To elucidate changes relative to compatibility with intermediate and definitive hosts affecting Schistosoma mansoni since it was introduced to the New World, the compatibility of S. mansoni from Africa (the Cameroons), from the Caribbean (Guadeloupe), and those resulting from experimental crosses with the gastropods Biomphalaria glabrata and B. pfeifferi has been studied. Results show that S. mansoni, regardless of its origin or its usual snail host, always infects B. pfeifferi more successfully than B. glabrata. The success rate with B. pfeifferi is 100% with 5 miracidia of S. mansoni from Guadeloupe and 97% with 5 miracidia from the Cameroons. On the other hand, in B. glabrata infection rate was 54% with 5 miracidia from Guadeloupe and 0% with 5 miracidia from the Cameroons (a rate of 19% is reached when using 10 miracidia). Hybrid miracidia infect B. pfeifferi and B. glabrata with a success rate of 60 and 86%, respectively, which are intermediate between those of the parent strains. Studies of S. mansoni development in Rattus rattus show that there is better infectivity and survival for the Caribbean strain than the Cameroon strain: the percentage worm recovery 4 weeks after exposure is 34% for S. mansoni from Guadeloupe, 14% for S. mansoni from the Cameroons, and 31% for the hybrids. The mortality rate beween 4 and 12 weeks after exposure is 51% for S. mansoni from Guadeloupe, 87% for S. mansoni from the Cameroons, and 31% for the hybrids. Twelve weeks after exposure, eggs of S. mansoni from Guadeloupe and those of the hybrids are more numerous than the eggs of S. mansoni from the Cameroons in the feces and all the organs of R. rattus. Results obtained with schistosomes from crosses provide genetic evidence of the differences observed between the parent strains. Thus, the environmental conditions and the relative isolation of the “American” schistosomes have not affected the parasite's affinity for its original intermediate host, B. pfeifferi, have led to better adaptation to the autochtonous host, B. glabrata, and have resulted in a greater degree of compatibility with R. rattus.
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