[1] Ctr Dis Control & Prevent, Div Viral & Rickettsial Dis, Atlanta, GA 30333 USA
[2] Washington & Jefferson Coll, Washington, DC USA
[3] Univ Penn, Sch Vet Med, Philadelphia, PA 19104 USA
[4] Ctr Dis Control & Prevent, Div Vector Borne Infect Dis, Ft Collins, CO USA