West Nile Virus in Horses


The first isolated case of West Nile was in Uganda in 1937.  West Nile virus (WNV) is a virus transmitted by various species of mosquitoes and cause inflammation of the brain and spinal cord.  West Nile virus was first recognized in the Western hemisphere in September 1999. By 2002, there were over 15,000 horses that were diagnosed with the virus in 41 states.

The strains of WNV present in North America are capable of causing disease in certain domestic and exotic species of birds, especially crows and blue jays.  Humans and horses can also be infected with WNV.  WNV infection in mammals does not result in large amounts of the virus in the bloodstream.  There are only very small amounts of the virus in the blood of the infected horses; mosquitoes are unable to transmit the virus from horse to horse or from horse to human.  The virus is transmitted when a mosquito takes a blood meal from an infected bird then feed on a horse.


When a horse is infected with WNV, the clinical signs can vary in range and severity.  The most frequently observed clinical signs include:

  • Lack of coordination, especially of the hind limbs.
  • Twitching of muscle and lower lip
  • Twitching of the muscles in the neck, shoulders and pectoral region
  • Stumbling
  • Toe dragging or leaning to one side

There is no specific anti-viral treatment for WNV.  Management should focus on controlling pain and inflammation.

A vaccine was licensed by the USDA’s Center for Veterinary Biologics for use in healthy horses.  Horses that are vaccinated against Eastern Western or Venezuelan Equine Encephalitis are not protected against WNV.

If you have any further questions regarding the West Nile Virus or would like to schedule an appointment to have your horse vaccinated for WNV, please contact our office at 505-869-2627 and we will be glad to help you.