A commercially viable recombinant subunit vaccine to prevent West Nile virus infection is the overall objective of this project. West Nile virus was isolated for the first time in North America in 1999. In less than three years, the geographical distribution of the virus expanded dramatically in the United States. A vaccine will be an important complement to other public health measures as this emerging pathogen becomes endemic in this continent. Phase I research studied immunogens based on the envelope protein of a U.S. West Nile virus isolate. We prepared recombinant protein and synthetic peptide antigens, and initiated immunization studies. Mice were vaccinated with the recombinant antigens, antibody responses were evaluated, and protective immunity was characterized. Phase II experiments will vaccinate horses with a promising candidate vaccine, and demonstrate that vaccination elicits protective antibodies. Experiments are designed to elicit high titer antibodies using our recombinant envelope protein antigen with aluminum hydroxide adjuvant. Subsequent experiments will determine if antibodies induced in horses can: (1) neutralize West Nile virus in vitro; (2) protect passively immunized mice from West Nile virus. This project will provide veterinarians, clinicians and public health officials with new means to manage West Nile virus outbreaks.