250 DISEASES DUE TO HELMINTHS 



(5) Infection only takes place through (5) Infection actually takes place both 

 the skin. by the mouth and through the skin. 



Recently contaminated moist earth or 

 water is not infective. 



(6) Infection in towns is due to con- (6) Infection in towns is acquired from 

 tact with recently contaminated moist unfiltered water which is still sup- 

 earth or water. plied, even in Cairo, in addition to 



filtered water, and is delivered by a 

 separate system of pipes. 

 {7) Eradication depends upon educa- (7) Eradication can be effected without 

 tion and complete sanitary control the co-operation of infected in- 



throughout the country. The sus- dividuals by destroying the mollusc 



tained co-operation of the affected and intermediaries, 



individual is essential. 



The practical conclusions are as follows: — 



That unfiltered water taken from canals, ditches, or birkets would 

 be rendered safe : — 



(i) If kept beyond the survival period of the cercaria, i.e., forty- 

 eight hours. 



(2) If heated to 50° C, a temperature at which the cercaria is imme- 

 diately killed. 



(3) If previously treated with those chemicals that are lethal to the 

 cercaria. 



The use of tabloids of sodium bisulphate, 16 grains to a pint of 

 water, giving a dilution of 1-567, is recommended. 



The following points should be attended to : — 



(i) Personal contact of any kind with unfiltered water is dangerous. 

 The surface of the water is the most likely to be infective as the cercariae 

 congregate there. An intake pipe should always be led, therefore, 

 to the centre of the stream, and should draw the water from near the 

 bottom and at a place where there is little or no vegetation. 



(2) It is essential in drawing water for storage, in order to destroy 

 the bilharzia cercaria, that no infective mollusc be admitted. This 

 can be ensured by screening the intake pipe with gauze having about 

 sixteen meshes to the inch. The common mosquito gauze or prosphor- 

 bronze wire gauze is very serviceable. 



(3) The water in the wells and " sakias " may be regarded as much 

 safer than that from other sources. Hitherto molluscs have not been 

 found in these wells. 



(4) Shallow barrel sand filters are open to suspicion. It has been 

 found experimentally that after fifteen minutes cercaria succeed in 

 passing in large and increasing numbers through four inches of desert 

 sand. 



(5) Although the reproductive activity of bilharzia in molluscs is 

 probably most intense during the summer months, the occurrence of 

 mature cercariae in infected molluscs in February shows that there is 

 a certain liability to the infection throughout the year. 



