Horse Health 411: Not All West Nile Vaccines Are the Same

Learn more about different types of vaccines and which kind might best protect your horse.

From AQHA Corporate Partner Zoetis

As mosquito season sets in, so does the threat of West Nile virus to horses. Last year, the United States reported 373 West Nile virus cases in horses.1 Texas and Oklahoma topped the charts with 60 and 41 cases, respectively.1 What can you do to help protect your horse during mosquito season this year? It seems like a combination vaccine would be the most convenient and effective way to help protect your horse. After all, a single injection helps provide a wide range of protection.

Even if your horse already received a combination vaccine this spring, the option of giving a monovalent West Nile vaccine with a combination of other boosters in the fall may help provide the added protection your horse needs to stay healthy.

Researchers recently tested horses' serologic (blood) response to six West Nile virus vaccination regimens and found some differences in their immune responses.2

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Researchers separated 280 West Nile virus-free Quarter Horses into three groups to study the effect of:

    • Vaccinating horses with a combination vaccine that contained West Nile virus, eastern equine encephalomyelitis virus, western equine encephalomyelitis virus and tetanus antigens in a single shot
    • Vaccinating horses with a West Nile virus monovalent vaccine (which helps protect against one disease) administered separately but at the same times as a combination vaccine that contained EEE, WEE and tetanus antigens
    • Treating horses in one group with saline, as the unvaccinated controls

All horses were vaccinated twice according to their respective label direction, 21 days apart, by Dr. Kevin G. Hankins, a senior veterinarian in Zoetis Equine Veterinary Operations, and his colleagues.

Blood samples were collected on six different occasions throughout the 42-day study to evaluate the horses' WNV antibody titer levels (concentration of specific antibodies in the blood that recognize a particular agent).

All of the vaccinated horses showed elevated WNV antibody titers compared with the control horses, meaning that all of the WNV vaccines produced an immune response. However, by Day 28, the researchers found that horses vaccinated with a combination WNV vaccine had antibody titers one-third to one-fourth of those in horses vaccinated with a product containing only WNV given at the same time as a combination EEE, WEE and tetanus vaccine.

"We thought that the monovalent WNV vaccine would produce a higher antibody titer level but did not think it would be three- to four-fold higher than the combination WNV vaccines," Dr. Hankins says.

The researchers believe the reduction in antibody titers could be attributable to antigen interference, antigen load (exposure to substances that trigger an immune response) or other unknown factors. Regardless of the cause, Dr. Hankins suggests that veterinarians should consider the possible consequences of a lower WNV antibody response with combination vaccines when developing and implementing vaccine programs for horse owners.

So are combination vaccines still effective?

“Absolutely,” Dr. Hankins says. “This study was not to show that the combination vaccines were not effective but that the monovalent vaccines might provide a higher degree of antibody response. It's just that when you have horses that are at risk for WNV infection, it might be advisable to use a monovalent WNV vaccine to help provide a higher antibody titer."

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According to the American Association of Equine Practitioners guidelines, West Nile is considered a core vaccination requirement, along with vaccinations for eastern equine encephalomyelitis, western equine encephalomyelitis, tetanus and rabies.3

“We have a disease that is here to stay, an effective vaccine but no treatment in the case of infection,” Dr. Hankins says. “That makes vaccination a cheap insurance policy.”

Explore the risks

Unvaccinated horses can be at serious risk of contracting diseases, particularly mosquito-borne illnesses, even in areas where the disease may not be common. With annual vaccination, the risk of your horse contracting West Nile can be up to 30 times lower.4-6

With the WEST NILE-INNOVATOR® app available from Zoetis for free in the Apple® App Store, horse owners can see why vaccinations are worth the investment. They can also evaluate the economic risk and expense of not vaccinating against certain mosquito-borne illnesses.

In addition to vaccination, it’s also important for horse owners to do their part to help reduce mosquito populations around the farm. Destroy any mosquito breeding habitats by removing all potential sources of stagnant water and cleaning and emptying any water-holding containers.

“No matter the location, horses are at risk for West Nile virus,” Dr. Hankins says. “Mosquito avoidance through environmental preventive measures and vaccination helps offer the best prevention.”

With guidance from a veterinarian on vaccination, owners can make informed decisions to help protect horses against these costly diseases.

For more information on the WEST NILE-INNOVATOR app, contact your Zoetis representative, call 888-ZOETIS1 (888-963-8471) or visit

To ensure peace of mind, WEST NILE-INNOVATOR vaccines are part of the Equine Immunization Support Guarantee from Zoetis.* This guarantee provides up to $5,000 for reasonable diagnostic and treatment costs if a horse properly vaccinated by a veterinarian contracts the corresponding equine disease.

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*This guarantee does not apply to, and Zoetis shall not be liable for, any (x) damages caused as a result of the improper handling, misuse or abuse of the vaccines that are the subject of this guarantee, or the willful misconduct or negligence of any third party, or (y) any indirect, punitive, special, incidental or consequential damages. Zoetis reserves the right to modify or cancel the terms and conditions of this guarantee.
1 U.S. Geological Survey. West Nile Virus Maps - Veterinary - USA. Available at: Accessed March 26, 2014.
2 Cortese V, Hankins K, Holland R, Syvrud K. Serologic Responses of West Nile Virus Seronegative Mature Horses to West Nile Virus Vaccines. J Equine Vet Sci 2013;33:1101-1105.
3 American Association of Equine Practitioners. Core Vaccination Guidelines. 2008. Available at: Accessed February 26, 2014.
4 Salazar P, Traub-Dargatz JL, Morley PS, et al. Outcome of equids with clinical signs of West Nile virus infection and factors associated with death. J Am Vet Med Assoc 2004;225(2):267-274.
5 Epp T, Waldner C, Townsend HG. A case-control study of factors associated with development of clinical disease due to West Nile virus, Saskatchewan 2003. Equine Vet J 2007;39(6):498–503.
6 Epp T, Waldner C, West K. Efficacy of vaccination for West Nile virus in Saskatchewan horses, in Proceedings. 51st Annual Convention of the American Association of Equine Practitioners 2005; 180-182.