Production of Schistosoma haematobium and Schistosoma mansoni cercariae in Tanzania

Abstract

Few Schistosoma haematobium cercariae emerged in darkness, but large numbers did emerge when snails were illuminated after a period in darkness, and the pattern of output was modified in response to alterations in the cycle of illumination. A rise in temperature from 23 °–30 °C stimulated the cercariae to emerge, but the effect of heat was subsidiary to that of illumination. In the laboratory the patterns of output of Schistosoma mansoni and S. haematobium cercariae were constant but differed from one another. The proportions of the day's output of S. mansoni cercariae shed in successive hours were 16, 33, 29, 15, and 5%, and only 5% was produced after the fifth hour. The mean daily output of S. mansoni from single experimentally infected Biomphalaria sudanica varied between 20 and 1,361; 58% of 171 counts were below 501 cercariae, 80% less than 1,001, and 90% less than 1,501. The proportions of the day's total output of S. haematobium cercariae shed in successive hours were 6, 11, 15, 32, 19, and 11%, and 7% was shed in the following 4 hours. The mean daily output of individual naturally infected Bulinus nasutus varied between 14 and 4,119; 34% of 205 daily counts was less than 501 cercariae, 55% less than 1,001, 76% less than 1,501, and 91% less than 2,001. Output from individual snails often varied from day to day by 200–500 cercariae. Bulinus nasutus remained infected for up to 9 weeks; B. sudanica for a maximum of four weeks.