Health & Wellness

West Nile Virus and Your Horse

By: Freya Stein, DVM, McKinlay & Peters Equine Hospital

West Nile Virus has appeared in the Northwestern horse population. These are the first horses in the Inland Northwest to test positive  and die from West Nile Virus. As horse owners we are understandably concerned about our beloved steeds. The best way to protect your horse is through knowledge of the disease and its prevention. The virus was first detected in Uganda in 1937 and arrived in the eastern U.S. in 1999, quickly spreading across the country and causing disease in humans, birds and horses, with rare reports of disease in dogs and alpacas in the years since. Birds are the host for the virus (meaning the virus can live and replicate in the animal), and mosquitoes acquire the virus from biting infected birds. Mosquitoes, in turn, infect humans and horses, neither of which are capable of transmitting the virus to other animals. Thus, WNV is not a communicable disease and affected horses are not contagious to other horses or humans.

The virus can cause a range of neurological signs, resulting in anything from mild, transient illness to death. The virus directly infects and destroys neurons and supporting cells, as well as causes inflammation within the central nervous system, eventually causing flaccid paralysis. Clinical signs of disease include depression, widespread muscle twitching (particularly noticed in the muzzle), and weakness and instability. The gait will often appear abnormal and range from dragging the toes to swaying from side to side at the walk and even falling down. Depending on the virus strain, some horses will develop Cataplexy, a type of transient semi-sleep state that is triggered by eating or exercise. Severely affected horses can lose the ability to stand or even lift their head or neck. Horses that reach this stage of disease are much less likely to recover, but more mildly affected horses who remain standing will usually recover with appropriate supportive care. However, return to full function may take weeks to months, and some horses will retain a degree of weakness and/or gait deficit. Case fatality rates in horses have ranged from 30-40%.

There is no specific treatment for WNV, and care involves reducing inflammation and supporting normal function. Anti-inflammatory drugs such as Bute or Banamine are frequently used, as is DMSO. Vitamin E is given orally as a neuroprotectant. For horses that are not drinking on their own, intravenous fluids are used to maintain hydration. Horses unable to stand can be placed in a sling to prevent muscle damage from lying on the ground for an extended time.

Vaccination is key in disease prevention. There are multiple vaccines available, all of which require an initial vaccination followed by a booster vaccine approximately one month later, then once yearly booster vaccinations thereafter. Horses should ideally be vaccinated in the spring to have maximum protection prior to mosquito emergence. While all horses are at risk, young horses less than 5 years old and geriatric horses are at particular risk. Foals born to mature and vaccinated mares are protected until about 4 months of age, at which point they should begin their vaccination series. If your horse has not been vaccinated yet this year, it’s not too late!

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