In field studies conducted to investigate the factors governing the infection of vertebrate hosts with Schistosoma mansoni, 205 mice were exposed for 1 hour to water in a natural pond in Sabará, State of Minas Gerais, Brazil. About 100 to 150 Australorbis glabratus with S. mansoni cercariae infection rates of 50–70% could be found per square meter of the pond. Seven weeks after exposure, the mice were killed, and the schistosomes were collected by perfusion, sexed, and counted. The percentages of mice which acquired infection, related to the time of exposure, were as follows: 26.4 (morning), 88.9 (afternoon) and 65.5 (night). The mean number of schistosomes recovered from the infected mice was rather small (3.42, 5.24, and 3.68, for animals exposed in the morning, afternoon and at night, respectively); about half of the animals harbored three or less worms. Only in three mice did the worm load exceed 20 schistosomes.
Under the conditions of the present study we had anticipated that the worm load of exposed mice would be very high. However, the results obtained strongly suggest the existence of predatory factor(s) limiting the number of cercariae and hindering the infection of mice.
Address: Instituto de Biologia, Faculdade de Filosofia, Caixa Postal 253, Belo Horizonte, Brazil.