As of the beginning of December 2004, 3,523 horses in Mexico had been tested for West Nile virus (WNV) antibodies. Of those, 29% (1,023) were positive, but none had displayed any symptoms of West Nile Virus infection. What’s up’
Prior exposure to a closely related virus, St. Louis Encephalitis virus (SLE), is known to cross protect against West Nile infection, and positive titers for this virus are commonly found in Mexican horses in some areas. However, previous large surveys of horses in Mexico that checked for antibodies to both viruses found similar fairly high exposure rates with West Nile, but none of those horses were positive for SLE so there’s more going on.
Formal studies of WNV in horses are sorely lacking, but we do know that horses deliberately exposed to WNV-infected mosquitoes, even with the detection of the virus circulating in the blood, don’t necessarily show symptoms. This is similar to the situation in people, where only a relatively small percentage of those exposed develop neurological signs.
A common question concerns how much natural exposure and the development of a natural immunity may be contributing to the overall drop in equine WNV cases. However, that’s a question that’s virtually impossible to answer. No one has followed horses that converted to positive titers after a natural exposure to see if they remain protected, nor do we have a reliable way to experimentally produce WNV encephalitis to check to see if the horses are protected. Experts do agree that it’s likely a naturally exposed horse will have immunity, possibly a lifelong immunity.
Where does that leave the owner when trying to decide whether or not to vaccinate’ Even if only a small percentage of exposed horses will develop neurological disease, it’s still too serious an illness to ignore.
As vaccine technology improves, the available options should be both more effective and less risky. Many owners will decide, quite reasonably, they would rather vaccinate than take a chance.
Vaccination is risky, though, in ill or stressed horses, and those with a prior history of severe vaccine reactions. In those instances, if a check of antibody titers shows the horse has already been naturally exposed, the risk-to-benefit ratio may favor not vaccinating.
Vaccinating is always a decision to be made on an individual basis ro each horse and in consultation with your treating veterinarians.
Timing Vaccines
West Nile is a seasonal disease, and case numbers peak when mosquito populations are highest. Common sense dictates that you time your initial WNV vaccination series or boosters so that the horse is best protected during the mosquito season. Despite this, people vaccinate their horses against WNV in winter.
To avoid unnecessary vaccinations — which are all risk and no benefit — and to provide your horse with peak protection when he needs it, look at the case numbers for your state (contact your state horse extension specialist or your veterinarian). Time your vaccinations so the protection is in effect around the time of year when the first cases start to appear.
You’ll need to allow six weeks after the first vaccination in the Ft. Dodge initial vaccine series; three to four weeks after the first vaccination in the Merial initial series; at least two weeks after a booster in a previously vaccinated horse. For example, if cases begin to appear in your area in early June, booster previously vaccinated horses in May. Plan the first dose of an initial vaccine series with the Ft. Dodge vaccine in mid-April or the first dose of the initial series of the Merial vaccine in early May. Vaccinating too early or too late is a waste of money and an unnecessary burden on your horse’s immune system.
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