Antagonism between parasites within snail hosts impacts the transmission of human schistosomiasis.

Abstract

Human disease agents exist within complex environments that have underappreciated effects on transmission, especially for parasites with multi-host life cycles. We examined the impact of multiple host and parasite species on transmission of the human parasite <i>Schistosoma mansoni</i> in Kenya. We show <i>S. mansoni</i> is impacted by cattle and wild vertebrates because of their role in supporting trematode parasites, the larvae of which have antagonistic interactions with <i>S. mansoni</i> in their shared <i>Biomphalaria</i> vector snails. We discovered the abundant cattle trematode, <i>Calicophoron sukari,</i> fails to develop in <i>Biomphalaria pfeifferi</i> unless <i>S. mansoni</i> larvae are present in the same snail. Further development of <i>S. mansoni</i> is subsequently prevented by <i>C. sukari</i>'s presence. Modeling indicated that removal of <i>C. sukari</i> would increase <i>S. mansoni</i>-infected snails by two-fold. Predictable exploitation of aquatic habitats by humans and their cattle enable <i>C. sukari</i> to exploit <i>S. mansoni</i>, thereby limiting transmission of this human pathogen.