148 ANIMAL LIFE AND HUMAN PROGRESS 



the duration of life of the cercaria as a free-swimming organism 

 does not exceed forty-eight hours. If, then, water containing 

 the infective stage of Bilharzia is simply stored for about 

 two days it becomes free from infection. 



A study of the fresh-water snails now known to spread 

 the disease in Egypt has revealed certain biological facts 

 which, if properly used, may result in a marked diminution 

 if not in the eradication of bilharziasis over a wide area. 

 Unlike the Melania and Katayama snails which spread disease 

 in the Far East, the Bilharzia carriers in Egypt are not 

 provided with an operculum by means of which they can 

 close their shells and so withstand drying for a long period. 

 Exposure and the Egyptian sun ensure their death in a few 

 days. 



In Egypt these carriers of Bilharzia live and breed chiefly 

 in the irrigation channels, the water running in which is 

 entirely under the control of the Irrigation Department 

 throughout the country. In the summer months the flow is 

 carefully regulated and the water is periodically cut off entirely 

 for some days. During such stoppages the snails which carry 

 the Bilharzia die off, in enormous numbers, from drying. Un- 

 fortunately the beds of the irrigation channels are often uneven 

 from the removal of mud for brickmaking etc., and pools 

 result. In these residual patches of water large numbers of 

 the Bilharzia-carriers congregate and so survive until the 

 flow of water is again established. I am convinced that, 

 without interfering with the general scheme, the summer 

 rotations might easily be utilised, with slight adjustments, 

 to bring about the complete eradication of the Bilharzia 

 intermediaries. Where the alignment of the canal cannot be 

 readily altered the pools could be effectively dealt with by 

 chemical means. Certain of the common chemical manures 

 are highly toxic to these fresh-water molluscs, and yet no 

 possible harm could result to the crops from their use. 



During the summer months there is absolutely no rainfall 

 in Egypt, and the existence of the Bilharzia-carriers is de- 

 pendent entirely upon " controlled " water. It is therefore 

 surely incumbent upon the Irrigation Department in Egypt 



