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A blocking ELISA targeting an immunodominant West Nile epitope on the West Nile Virus NSI protein was assessed for the detection of West Nile-specific antibodies in blood samples collected from 584 sentinel chickens and 238 wild birds collected in New Jersey from May-December 2000. Ten mallard ducks (Anus platyrirynchos) experimentally infected with West Nile virus and six uninfected controls were also tested. The ELISA proved specific in detecting WNV antibodies in 9/10 chickens and 414 wild birds previously confirmed as positive by Plaque Reduction Neutralization test (PRNT) at the Center for Disease Control, Division of Vector Borne Diseases, Fort Collins, CO, USA (CDC). Nine out of the ten experimentally infected mallard ducks also tested positive for WN antibodies in the blocking ELISA, while 616 uninfected controls did not. Additionally, 1705 wild birds, collected in New Jersey from December 2000-November 2001 and Long Island, New York between November 1999 and August 2001 were also tested for WN antibodies by the blocking ELISA. These tests identified 30 positive specimens, 12 of which had formalin-fixed tissues available to allow detection of WN specific viral antigen in various tissues by WNV-specific immunohistochemistry. Our results indicate that rapid and specific detection of antibodies to WN virus in sera from a range of avian species by blocking ELISA is an effective strategy for WN Virus surveillance in avian hosts. In combination with detection of WN-specific antigens in tissues by immunohistochemistry (IHC) the blocking ELISA will also be useful for confirming WN infection in diseased birds. Key Words: West Nile Virus-Blocking ELISA-Immunocytochemistry-Wild birds-Arbovirus-Serum-blood spot. Vector-Borne Zoonotic Dis. 3, 99-110.