Recombinant canarypoxvirus vaccine carrying the prM/E genes of West Nile virus protects horses against a West Nile virus-mosquito challenge.

An ALVAC (canarypoxvirus)-based recombinant (vCP2017) expressing the prM and E genes derived from a 1999 New York isolate of West Nile virus (WNV) was constructed and assessed for its protective efficacy in horses in two different experiments. In the first trial, a dose titration study was conducted to evaluate both serum neutralising antibody responses to WNV and duration of immunity. In the second trial the onset of protection was determined. Twenty-eight adult horses received two doses of vCP2017 administered intramuscularly at 5-week intervals and sixteen horses comprised age-matched non-vaccinated controls. Individual sera were taken periodically and tested for neutralising antibodies against WNV. Horses were challenged by allowing WNV-infected Aedes albopictus mosquitoes to feed on them two weeks (second trial) or one year (first trial) after the second vaccination. After challenge, horses were monitored for clinical signs of disease, and blood samples were collected for detection of WNV viremia and antibody. In both trials, all vaccinated horses developed neutralising antibodies against WNV. None of the vaccinated or control horses developed clinical signs of WNV disease upon challenge. None of the nine horses challenged 2 weeks after primary vaccination and only one of the ten vaccinated horses challenged 1 year after vaccination developed detectable viremia after challenge, whereas more than 80% of the controls became infected. Results from these studies demonstrated that a primary course of two doses of vCP2017 provides both antibody response and an early immunity in horses against WNV viremia.