Schistosoma mansoni is the most widespread of the human-infecting schistosomes, present in 54 countries, predominantly in Africa, but also in Madagascar, the Arabian Peninsula, and the Neotropics. Adult-stage parasites that infect humans are also occasionally recovered from baboons, rodents, and other mammals. Larval stages of the parasite are dependent upon certain species of freshwater snails in the genus Biomphalaria, which largely determine the parasite's geographical range. How S. mansoni genetic diversity is distributed geographically and among isolates using different hosts has never been examined with DNA sequence data. Here we describe the global phylogeography of S. mansoni using more than 2500 bp of mitochondrial DNA (mtDNA) from 143 parasites collected in 53 geographically widespread localities. Considerable within-species mtDNA diversity was found, with 85 unique haplotypes grouping into five distinct lineages. Geographical separation, and not host use, appears to be the most important factor in the diversification of the parasite. East African specimens showed a remarkable amount of variation, comprising three clades and basal members of a fourth, strongly suggesting an East African origin for the parasite 0.30-0.43 million years ago, a time frame that follows the arrival of its snail host. Less but still substantial variation was found in the rest of Africa. A recent colonization of the New World is supported by finding only seven closely related New World haplotypes which have West African affinities. All Brazilian isolates have nearly identical mtDNA haplotypes, suggesting a founder effect from the establishment and spread of the parasite in this large country.

Susceptibility experiments were carried out with a Biomphalaria straminea-like planorbid snail (Biomphalaria aff. straminea, species inquirenda) from Espinillar, near Salto (Uruguay), in the area of the Salto Grande reservoir, exposed individually to 5 miracidia of Schistosoma mansoni (SJ2 and BH2 strains). Of 130 snails exposed to the SJ2 strain, originally infective to Biomphalaria tenagophila, 30 became infected (23%). The prepatent (precercarial) period ranged from 35 to 65 days. The cercarial output was irregular, following no definite pattern, varying from 138 to 76,075 per snail (daily average 4.3 to 447.5) and ending up with death. Three specimens that died, without having shed cercariae, on days 69 (2) and 80 after exposure to miracidia, had developing secondary sporocysts in their tissues, justifying the prospect of a longer precercarial period in these cases. In a control group of 120 B. tenagophila, exposed to the SJ2 strain, 40 became infected, showing an infection rate (33.3%) not significantly different from that of the Espinillar snail (chi 2 = 3.26). No cercariae were produced by any of the Espinillar snails exposed to miracidia of the BH2 strain, originally infective to Biomphalaria glabrata. Four specimens showed each a primary sporocyst in one tentacle, which disappeared between 15 and 25 days post-exposure, and two others died with immature, very slender sporocysts in their tissues on days 36 and 54. In a control group of 100 B. glabrata exposed to BH2 miracidia, 94 shed cercariae (94%) and 6 remained negative. Calculation of Frandsen's (1979a, b) TCP/100 index shows that "Espinillar Biomphalaria-SJ2 S. mansoni" is a vector-parasite "compatible" combination.(ABSTRACT TRUNCATED AT 250 WORDS)

Experiments to determine the susceptibility of Biomphalaria tenagophila from 20 localities over the species range exposed to a strain of Schistosoma mansoni showed widely varying infection rates, from 0% to 91.5%, this latter having been observed in a sample from an area where the natural infection rate of B. tenagophila has been the highest in the country. Comparison with similar studies involving B. glabrata seems to indicate that a process of adaptation between S. mansoni and B. tenagophila is evolving, the 2 organisms having reached a high degree of compatibility in a few areas. The present results point to the possibility of expansion of schistosomiasis to a wide South American area where B. tenagophila occurs.

The sperm of the blood fluke Schistosoma mansoni consist of a bulbous head 8 by 2 mum, with a rounded anterior tip and tapering posterior region, followed by a relatively short flagellum ca. 20 mum long. Electron microscopic observations revealed that these sperm are devoid of an acrosome, while a few undifferentiated mitochondria accumulate at the anterior part of the head. The nucleus appears dense, except for some electron-lucent patches. The flagellum starts at the basal body, posterior and slightly lateral to the nucleus, and the axial complex is of the 9 + 0 type. A layer of microtubules runs longitudinally, just beneath the plasmalemma, from the anterior part of the head to the initial part of the flagellum, where they overlap with the axial complex. It is suggested that this relatively rudimentary type of the S. mansoni sperm is probably related to the low activity required for fertilization.

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.
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