This article is to bring everyone up to date on the West Nile Virus (WNV). It is important to remember what WNV is and some of the symptoms. WNV is a virus that infects horses and humans through the bite of a mosquito. Mosquitoes and the wild bird population are the reservoirs for the virus. The primary species of birds are those of the raven family, i.e. crows, ravens, magpies and blue jays.

Once infected, the virus can cause encephalitis (inflammation of the brain), with the most common signs of infection in the horse being: stumbling or incoordination , weakness of the limbs, partial paralysis, muscle twitching (“the Shakes”), hypersensitivity to touch, and death, killing about 35% of diagnosed horses.

There is a vaccine that is conditionally approved for the use in horses and has recently been found to be 94% effective in preventing the disease. The duration of immunity is one year, so a horse needs to be vaccinated once a year prior to mosquito season to be reasonably sure they are protected.

The virus was first identified in the U.S. in 1999 in New York and has since spread to 43 states. Of those 43 states, all but 2 states have had infected horses. In the 63 counties in the state of Colorado, 33 have reported at least one horse positive for WNV. Of the 749 horses tested in the state, 378 horses have tested positive for WNV. Weld County has had the most horses test positive with 99 and Larimer County had 29 positive cases for a total of 128 horses testing positive in this area. Just over one third of the horses testing positive for WNV in our state are in our back yard!

Since WNV is here to stay, what can we do? The first thing to do is to vaccinate your horses against the virus and do it before mosquito season starts. If your horses were not vaccinated this summer/fall, the vaccination series needs to be started in January and February for your horse to build up sufficient immunity before mosquito season. If your horse was vaccinated previously, re-vaccination should be done in February, March, and April. The second thing to do is to reduce you and your horse’s exposure to mosquitoes (i.e. spraying yourself and your horse with a product containing DEET, eliminate standing water). There are several ways of doing this but since none are fool proof your best bet is to be sure your horse is protected through vaccination.