RESUMO
The digenean trematode Schistosoma mansoni is responsible for chronic schistosomiasis worldwide, and in Brazil alone an estimated 35 million people are at risk. To evaluate epidemiological patterns among human definitive hosts, we assessed genetic diversity and population subdivision of S. mansoni infrapopulations in human hosts from the highly endemic village of Virgem das Graças in the state of Minas Gerais, Brazil. We believe this is the largest such survey to date. Genetic diversity of parasites, measured over eight polymorphic microsatellite loci, was relatively high and standard measures of inbreeding indicated that the population was panmictic. Furthermore, there was no significant isolation-by-distance of parasite infrapopulations, and measures of population subdivision indicated significant but low to moderate levels of population differentiation. We conclude that patients within this village sample from a broad range of schistosome genetic diversity and effectively act as "genetic mixing bowls" for the parasites. These results contrast with those previously observed in the Brazilian village of Melqui ades and thus provide the opportunity for comparisons of environmental and epidemiological differences that are likely to influence host-parasite coevolution and parasite transmission.
Assuntos
DNA de Helmintos/análise , Variação Genética , Interações Hospedeiro-Parasita , Polimorfismo Genético , Schistosoma mansoni/genética , Animais , Biomphalaria/parasitologia , Brasil , Humanos , Estágios do Ciclo de Vida , Camundongos , Camundongos Endogâmicos BALB C , Contagem de Ovos de Parasitas , Técnica de Amplificação ao Acaso de DNA Polimórfico , Esquistossomose mansoni/parasitologia , Esquistossomose mansoni/transmissãoRESUMO
Biomphalaria glabrata snails infected with Schistosoma mansoni were collected during consecutive seasons from a site in Brazil known to have a very high percentage of infected snails. Schistosoma mansoni cercariae from single snails were used to infect individual mice, and the recovered adult worms were genetically assessed using a mtVNTR marker. The number of unique parasite genotypes found per snail was compared to expected abundance values, based on the infection prevalence at the site, to determine the distribution of S. mansoni infections within the snail population. The observed distributions and those from previous studies were used to examine the relationship between schistosome prevalence and aggregation across a wide range of prevalence values. Our analysis showed that prevalence was inversely related to the degree of parasite overdispersion, and at high prevalence, S. mansoni infections were randomly distributed among snails.
Assuntos
Biomphalaria/parasitologia , Schistosoma mansoni/isolamento & purificação , Esquistossomose mansoni/epidemiologia , Animais , Brasil/epidemiologia , Impressões Digitais de DNA , DNA Mitocondrial/química , DNA Mitocondrial/genética , Feminino , Masculino , Camundongos , Repetições de Microssatélites/genética , Prevalência , Schistosoma mansoni/genética , Estações do AnoRESUMO
A total of 256 sites in 11 habitats were surveyed for Biomphalaria in Melquiades rural area (State of Minas Gerais) in August and November 1999 and in March 2000. Of the 1,780 Biomphalaria collected, 1,721 (96.7%) were B. glabrata and 59 (3.3%) B. straminea. Snails were found in all habitats except in wells, with the largest mean numbers in tanks, seepage ponds and canals, and the smallest numbers in springs, rice fields and fishponds. People's knowledge of the occurrence of Biomphalaria at the collection sites and the presence of Biomphalaria ova were strongly correlated with the occurrence of snails, and distance between houses and collection sites, as well as water velocity were inversely correlated with Biomphalaria occurrence (p < 0.001). The strongest predictor o f Biomphalaria occurrence was the presence of tilapia fish in fishponds. Fourteen Biomphalaria (0.8% of all snails) found at 6 sites were infected with Schistosoma mansoni. Suggestions are made for the utilization of local people's knowledge in snail surveys and further studies are recommended on the possible use of tilapia for biological control of Biomphalaria in fishponds, as well as modeling of S. mansoni transmission and reinfection.
Assuntos
Biomphalaria , Meio Ambiente , Água , Animais , Brasil/epidemiologia , Vetores de Doenças , Humanos , Controle Biológico de Vetores/métodos , Densidade Demográfica , Comportamento Predatório , Saúde da População Rural , Esquistossomose mansoni/epidemiologia , Esquistossomose mansoni/transmissão , Tilápia/parasitologiaRESUMO
Naturally infected Biomphalaria glabrata snails were collected at two sites near Belo Horizonte, Brazil, and Schistosoma mansoni cercariae isolated from single snails were used to infect individual mice. Genetic comparison of single worm DNA was accomplished by hybridization of Southern blots to a polymorphic repetitive DNA element. Genetic profiles of parasite individuals revealed a diverse array of parasite genotypes in naturally infected intermediate hosts. The observed distribution of schistosome genotypes among intermediate hosts indicates that over half of the infected snails harbour multiple miracidia. Snails were more likely to carry multiple infections than expected by chance. This degree of overdispersion combined with high levels of genetic variability facilitates multi-genotype transmission and helps maintain parasite genetic diversity.