaria very slowly sinks. When seen with a hand lens their 

 attitudes recall slightly minute mosquito larvae. As a rule they 

 frequent the surface, but when a small mammal like a mouse is 

 placed in the water they at once attack the skin. As successful 

 infection resulted in a young mouse after only ten minutes' im- 

 mersion on a single occasion they appear to be able to pierce the 

 skin very rapidly. 



In ordinary tap-water freshly discharged cercariae 

 of'li^fe^ usually live about twenty-four hours. A consider- 

 able number survive thirty-six hours, but none 

 has ever been found alive after forty-eight hours. They al'e 

 apparently unable to obtain nourishment from water. An infected 

 mollusc will apparently continue to discharge active cercarise for a 

 long period. On two occasions infected Planorbis boissyi were 

 kept in tap-water, which was renewed daily for three weeks. 

 Large numbers of cercariae were discharged into the water 

 every day. 



