Revealing the mechanistic determinants of host specificity: using the parasite Schistosoma mansoni and the snail Biomphalaria obstructa as a model system

The majority of parasite species exhibit host specificity. In general, our knowledge of why a particular parasite can infect some host species but fails to infect others is poor. To address this basic question, I will study the parasite Schistosoma mansoni which infects 200 million people in both Africa and the Neotropics. Its larval development occurs in certain species of snails of the genus Biomphalaria, but not in others:  the Neotropical snail B. glabrata supports S. mansoni development whereas the closely-related North American species B. obstructa, does not. I will undertake a series of laboratory experiments to explore the underlying reasons for why B. obstructa is unable to serve as a host. To do this, I will manipulate snails to disable their protective hemocyte populations, as well as expose snails to other types of immunosuppressive parasites such as echinostomes and nematomorphs. Using RNA interference I will isolate and disable specific snail genes known to be involved in defense to see if this renders snails susceptible to infection. I will also take into consideration environmental conditions that could influence the snail's susceptibility, such as, temperature, pH, and levels of pollution in the water.  These experiments will help define, for a medically important parasite, the basic factors that dictate host specificity and that prevent a North American snail from hosting this human parasite.