DISTRIBUTION OF SCHISTOSOME GENETIC DIVERSITY WITHIN MOLLUSCAN INTERMEDIATE HOSTS

Naturally infected Biomphalaria glabrata snails were collected at two sites near Belo Horizonte, Brazil, and Schistosoma mansoni cercariae i solated from single snails were used to infect individual mice. Geneti c comparison of single worm DNA was accomplished by hybridization of S outhern blots to a polymorphic repetitive DNA element. Genetic profile s of parasite individuals revealed a diverse array of parasite genotyp es 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 m ore likely to carry multiple infections than expected by chance. This degree of overdispersion combined with high levels of genetic variabil ity facilitates multi-genotype transmission and helps maintain parasit e genetic diversity.