216 THE FLUKES 



" stones " resulting from the inflammation is sometimes resorted 

 to but in most cases this is said only to aggravate the disease. 

 Even without reinfection some of the worms may continue to 

 live and produce eggs for years, but in most long-standing cases 

 reinfection probably occurs frequently. 



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 dise^ase can be eradicated without such 

 cooperation 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 b}^ 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 pro- 

 tected 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. 



In rural agricultural districts prevention of infection is de- 

 pendent on the intermittent flow of water in irrigation canals, 

 under government control. The snails which serve as inter- 

 mediate hosts for Schistosotna arc 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 extensive 

 experimentation. Infected water to be used for washing can be 



