Do Horses Get Mild West Nile Virus Infection?
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If you get bitten by a mosquito carrying West Nile virus, one of three main things could happen. The best case scenario, which happens in about 80% of people that are infected, is nothing happens. Most of the other 20% people that get sick only develop mild disease, including fever, headache, body aches and similar flu-like symptoms. These cases are usually not diagnosed as West Nile virus infection because signs are vague, typically mild and often last only a few days. Approximately 1 in 150 people that are infected will develop neurological disease. This is the most serious and potentially fatal form of West Nile virus infection.
In horses, we really only recognize the two extremes - no disease or neurological disease. Does this mean the mild form (sometimes call West Nile fever) doesn't happen in horses? It probably does happen, it's just that we rarely notice or diagnose the problem. Mild flu-like disease is easily missed in horses, particularly horses that are housed outside. If these signs are noticed, West Nile virus doesn't often jump to mind and testing is not common. It's likely that many horses actually develop this mild form of disease, but the only way to know for sure would be to test more horses with mild, non-specific disease in areas where West Nile virus occurs during the typical West Nile virus season. This might be hard to justify, since horse owners would have to pay for the testing and the results would not change what's done with their particular horse. If horses that were infected and developed mild disease had life-long immunity to the virus after infection, then that would be a reason to test (because they would no longer be at risk of the more severe form), but we don't have enough evidence to say that's the case.
So, if your horse develops mild signs of illness such as fever, in a West Nile virus area, and during the right time of year, it certainly could be a mild case of West Nile virus infection - but it could also still be a lot of other things. Horses that develop fevers or other signs should be watched closely to make sure more severe disease is not developing, and getting a veterinarian to examine the horse for other (possibly more subtle) signs is never a bad idea. Testing may not change what you do, but it might help you and your veterinarian understand whether West Nile virus exposure is a problem in your area and explain why your horse got sick.
