216B THE FLUKES 



Now that the life history and modes of infection are known, 

 definite preventive measures can be taken. Prevention of con- 

 tamination of drinking water by infected urine is, of course, the 

 ideal preventive measure, but in countries where the disease is 

 most prevalent, as in Egypt, cooperation of the natives in such 

 a matter is more than can be expected. Leiper has pointed out, 

 however, that the disease can be eradicated without such coop- 

 eration by other means, depending upon local conditions. In 

 large towns and cities it is practical to destroy the free-swim- 

 ming infective stage of the worm by filtering or impounding 

 water, while in rural districts the worm must be deprived of 

 its intermediate host. Cairo, for example, obtains its water 

 supply from the Nile, part of it being unfiltered. Water used 

 for irrigation purposes above Cairo, and frequently contaminated, 

 is turned back into the river and is probably the chief source of 

 infection at Cairo, where 10,000 children are said to become in- 

 fected annually. In towns where filtering is impractical the 

 water could be rendered uninfective by impounding it in protected 

 reservoirs for 48 hours, since the cercarise die in this time. The 

 objection to this is that the water loses valuable sediment, but 

 it is doubtful whether the agricultural loss from lowered vitality 

 resulting from Schistosoma infection is not greater than is the 

 loss in fertility from impounding water. 



It seems very probable that the use of copper sulphate for 

 destroying snails in ponds or irrigation ditches in agricultural 

 districts, as outlined on p. 211, will prove to be an effective means 

 of controlling the disease. At present a preventive measure used 

 in Egypt is the intermittent flow of water in irrigation canals, 

 under government control. The snails which serve as inter- 

 mediate hosts for Schistosoma are said to die if the water in which 

 they live is dried up. It is customary for water to be turned out 

 of most irrigating canals for periods of 15 days at a time, which 

 Leiper says would be sufficient to destroy molluscs in them except 

 in puddles left by an uneven floor, which must be treated by 

 chemicals, or the floors leveled. However, in view of the remark- 

 able resistance which most snails have to drouth and to other 

 adverse conditions, this conclusion ought to be proven by esrtensive 

 experimentation. Infected water to be used for washing can be 



