Resumo ARTIGO Abstract A malacological survey to detect foci of transmission of schistosomiasis and other parasitic diseases was undertaken into water-courses from 13 municipalities of microregion of Belo Horizonte, MG, Brazil. From 1990to 1996 were found infected by trematodes: Biomphalaria glabrata (7,920), infected by Schistosoma mansoni (1.9%), Echinostomatidae (1.2%), Strigeidae (0.6%), C e r c a ria minense (0.1%) and Dero g e n i d a e ( -0.1%); B. s t ra m i n e a (4,093) infected by Strigeidae (0.6%), Echinostomatidae (0.2%), B. tenagophila (1,338), infected by Strigeidae (0.1%) and P hysa marm o ra t a

The ratios of male to female worms of Schistosoma mansoni were determined in mice infected with cercariae from LE, SJ and AL strains shed by mollusc hosts of the parasite in Brazil. The sex ratios of worms in the animals were similar with cercariae from Biomphialaria glabrata and B. tenagophila varying from 1.1:1 to 1.6:1 with LE and AL strains and 1:1.1 with SJ. In the animals infected with cercariae from B. straminea the ratio of male to female worms was similar to those obtained using cercariae shed from the other two species of molluscs, 1.5:1 with LE strain. Inoculations by AL and SJ cercariae resulted in sex ratios of 3.1:1 and 6:1 respectively. The normal sex ratios of worms established in Brazil in animals inoculated with cercariae from B. glabrata and B. tenagophila is from 1:1 to 2:1. The higher number of male worms that developed from cercariae of the AL and SJ strains obtained from B. straminea indicate a lower compatibility of the snail concerning these strains of S. mansoni.

In these experiments the ratio of male to female S. mansoni larvae in B. glabrata from Belo Horizonte and Ribeirão das Neves Minas Gerais, Brazil, either reared in laboratory or collected in the field, varied from 1:1 to 1:1.3 or 1.4:1. Cercariae of LE strain of Schistosoma mansoni, shed by 39 snails maintained at 25 +/- 0.5 degrees C were used to infect mice on a weekly basis. Subsequent perfusion resulted in 76.6% male and 23.4% female worms. The cercariae produced by 32 infected snails maintained at 27 +/- 0.5 degrees C were inoculated into mice and produced 43.4% male and 56.6% female worms (p < 0.05). Cercariae eliminated by snails collected in Barreiro and Ressaca, Belo Horizonte, during hot months, produced 45.7 to 47.7% male and 52.3 to 54.3% female worms. A lower number of cercariae shed by snails collected in Gorduras, Belo Horizonte, at 20 +/- 3.0 degrees C, produced 51.6% male and 48.4% female worms. Thus, in this region the infection of vertebrate hosts with S. mansoni cercariae would be more severe in the summer due to the higher level of parasites and the number of eggs.