Interspecific antagonism and virulence in hosts exposed to two parasite species.

Co-infection of host organisms by multiple parasite species has evolutionary consequences for all participants in the symbiosis. In this study, we co-exposed aquatic-snails (Biomphalaria glabrata) to two of their trematode parasites, Schistosoma mansoni and Echinostoma caproni. In co-exposed snails, E. caproni prevalence was 63% compared to only 23% for S. mansoni. Co-exposed E. caproni-infected snails exhibited reduced fecundity, higher mortality, and higher parasite reproduction (higher virulence) compared to hosts exposed to echinostomes alone. Conversely, co-exposed S. mansoni-infected snails released fewer parasites and produced greater numbers of eggs compared to hosts exposed to S. mansoni alone. These results suggest that co-exposure not only influences the establishment (presence or absence) of particular parasite species, but also impacts host life history, parasite reproduction, and the virulence of the interaction.

Multi-species interactions among a commensal (Chaetogaster limnaei limnaei), a parasite (Schistosoma mansoni), and an aquatic snail host (Biomphalaria glabrata).

This study assessed the effects of a commensal, Chaetogaster limnaei limnaei, and a parasitic trematode, Schistosoma mansoni, on infection patterns and life-history responses in the aquatic snail Biomphalaria glabrata. Prevalence of infection was significantly higher in snails that were devoid of C. limnaei limnaei relative to those that were colonized by the commensal, indicating that the oligochaete may protect the host from trematode infection. This finding appeared to be the direct result of the commensal as opposed to indirect stimulation of the immune system, as hemocyte numbers did not differ between C. limnaei limnaei-colonized and noncolonized snails. Snail growth and reproduction were affected by the presence of C. limnaei limnaei and exposure to S. mansoni. Two-way ANOVA revealed a significant effect of both C. limnaei limnaei presence and trematode exposure on B. glabrata growth over the 5-wk study with C. limnaei limnaei-colonized and parasite-infected snails demonstrating the greatest growth. Snails exposed, but uninfected, by S. mansoni demonstrated the lowest growth regardless of commensal colonization. Chaetogaster limnaei limnaei colonization had no effect on egg production, but S. mansoni-infected snails produced significantly more eggs than individuals from other treatment groups. Survival remained over 85% in all treatment groups. The ecological implications of these results are discussed.