When we only had one vaccine for West Nile virus (WNV) in horses, vaccination was quite simple. Then when different types of vaccines came along, the waters got a little muddier, especially for pregnant mares in which breeders feared risking not one, but two equine lives with new vaccine technologies.

At the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev., a study discussed the efficacy of using a chimera West Nile virus vaccine to booster immunity in mares previously immunized against West Nile virus. The chimera vaccine used in the study, Prevenile, is made by replacing structural genes of the human yellow fever vaccine virus with those of the West Nile virus. The immune system acts against the West Nile virus proteins just like in a wild-type infection, but the chimera can safely replicate in the horse, producing a strong, protective immune response.

"When reviewing the literature, there was no clear evidence that this type of vaccine would be efficacious to booster pregnant mares (although in previous vaccine licensure studies, no adverse events were reported in pregnant mares)," said presenter Joseph Manning, DVM, of Equine Sports Medicine in Weatherford, Texas.

Lead author Cynthia V