West Nile Who and When to vaccinate

Protection against West Nile induced encephalomyelitis requires attention to insect control and an effective vaccination program. There are four USDA licensed vaccines for horses. Two inactivated vaccines, a nonreplicating canary pox-vectored vaccine and an inactivated flavivirus chimeric vaccine.

The following four USDA inactivated WNV vaccines are available;

1. Inactivated – two to three dose initial series, depending on age at initial vaccination; annual booster
2. Canarypox vectored vaccine – two to three dose initial series depending on age at initial vaccination; annual booster
3. Inactivated flavivirus chimera vaccine, yellow fever vectored – two to three dose initial series depending on age at initial vaccination; annual booster

Broodmares ideally should receive their vaccination prior to breeding whenever possible. A booster vaccination is recommended 4 to 6 weeks prior to expected foaling. The booster prior to foaling will provide WNV specific IgG antibodies in the colostrum.

Researchers have demonstrated that foals which have received colostrum from WNV vaccinated mares were capable of producing WNV specific antibodies when vaccinated at 180 days of age. Current recommendations are a three-dose series for WNV vaccination to begin at 4 to 6 months of age. Foals delivered by unvaccinated mares (or an unknown vaccination status) should receive WNV vaccination at 3 to 4 months of age. The ideal plan for these foals is to complete the series prior to mosquito season. An example of the protocol is as follows;

June Foal is born
September 3 months of age


October 4 months of age

2nd WNV

December 6 months of age

3rd WNV

If the foal (delivered by a mare with a history of unvaccinated or unknown vaccination status) is being vaccinated during the mosquito season, the vaccination protocol should be completed in an 8 week interval (versus 6 month protocol). An example of this protocol is as follows;

March 1st Foal is born
March 22nd 3 weeks of age

1st WNV

April 12th 6 weeks age

2nd WNV

April 26th 8 weeks of age

3rd WNV

Contact the Equine Veterinarians at Arizona Equine for more information.


Cortese, V., Hankins, K.G., Holland, R., et al. (2013). Serologic responses of West Nile virus seronegative mature horses to west Nile Vaccines. Journal of Equine Veterinary Science; 33:1101-1105.

Davis, E. (2014). Equine Immunology and Vaccine Strategies. American Association of Equine Practitioners. Vol. 60, pp. 390-400.

Turner, J.T., Waggoner, J.W., Rose, S.S., et al. (2008). West Nile antibody titers and total immunoglobulin G concentrations in foals from mares vaccinated in late gestation. Journal of Equine Veterinary Science; 28:17-21.

Vest, D.J., Cohen, N.D., Berezowski, C.J., et al. (2004). Evaluation of administration of West Nile virus vaccine to pregnant broodmares. Journal of the American Veterinary Medical Association; 225: 1894-1897.

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