West Nile Virus Prevention
by Beth A. Valentine, DVM, PhD

The most effective prevention of WNV involves both measures to minimize exposure and vaccination.

Mosquito Control
The most important part of prevention is to minimize exposure to infected mosquitoes:

  • Enforce a program of mosquito control on your farm. Eliminate standing water in tires, buckets, gutters, and the like in which mosquitoes breed. If you have a pond on your property, talk to your local experts about the best means to control mosquito breeding without risking your horses' health.

  • Obviously you can't drain your water troughs, but do check troughs regularly and clean them out if you detect the small worm­like black wrigglers that are mosquito larvae. Some folks put goldfish or other fish in their troughs or ponds to eat the mosquito larvae. Others pour a layer of vegetable oil on the surface of troughs to keep mosquitoes from being able to lay eggs in the water or to keep those already in the water from hatching and flying out.

  • Use fly repellents on yourself and your horses, and utilize insecticidal compounds when needed. Always confer with your veterinarian or other equine experts before exposing horses to insecticides not specifically approved for stable use. Screening the stall and barn windows will help. Although keeping horses in at dusk and dawn, when many kinds of mosquito are most active, is often recommended, WNV is carried by day-feeding mosquitoes as well.

Although a vaccine is not yet available for people, one has been developed for horses. Fort Dodge released a vaccine last year under what is called a conditional license, meaning the vaccine is safe and we have a reasonable expectation, but as yet no proof, that it works. The vaccine was released because it was sorely needed with no time to prove its efficacy. The company is working to determine its efficacy, as well as the best schedule for vaccination of horses, and hopes to have this information available before the end of this year.

This vaccine has been given to thousands of horses with few side effects. Although some vaccinated horses have developed WNV, most of those either had only recently been vaccinated or had received only one of the required two shots to be given three to four weeks apart. In one case, a yearling in Kentucky had received both vaccinations several weeks before the horse became infected. Clearly no vaccine is 100% effective. Factors such as the stress of shipping or other illness, other medications given, or even heavy exercise at the time of vaccination can affect the horse's ability to mount an effective immune response following vaccination.

Many horse owners and even veterinarians are in a quandary as to whether to vaccinate for WNV. Because this vaccine has a conditional license, it may be given only by a veterinarian. The cost of the farm call combined with the cost of the vaccine may seem exorbitant, especially if WNV is not yet in your area. Some people are also concerned that we are giving too many vaccines to our horses, introducing too many foreign proteins into their bodies.

I am a firm advocate of vaccinating horses only for what they need, not with every vaccine available. Yet I have vaccinated my horses for WNV, even though WNV is, as yet, nowhere near the state of Oregon. Why did I vaccinate? Because I have a reasonable expectation that the vaccine will be as effective as other equine encephalitis vaccines, and because the probability is high that WNV will reach Oregon at some point. Waiting for the virus to reach your area before vaccinating means you would be vaccinating in the face of possible exposure, when it may be too late. Horses that have had the initial series of two shots will require boosters, possibly every six months, but should have far greater immunity than horses only recently vaccinated.

Few WNV infected horses have been reported as of September, 2002 in the East Coast states, where the virus initially emerged. This fact suggests that exposure and/or vaccination has resulted in some degree of protection for horses in those areas, as dead birds with WNV are still being reported from those states.

Other important measures

  • Vaccinate your horses every spring for any other encephalitis viruses that are likely to be in your area. Effective vaccines are available for EEE, WEE, VEE. The effectiveness of a recently released vaccine for EPM is in doubt.

  • If you live in an area with rabies infection, be sure to keep your horses' rabies vaccinations up to date. The signs of WNV are indistinguishable from those of rabies. Since rabies is transmitted from horses to people, handling and treating a possible rabies horse is dangerous, so you can hardly blame your veterinarian for declining to do so. An unvaccinated horse exhibiting the classic signs of rabies is likely to be put down. What a shame to find out afterwards that it had WNV, from which it might have recovered.

  • Government agencies are working hard to institute mosquito control measures in affected areas and to track and control the spread of WNV. Sentinel chicken flocks are being maintained and tested for exposure to WNV in new areas, including more than 600 flocks stationed by the Canadian government along the border from Saskatchewan to the Atlantic. Unfortunately, horses, and some wild birds seem to be more susceptible to WNV than chickens, so most areas are also have surveillance measures that include testing of dead blue jays, crows, and magpies.

  • Remain informed. The following sites are excellent sources of updated information:


    New York State

    Pennsylvania State

    Centers for Disease Control


  • Alert health officials regarding any suspicious bird die-offs in your area. Handle dead birds only with gloved hands. Call your veterinarian immediately if your horse develops a fever or any problems that could relate to brain or spinal cord infection.

Above all, remain calm. The risk of infection from WNV is here, but the risk of your horse contracting fatal colic is higher than the risk of death due to WNV. Maintaining as safe and healthy an environment and diet for your horses as possible will go a long way toward protecting them from West Nile virus infection.

West Nile Virus and Its Signs


Beth A. Valentine, DVM, PhD, is an equine pathologist at Oregon State University, Corvallis, and co-author of Draft Horses, an Owner's Manual.

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23 April 2012 last revision