396 INFESTATION OF FRESH-WATER SNAILS. 



None cf these birds does any harm to the agriculturalist, stock 

 farmer, or fruit farmer, consequently it is nothing less than a 

 crime to take the lives of such valuable feathered allies, which 

 wage incessant war on snails and a host of other pests. 



THE EXPERIMENTAL INFESTATION OF FRESH-WATER 



SNAILS, WITH SPECIAL REFERENCE TO THE 



BILHARZIA PARASITE. 



BY 



F. G. Cawston, M.D. Cantab. 



Read July 11, 1921. 



One of the most interesting parts of the study of the life- 

 history of the Bilharzia and allied parasites is that which deals 

 with the survival of the parasites in their various stages. 



There is no doubt that those Bilharzia worms which have 

 reached maturity in the liver can live in the blood-system of a 

 human being for from ten to twenty years, and are sometimes 

 found alive after the death of their host; Fasciola parasites 

 more often cause wasting and death of the infested oxen and 

 sheep. 



The eggs which escape from the system of a bilharzia patient 

 may remain unhatched for several clays, but die as soon as they 

 become dry. I have kept them alive in urine for several days, 

 but they hatch as soon as they reach fresh water on a warm day. 



The miraciclia which escape from these eggs are just visible 

 to the naked eye, and can be seen swimming about for several 

 hours but will not survive more than twenty-four hours and are 

 harmless to human beings, as they cannot prolong their existence 

 outside a suitable fresh water snail. 



In 1915 and 1916 I carried out some experiments at Maritz- 

 burg with the object of experimentally infecting P/n/.sopsis africana 

 and Limnaca natalensis, using snails I had collected from the 

 Umsindusi. 



In December, 1917, I repeated these experiments at Potchef- 

 stroom, using Isidora schackoi Jickeli from the golf links. This 

 snail sometimes harbours a schistosome, C. gladii. Thirty-eight 

 young Isidora were selected and placed in water swarming with 

 miracidia. None were examined for sixteen days, by which time 

 bilharzia infestation should have been clearly marked, but all 

 were dissected before the twenty-eighth day. Thirty-one were free 

 from cercariae, while the remaining seven showed signs of early 

 infestation with Cercaria f random, an amphistome which infested 

 practically all the mature Isidorae in the pool from which these 

 snails were collected. 



On May 14th some water containing numerous schistosomes 

 from PJn/sopsis africana from Mrs. Oldham's pool in Sydenham 



