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Tracking Eastern Equine Encephalitis Virus Perpetuation in the Northeastern United States by Phylogenetic Analysis

Epidemics and epizootics of eastern equine encephalitis virus (EEEV) occur sporadically in temperate regions where transmission is seasonal from late summer to early fall. These outbreaks may derive from virus that overwinters locally or perhaps results from reintroduction of virus from other sites. To evaluate these possibilities, we compared the phylogenetic relationships of EEEV isolates obtained from mosquitoes collected during statewide arbovirus surveillance in Connecticut, in addition to isolates from concurrent outbreaks in southern New Hampshire and upstate New York. In Connecticut, viral isolates grouped into temporally discrete clades by year of isolation or over 2 years of sampling. Two or more clades arose in 2000, 2001, 2003, 2004, and 2006, possibly the result of separate introduction events into the state, whereas viruses from upstate New York and New Hampshire segregated into single clades that persisted for 2 or more years. New Hampshire viruses shared recent common ancestry to those isolated in Connecticut suggesting viral dispersal among these regions. These results provide additional evidence for independent episodes of EEEV overwintering in northern foci.

Received March 17, 2008. Accepted for publication May 8, 2008.

Acknowledgments: The authors thank Andrew Main and Shirley Tirrell for assistance in virus isolation and identification and Christine Bean, SueAnn MacRae, and Denise Bolton for laboratory testing of EEEV in New Hampshire. We also wish to acknowledge the technical assistance of our support staff: Shannon Finan, John Shepard, Michael Thomas, Nicholanna Halladay, Bonnie Hamid, Jodie Ortega, and Amanda Rahmann.

Financial support: This work was supported in part by grants from the Centers for Disease Control and Prevention (U50/CCU116806-01-1) and the US Department of Agriculture (58-6615-1-218, CONH00768, and CONH00773).

^* Address correspondence to Philip M. Armstrong, The Connecticut Agricultural Experiment Station, 123 Huntington St., P.O. Box 1106, New Haven, CT 06504. E-mail: philip.armstrong{at}po.state.ct.us

Authors’ addresses: Philip M. Armstrong, Theodore G. Andreadis, and John F. Anderson, The Connecticut Agricultural Experiment Station, 123 Huntington St., P.O. Box 1106, New Haven, CT 06504, Tel: 203-974-8461, Fax: 203-974-8502, E-mail: philip.armstrong{at}po.state.ct.us. Jason W. Stull, Department of Health and Human services/Bureau of Disease Control, 29 Hazel Drive, Concord, NH 03301. Christopher N. Mores, Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Skip Bertman Drive, Baton Rouge, LA 70803.