Experimental studies of the ecology and evolution of eastern equine encephalitis virus and implications for its emergence and classification
Eastern equine encephalitis virus strains from North (NA EEEV) and Central/South America (SA EEEV) have developed markedly different epidemiologic, pathogenic, antigenic, and genetic profiles, have distinct geographic distributions, and potentially occupy unique vector and vertebrate ecological niches. The goal of my research was to clarify the extent to which these viruses have diverged by further understanding their evolutionary history and adaptation to different ecological niches, and the impact that this divergence has had on their ability to emerge in reciprocal environments. My studies were designed to examine each of the three main aspects of the arboviral transmission cycle: the virus, the vertebrate host, and the mosquito vector. To investigate the evolutionary history and genetic divergence of NA and SA EEEV, I conducted a phylogenetic and Bayesian coalescent analysis of the structural polyprotein genomic region (26S) of all available SA EEEV, and additional NA EEEV, isolates spanning a broad geographic and temporal spectrum. In accordance with support provided by the evolutionary and phylogenetic analyses, I sought to apply a more direct and experimental approach to explore the adaptation of NA and SA EEEV to the use of different vertebrate host species. Wild cotton rats (Sigmodon hispidus) and house sparrows (Passer domesticus) were collected in Galveston and Houston, Texas, respectively, and evaluated for their potential to serve as amplification and/or reservoir hosts for NA and SA EEEV. Juvenile cotton rats experienced complete mortality with both NA and SA EEEV and provided me with a unique opportunity to compare the pathology resulting from NA and SA EEEV infection a wild vertebrate species. In order to better understand the directionality of NA EEEV divergence and adaptation and to further clarify the vector ecology of SA EEEV, I evaluated the relative susceptibilities of the NA enzootic vector, Culiseta melanura, and the presumed enzootic vector for SA EEEV, Culex taeniopus, and the probable epizootic EEEV mosquito vectors, Aedes (Ochlerotatus) taeniorhynchus and Ae. (Och.) sollicians, to sympatric and allopatric EEEV strains. Taken together, the results of my dissertation research emphasize the striking extent of evolutionary divergence between NA and SA EEEV and provide a greater understanding of the directionality of NA EEEV adaptation to North America subsequent to its divergence from an ancestral EEEV in Central/South America. My research has also clarified the vector and vertebrate usage of both NA and SA EEEV, providing support for the use of mammalian vertebrate host species by SA EEEV and highlighting its emergence potential in a novel North American environment.