Structuring of schistosomes and snails: Genetic insights and epidemiological consequences

Elizabeth A Thiele, Purdue University

Abstract

Schistosoma mansoni is the digenean trematode responsible for chronic schistosomiasis in South America and the Caribbean and is one of several species of schistosomes that infect an estimated 200 million people across 74 countries worldwide. Lack of a vaccine, inconsistent sociopolitical will to maintain sanitation and control measures, and anthropogenic ecological disturbances that expand habitat range of the aquatic snail intermediate host (Biomphalaria glabrata in the Americas) maintain a continuous threat of infection and resurgence or introduction of disease transmission in tropical areas. Elucidating host and parasite genetic distribution in both space and time is important for understanding and gaining predictive power over disease epidemiology, particularly as it relates to influences of habitat on parasite transmission at a local scale. Moreover, understanding host-parasite distributions is necessary for assessing the coevolutionary potential of host-parasite interactions. Using Brazilian populations of S. mansoni and B. glabrata, I have assessed spatiotemporal patterns of parasite and snail genetic variation at the level of individual villages in order to better understand the interactions of environment and host movement on parasite transmission, as well as to further our knowledge of natural levels of host and parasite variation at local scales. Assessments of neutral genetic variation within and among both snail host populations and parasite populations collected from infected humans indicate that human definitive hosts are the primary drivers of parasite gene flow within these Brazilian villages. Furthermore, relative to their snail intermediate hosts, parasites appear more genetically variable and "well-traveled", setting the stage for potential local coadaptation between schistosomes and snails. However, all villages are not created equal and these findings provide preliminary indications that patterns of parasite transmission can differ between villages in a manner that may be linked to environmental features, such as spatial availability and connectivity of water. Moreover, it is increasingly recognized that parasites are overdispersed within their snail hosts and that interactions between co-infecting parasites can influence infection outcomes and, thus, transmission dynamics. To facilitate experimental investigations of intraspecific interactions between co-infecting schistosomes, I have tested and applied a molecular method allowing comparisons of the relative transmission potential of competing larval parasite strains. Overall, this work has demonstrated the need for further local-level population genetics investigations that apply more explicit tests of the influence of both social and ecological habitat on macroparasite transmission, as well as provided a new tool for empirical assessments of interactions and coevolutionary consequences of coinfection.

Degree

Ph.D.

Advisors

Minchella, Purdue University.

Subject Area

Ecology|Genetics|Epidemiology

COinS