|West Nile Virus Prevention|
by Beth A. Valentine, DVM, PhD
The most effective prevention of WNV involves both measures to
minimize exposure and vaccination.
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
- 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
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
- Remain informed. The following sites are excellent sources of updated
- 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
West Nile Virus and Its Signs
Valentine, DVM, PhD, is an equine pathologist at Oregon State University,
Corvallis, and co-author of
Horses, an Owner's Manual.