Observations on two biological races of Schistosoma mansoni

Abstract

A comparative study of the BH strain of Schistosoma mansoni from Belo Horizonte, Minas Gerais state, infective to Biomphalaria glabrata from the same locality, and the SJ strain from São José dos Campos, São Paulo state, infective to B. tenagophila from the latter locality, showed the following differences: 1. Length of adult worms and size of eggs significantly larger in the BH strain. 2. Higher infection rates in the B. glabrata-BH strain association than in the B. tenagophila-SJ strain association, following exposure of each snail to 1 or 10 miracidia. 3.Longer prepatent period (from penetration of miracidium to first shedding of cercariae) in the B. tenagophila-SJ strain association. 4. Infection of both Biomphalaria species when exposed to hybrid miracidia from crosses between the two strains, at lower levels than those resulting from exposure of each snail species to miracidia of the pure sympatric strain. (Both Biomphalaria populations are practically refractory to infection with the allopatric strain). These results are interpreted as pointing to a better host-parasite adjustment in the B. glabrata-BH strain association than in the B. tenagophila-SJ association. The interfertility between the two strains, which produced viable hybrids infective to both Biomphalaria species, supports the conclusion that the observed differences are merely intraspecific, and that the two strains may be considered distinct biological races of Schistosoma mansoni.<br>O estudo comparativo da cepa BH de Schistosoma mansoni, oriunda de Belo Horizonte, Minas Gerais, e infectante para Biomphalaria glabrata da mesma localidade, e da cepa SJ, oriunda de São José dos Campos, São Paulo, infectante para B. tenagophila desta localidade, revelou as seguintes diferenças entre elas: 1. Comprimento dos vermes adultos e tamanho dos ovos significamente maiores na cepa BH. 2.Índices de infecção mais altos na associação B. glabrata-cepa BH do que na associação B. tenagophila-cepa SJ, após exposição de cada molusco tanto a 1 quanto a 10 miracídios. 3. Período pré-patente (da penetração do miracídio à liberação das primeiras cercárias) mais longo na associação B. tenagophila-cepa SJ. 4. Infecção de ambas as espécies de Biomphalaria quando expostas a miracídios híbridos oriundos do cruzamento entre as duas cepas, em níveis mais baixos que aqueles resultantes da exposição de cada espécie a miracídios da cepa simpátrica pura. (As duas populações de Biomphalaria são Praticamente refratárias á infecção com a cepa alopátrica). Esses resultados são interpretados como indicativos de melhor ajustamento hospedeiro-parasito na sssociação B. glabrata-cepa BH do que na associação B. tenagophila-cepa SJ. A interfertilidade das duas cepas, que produziram híbridos viáveis infectantes para as duas espécies de Biomphalaria, permite concluir que as diferenças observadas são meramente intraespecíficas, e que as duas cepas podem ser consideradas como distintas raças biológicas do S. mansoni

Schistosoma mansoni Sambon, 1907: morphometric differences between adult worms from sympatric rodent and human isolates

“…Such observations are in agreement with preceeding morphological and biological studies providing the existence of intraspecific variations (strains) (Saoud 1965, Magalhães & Carvalho 1973, Frandsen 1979, Paraense & Corrêa 1981. More recently, we have showed that adult worm length is the only difference between sympatric isolates from rodent and human (Machado-Silva et al 1994).…”

“…Biological studies on the compatibility between S. mansoni strains and their intermediate host, indicate that in the most compatible relationship the time required for the development in the snail host is shorter (Frandsen 1979, Paraense & Corrêa 1981. Besides, cercariae produced have a greater capacity of infection (Zanotti-Magalhães et al 1991).…”

“…BH (Belo Horizonte, MG) strain of S. mansoni has adult male worms bigger than SJ (São José dos Campos, SP) strain and a better relationship with the snail host (Paraense & Corrêa 1981). Some articles demonstrate this same aspect in the compatibility between a rodent strain and a invertebrate host (Bastos et al 1979).…”

Biological implications of the phenotypic plasticity in the Schistosoma mansoni - Nectomys squamipes model

“…It is worth emphasizing that all strains compared favorable with experiments using mice models. Adult worms attain "normal" dimensions 16,27,29 and supernumerary testes lobes are found 14,17,21,27 . The biological characteristics were not affected, because the pre-patent period, tissue egg distribution and egg laying reproduced observations in mice 15,21,28 .…”

The schistosome vectors in the Americas

“…Finally, additional experiments (Paraense & Corrêa 1981) led to the conclusions that the SJ schistosome is less adapted to its vector snail than the BH parasite to its own; that they are interfertile, producing hybrids infective to BH and SJ snails at lower rates than in the sympatric combinations; and that they are to be considered biological races rather than mere strains. Now hybridization of the two races is surely taking place in nature as a result of human migration.…”