The pathogenesis of three different Schistosoma mansoni strains from the Brazilian states of Minas Gerais (BH strain) and S？o Paulo (SJ and SD strains) was evaluated in experimentally infected mice. Observations of the most severe clinical cases among local patients treated (SD strain) in the city of Campinas (S？o Paulo, Brazil) formed the basis of this study. Mice were used as definitive hosts and were infected with cercariae from Biomphalaria tenagophila (SJ and SD strains) and Biomphalaria glabrata (BH strains). The parameters analyzed were as follows: number of S. mansoni eggs in mice feces; number of granulomas per tissue area in liver, spleen, lungs, pancreas, and ascending colon; measurements of hepatic and intestinal granulomas; number of adult worms; and measurements of trematode eggs. The comparison among the three strains indicated that the SD strain, isolated in Campinas, presented a higher worm recovery relative to the number of penetrating cercariae. In addition, when compared to the SJ and BH strains, the SD strain demonstrated similar pathogenicity to the BH strain, with a greater quantity of granulomas in the viscera, as well as larger granulomas and eggs. Furthermore, a greater quantity of trematode eggs was also shed in the feces. 1. Introduction After malaria, schistosomiasis is the most important global parasitic disease and is a serious public health problem mainly in tropical and subtropical regions. In Brazil, the spread of schistosomiasis is caused by the single species occurring in the country, Schistosoma mansoni. It has been shown that the spread of S. mansoni is directly related to the economic history of the country via human migratory events from hyperendemic regions and the presence of three species of snails, which are the natural intermediate hosts of the parasite. Schistosomiasis, initially described as a typically rural disease, now has an urban character and is prevalent mainly on the outskirts of large cities in S？o Paulo (Brazil), the most populous state in Brazil. Another feature of the disease in S？o Paulo state is the absence of severe cases [1], which is favorable in one sense; however, due to the absence of clinical manifestations that bring patients to the public health clinics, schistosomiasis often remains undiagnosed, and the infected individuals may continue to contaminate fresh water sources where the intermediate hosts breed. In 1963 [2], the BH and SJ strains of Schistosoma mansoni were initially described based on the observation that Biomphalaria glabrata from Belo Horizonte (in the state of Minas
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