Do Horses Get West Nile Fever?
In humans, infection with West Nile virus have a few different possible outcomes:
- Nothing happens: This is probably the most common outcome. People get exposed to the virus by being bitten by an infected mosquito, but their immune system fights it off. They may develop antibodies against the virus but they don't get sick.
- West Nile fever (also called West Nile non-neuroinvasive disease): In this scenario, the infected person develops a fever, often along with a headache and joint or muscle pain. This accounts for most of the people that get sick after being infected.
- West Nile neuroinvasive disease: This is the big scary problem, though it fortunately only occurs in a small percentage of infected people. This form is characterized by neurological abnormalities such as meningitis, encephalitis and/or paralysis.
US data indicate that there were 720 cases of West Nile virus disease reported in 2009, 54% of which were non-neuroinvasive and 46% of which were neuroinvasive. While that may not seem to support my statement above that only a small percentage of infected people get neurological disease, you have to remember that if you don't get sick, you won't get tested. Further, if you just have a headache and muscle or joint aches, it's pretty unlikely you're going to get tested. Therefore, while a large percentage of reported cases were neurological, a very large percentage of non-neurological cases were presumably not identified because they were mild and testing wasn't performed.
Back to horses...
In horses, we pretty much only identify horses that are healthy but have been exposed (based on the presence of antibodies against the virus) and those with neuroinvasive disease. However, that doesn't mean that West Nile fever does not exist in horses. It probably does, we just don't diagnose it.
Horses that develop mild disease of short duration aren't always even examined by a veterinarian. If a veterinarian is called and the horse is just a little "off" with a fever and vague signs of illness, testing for West Nile virus will not be very high on the list of priorities (especially if the horse owner has to pay for testing). Most of these horses, if they are examined, probably get treated empirically with anti-inflammatories and rest, and they get better. So, we really have no idea whether this form of the disease exists or how common it is. In a lot of ways, it doesn't really matter. The main reasons that it would be nice to know whether horses with mild, non-specific disease have West Nile is to know that there is active West Nile virus transmission to horses going on in the area, and for a better general understanding of West Nile virus in horses.
If you have a horse that develops vague signs of disease during the typical West Nile season, especially if there are no major other risk factors for other diseases that typically cause fever and vague illness (e.g. going to shows, new horses in the barn), you should at least consider the possibility that West Nile virus is present. That doesn't necessarily mean you have to get the horse tested or do anything different, but it never hurts to have a better idea of to what infectious diseases your horses might be exposed.
Image source: www.lapublichealth.org (click image to enlarge)
