Karl Hoopes, DVM


West Nile virus has been confirmed this summer in mosquitoes and birds in Idaho, Utah and Wyoming, putting humans, horses and other animals at risk.

Birds are the primary reservoir for West Nile virus (WNV) and in birds the disease is usually fatal. The virus replicates quickly in birds, resulting in large amounts of virus in the bloodstream. A viremic bird bitten by a mosquito transfers WNV to horses, humans, and other mammals. Infected mammals typically experience low viral loads and mosquitoes are unable to transmit WNV from an infected mammal to an uninfected mammal.

Horses can exhibit clinical signs when they become infected by WNV, though many do not. However, WNV can cross the blood-brain barrier and cause disease in the brain and spinal cord. The disease is WNV encephalomyelitis and it ranges in severity. The most common signs include ataxia in the hind limbs as well as twitching of the muzzle and neck muscles. Other signs may include fever, wandering, impaired vision and generalized weakness. More severe signs can include depression, stumbling, paralysis, recumbency and death.

Geriatric horses or any horse that becomes recumbent (unable to get up) have a greater risk of death from WNV infection. Local and state health departments monitor bird and mosquito populations for WNV and weekly updates are available on each state’s health department website. In horses, WNV disease is diagnosed by the presence of clinical signs plus the detection of antibodies in the bloodstream. Clinical signs of WNV encephalomyelitis cannot be distinguished from other equine neurological diseases. A blood test is required to identify specific antibodies. Other equine neurological diseases, such as Equine Herpes Virus (EHV), can also be fatal and spread from horse to horse. This makes it important to distinguish WNV infection from other neurological diseases.

There is no specific cure for infection from WNV. Controlling pain and inflammation with anti-inflammatories and fluid therapy is most often used. Care early in the onset of the disease leads to more favorable outcomes. The mortality rate for infected horses is estimated at 35%. Approximately two-thirds of infected horses will recover. But 40% of those that survive continue to show clinical signs.

Equine WNV vaccines are available. Broodmares should receive the vaccine 4-6 weeks prior to their due date to ensure adequate antibody concentrations are in the colostrum for the foal. If the dam was given her pre-foaling vaccine, foals should start their vaccine series for WNV at 6 months of age. It is important to follow the manufacturer’s recommendation for dosing frequency when starting the series. Visit with your veterinarian to determine a vaccination schedule.

The American Association of Equine Practitioners recommends these steps to help eliminate mosquitoes from horse areas: eliminate standing water (old tires, etc.), clean water troughs at

least monthly, use larvicides to control mosquito populations, keep horses inside during peak mosquito times from dusk to dawn, use insect repellants on your horse specific for mosquitoes, remove birds from the stall area and protect yourself with proper clothing and mosquito repellent. Finally, if you see any clinical signs of WNV infection in your horse, call a veterinarian immediately.