SOME INHABITANTS OF MAN 147 



These experiments support irrefutably the common but 

 vaguely held view that infection occurs in many instances 

 duriag bathing. It had been shown that the young, newly 

 hatched larva is killed by acid of the dilution of 1 in 1000, and 

 from this it was concluded that infection by the mouth could 

 not take place owing to the lethal action of the gastric juice. 

 A study of the free-swimming cercariae showed, however, 

 that these attach themselves firmly to the Uning of the mouth 

 when given in drinking-water to monkeys, and that these 

 animals later succumb to intense infections of Bilharzia 

 worms. Infected water is therefore dangerous whether used 

 for drinking or for ablution. 



The measures which have been outlined above for the 

 control of fluke diseases in the Far East are obviously in- 

 applicable to bilharziasis. The collection of the moUuscan 

 intermediaries would be attended with grave risk owing to the 

 ease with which the parasites can penetrate the moist skin. In 

 a country like Egypt, where bilharziasis is so prevalent, it 

 would be impracticable owing to the enormous numbers of 

 irrigation channels and drains, in which the snails find ideal 

 breeding-places. The sterilisation by heat of all water for 

 drinking and ablution would likewise be impracticable. 

 Sedimentation and mechanical filtration through sand at first 

 sight offers a promising method of dealing with large quan- 

 tities of water, especially in view of its undoubted success 

 in mechanically detaining bacteria. Unfortunately the free- 

 swimming infective stage of Bilharzia actively maintains itself 

 near the surface of the water. It is not entangled in the 

 scum which successfully retains bacteria on the surface of a 

 sand-filter, for by its vigorous movements it not only quickly 

 pierces this but can migrate through the sand without in- 

 convenience. Preventive measures have therefore once more 

 to be based upon a study of the bionomics of the parasite and 

 of its intermediary host. 



After the cercaria has left its moUuscan host it does not 

 feed again until it has entered its definitive host — man. 

 During its stay in the water it is constantly moving and thus 

 rapidly exhausts its mitial energy. It has been found that 



