Scientists recently showed that the Fort Dodge Animal Health West Nile virus (WNV) DNA vaccine, which is licensed but not yet commercially available, should protect foals from the virus regardless of residual antibodies received at birth from their dams that often interfere with vaccine effectiveness (maternally derived antibodies, or MDA). Prior studies showed protection in yearlings and older horses. Yu-Wei Chiang, PhD, of Fort Dodge, presented results of immunity studies on Dec. 5, 2005, at the American Association of Equine Practitioners Convention in Seattle, Wash.

Chiang said the vaccine at its preliminary formulation was 83.3% effective in a challenge study of 30 seronegative foals (they had no MDA against WNV in their blood prior to two vaccine doses) that were eight to nine months old. “Eighty percent of the controls developed viremia (presence of virus in the blood) and 13% of the vaccinates showed viremia,” he said. None of the horses showed clinical signs.

The vaccine increased the foals’ ability to neutralize the virus and increased WNV-specific IgG antibody levels. These levels meant the vaccinated foals were able to produce an immune response that, according to Chiang, should prevent clinical WNV disease.

In the second study, Chiang challenged 29 foals (15 vaccinated foals and 14 unvaccinated controls, all three to 13 weeks old) using the same study design, but a majority of the foals in these groups had MDA. None of the challenged foals became sick