Changing pattern of schistosomiasis in Egypt 1935--79.

A village in the Nile surveyed for schistosomiasis by J. A. Scott in 1935 was surveyed again in 1979. The same number of people as in the 1935 survey were randomly selected for investigation by the same parasitological techniques as those used by Scott. The prevalence of Schistosoma mansoni infection had increased from 3.2% to 73%, whereas S. haematobium infection, which had been very common in 1935 (74%), had almost disappeared (2.2%). In the local district hospital since 1972 the percentage of urine specimens found to contain S. haematobium ova has dropped from 30 to 9%, while the percentage of stool specimens containing S. mansoni ova has increased from 2 to 22%. In the local irrigation canals snail intermediate hosts for S. mansoni have outnumbered those for S. haematobium by a factor of 5--40 in the past 7 years. Changes in the proportions of snail vectors appear to be related to construction of the Aswan High Dam and to changes in the water-flow patterns of the Nile. The change in the relative frequencies of the two infections had important public-health implications, since the hepatosplenic schistosomiasis caused by S. mansoni is more difficult to treat and is associated with more morbidity and mortality than the urinary schistosomiasis caused by S. haematobium.