West Nile Virus – what owners should know

West Nile Virus (WNV) is a viral disease previously only seen in Africa, western Asia, and southern Europe. Now it can also be found in the Middle East, Mediterranean region of Europe and the US. Prevention is the name of the game, so make sure you are one step ahead and protect your horse from this life-threatening disease.

West Nile Virus (WNV) is a viral disease that can cause encephalitis, an infection of the brain and the spinal cord. For the past four years, WNV has caused widespread disease in the United States. As of November 3, 2004, the number of equine cases was 1,243. In 2003 five thousand and fifty one equine cases were confirmed in 41 states; a large proportion were seen in the midwestern states, including Colorado, Missouri, Nebraska, New Mexico, Oklahoma and Texas. It is expected that WNV will continue to be a disease threat.
WNV is spread by the bite of an infected mosquito that acquires the virus from infected birds. The principal transmitter of WNV is the Northern House Mosquito (Culex pipiens). The incubation period of the WNV infection is usually 5-15 days. To date, WNV has never occurred in horses in the UK.
Many birds can be infected with WNV, but crows and blue jays are most likely to die from the infection. Horses, too, are prone to severe WNV infection. Although horses may be infected by the virus, there is no evidence that infected horses can spread the virus to uninfected horses or other animals. However, if at all possible, horses with suspected West Nile Virus should be isolated from mosquitoes and tested for the virus.

In horses that become clinically ill, the virus infects the central nervous system and causes symptoms of encephalitis. Clinical signs of encephalitis in horses include:

  • Ataxia (incoordination).
  • Circling.
  • Convulsions.
  • Depression.
  • Hyperexcitability.
  • Listlessness.
  • Loss of appetite.
  • Partial paralysis.
  • Stumbling.
  • Weakness of limbs.
  • Death.

Other diseases can cause a horse to have symptoms similar to WNV, including:

  • Rabies.
  • Botulism.
  • Equine protozoal myeloencephalitis (EPM).
  • Neurologic equine herpesvirus-1 infections.
  • Other mosquito-borne viral encephalitic diseases caused by Eastern, Western, and Venezuelan encephalitis viruses.

If you are worried that your horse may be exhibiting signs of encephalitis, contact your vet immediately. Only laboratory tests can confirm the diagnosis of WNV.

There is currently no licensed vaccine for horses in the UK. There is a vaccine available against West Nile Virus in the USA – horses are initially vaccinated twice, three to six weeks apart, with this vaccine. The use of this vaccine, however, is restricted, so owners have to contact their vet to find out if it’s available for use in their area.
Remember: Horses vaccinated against Eastern, Western, and Venezuelan equine encephalitis are NOT protected against WNV.
Currently, there is no specific treatment for WNV in horses, so prevention is essential (supportive veterinary care is recommended).
If your horse is in a WNV affected area, you can decrease the chances of your horse being exposed to WNV by taking preventative steps, eg:

  • Avoid turning on lights inside the stable during the evening and overnight as they will attract mosquitoes.
  • Keep an eye out for dead birds, such as crows, they should be reported to the local health department. Use gloves to handle dead birds.
  • Place bright lights outside the stable to attract mosquitoes away from your horse.
  • Remove all birds, including chickens, that are in or close to the stable.
  • Remove any sources of water where mosquitoes may breed, eg shallow standing water, used tires, manure storage pits, drainage areas with stagnant water. Note: Mosquitoes can breed in any water that stands for more than 4 days.
  • Stable your horse during periods of mosquito activity, ie dusk and dawn.
  • Use mosquito repellents available for use on horses. Make sure you read the product label before using and follow all instructions.

The Department for Environmental Protection, local Department of Health, local vet or mosquito and pest control company will be able to help with assessing mosquito exposure risks on your property and suggest appropriate control methods.