TRACKING EEE VIRUS EVOLUTION, OVERWINTERING, AND SPREAD IN NORTH AMERICA BY PHYLOGENETIC ANALYSIS

Phillip Armstrong

Department of Entomology and Center for Vector Biology and Zoonotic Diseases

The Connecticut Agricultural Experiment Station

123 Huntington St.

New Haven, CT06511

phillip.armstrong@ct.gov

ABSTRACT

Eastern equine encephalitis virus (EEEV) causes periodic outbreaks of severe disease in horses and humans in eastern North America. The mechanisms responsible for the seasonal reemergence of EEEV in endemic sites are not well understood but could derive from locally overwintering virus or from annual re-introduction of new virus strains. By phylogenetic analysis of EEEV sequences, we may track the spread, local persistence, and evolution of viral variants to better understand the origin of disease outbreaks. Prior studies show that North American strains of EEEV are genetically distinct from South American strains and likely represent separate species. North American viruses sampled over a 70 year period were found to exhibit limited spatial structure by phylogenetic analyses and tend to group by year of isolation with newer strains differing most from ancestral strains. Recent studies have focused on the patterns of viral clade replacement over time in northern and southern locations. Collectively, they indicate that EEEV variants may overwinter in the northeastern US but eventually go locally extinct after a period of 1-5 years. Northeastern populations of EEEV share recent common ancestry with strains circulating in the southeastern U.S. suggesting long-range viral dispersal among these locations required for new virus introductions. These findings may help explain the periodicity of EEEV outbreaks in northern regions that tend to occur over 2-5 year time intervals.