West Nile Virus

West Nile Virus:

Affecting Both Your Horse and Yourself

By:  Brie Taylor, DVM

 You’ve heard it in the news.  You’ve heard it around the county.  You’ve heard it from your veterinarian.  West Nile Virus is here in California and it is in Lake County.  It is a very serious illness and can be prevented with a proper vaccination protocol and mosquito control.

 Horses and humans are susceptible to infections from West Nile Virus (WNV) and primarily become infected via mosquito bites.  Although any age horse can become infected with WNV, it seems that the young and old are more prone to illness.  The virus is not contagious between horses or people due to the low levels of virus amplification within these species.  It is maintained in the environment primarily by transmission between birds and mosquitoes.

 WNV is an arbovirus that causes infection of the spinal cord and brain.  Clinical signs associated with WNV include:  fever, listlessness, depression, sudden or progressive ataxia (off-balance), dragging of limbs or placing them in awkward and abnormal positions, apprehension, periods of hyper excitability, and muscle rigidity.  It is possible for the disease to progress to recumbency in the horse and even death.  The fatality rate for horses exhibiting clinical signs of WNV is ~33%.  However, if a horse becomes recumbent, the fatality rate increases to >60% since horses are unable to remain down for long periods of time.  In those instances, euthanasia is indicated.

 There are other diseases that cause the same clinical signs mentioned above, including Equine Herpesvirus (Rhino), Equine Protozoal Encephalomyelitis (EPM), Rabies, spinal trauma, cancer, and others.  Therefore, further diagnostics are indicated before a definitive diagnosis of WNV is determined.  Many of these tests are specific blood tests, but also may include spinal radiographs and spinal taps.

 Treatment of WNV is supportive care only.  In most instances this means giving those affected an anti-inflammatory, keeping them in a small area to decrease the risk of stumbling and falling, and antibiotics if any wounds have occurred.  If a horse survives the initial phase, they usually get better over the next few days.  And in most cases, there are residual neurological effects for up to 6 months post-infection.  There is no cure or specific treatment for this disease.  Therefore, it is important to prevent it from occurring by maintaining a good vaccination protocol and mosquito control.  The best vaccination protocol includes:  initially vaccinating with two injections 30 days apart, followed by every 6 months, preferably beginning of spring and fall.  All of the West Nile vaccines are ~95% effective, which means it is still possible to become infected even if given the vaccine.  Thus, mosquito control is also important and protects against other neurologic viruses in addition to WNV.  Make sure that there is no standing water for long periods of time, control faucet leaks, place a fly sheet on your horse, or even apply Advantix (off-label) once monthly, which control ticks, fleas, and acts as a mosquito repellent.  For more information on vaccines and the use of Advantix, please ask your veterinarian.

 West Nile disease is an important topic, especially since it is present in Lake County and we have already confirmed one case this year.  It is a very debilitating illness with no cure that can cause death.  And don’t forget that people are susceptible as well, which makes it even scarier.  Give your horses the best chance by properly vaccinating them and keeping mosquitoes at bay.  And don’t forget to protect yourself and your children by wearing mosquito repellent.  If you would like more information on this topic, please contact your veterinarian or


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