Transmission dynamics of two strains of Schistosoma mansoni utilizing novel intermediate and definitive hosts

Parasites with complex life cycles must adapt to the genetic and phenotypic variability of their host. This study examines the interactions of a parasite with a homozygous genetic background when it encounters local and novel hosts. Both inbred and randomly outcrossed breeding strains of S. mansoni were challenged with novel hosts to determine whether a lack of genetic variability constrains parasite infectivity in non-local hosts. The B. glabrata snails used as intermediate hosts consisted of a novel line recently derived from Brazilian field-collected snails and a local line that has been maintained in the laboratory for the last 20 years. Progeny from each snail line were exposed to S. mansoni larvae from either parasite strain and were monitored over a 10 week period for differences in both snail and parasite life-history parameters. Among the life history traits, snails infected with the inbred parasite strain demonstrated a greater growth rate than snails infected with the outcrossed strain. Outcrossed parasites were observed to induce a greater host mortality rate than the inbred strain. As predicted there was a significant difference in parasite prevalence among the treatments, as local snails exposed to inbred parasites infected a greater percentage of hosts. Parasite reproduction among the groups varied as well, with outcrossed parasites demonstrating higher levels of cercariae production than inbred parasite strains. The parasite strain with higher levels of genetic variability was able to exploit its snail hosts more efficiently than the inbred strain. Local (Balb/c) and novel (C57) definitive hosts were exposed to cercariae progeny from each parasite-snail treatment. Outcrossed parasites had a greater infectivity in the definitive hosts. Parasite infectivity in mice was observed to be greater in parasites that were passaged through local snail hosts, compared to parasites passaged through novel snail hosts suggesting the strong influence of intermediate hosts on parasite fitness in definitive hosts. In summary, increased homozygosity within a strain of S. mansoni altered the transmission dynamics of the parasite within both of its obligatory hosts, and intermediate host genetic background impacted the fitness of the parasite in subsequent definitive hosts