Single-Dose WNV Booster Effective for Pregnant Mares (AAEP 2009)

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. Gutierrez, DVM, equine technical services veterinarian for Intervet/Schering-Plough Animal Health, reported the study had three objectives:

  • To see if pregnant mares with low antibody levels (titers) had at least a four-fold increase in these levels after boostering;
  • To see if high-titer mares had antivector immunity to this modified-live vaccine (to see if circulating antibodies inactivated the vaccine); and
  • To confirm that this vaccine can be used as a pre-partum vaccine to induce colostral immunity.

The researchers boostered 81 previously vaccinated mares four to eight weeks before foaling with one dose of the chimera vaccine, according to the label instructions. After 14 days the mares all showed a four- to eight-fold increase in antibody titers, thus the vaccine was not inactivated and the boosted immunity would also increase colostral immunity (immunoglobulins in the mare's first milk or colostrum, which will help protect the foal against the disease).

No adverse events were seen as a result of vaccination, and all mares delivered live foals.

"The take-home message is that vaccinating primed pre-partum broodmares with a single dose of a West Nile virus chimeric vaccine does produce a titer rise (increased immune response) and can be used to boost colostral antibodies," the authors summarized.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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