West Nile Virus and Your Horse

Note: this information is taken from Alberta Agriculture and Food's Ropin' the Web.

The West Nile virus (WNv) is a mosquito-borne virus that can cause swelling and inflammation of the brain and spinal cord in horses, birds and humans. The virus is named after the West Nile region of Uganda where the virus first appeared in 1937. Since the discovery of WNv, it has become widespread in Africa and Eurasia. WNv was identified in the New York area in 1999, and has since become established across the North American continent. It was first detected in Alberta in July 2003 in a wild bird. WNv is related to the viruses that cause St. Louis encephalitis and Japanese encephalitis.

Mosquitoes Spread the Virus

Only specific species of mosquitoes spread WNv. In Alberta, the species of mosquito of concern is Culex tarsalis, which becomes infected when feeding on infected wild birds. Wild birds are the primary reservoir of WNv. Most wild birds are not affected by the virus, but rather just carry the virus for a variable period of time. However, members of the Corvidae family, including crows, blue jays, magpies and ravens, are very susceptible to the effects of WNv. So too are some species of raptors. Often, dead crows are indicators of the arrival of WNv in a geographic area. There is no evidence that WNv can be spread from birds to humans or animals, or from horse to horse or humans.

Birds and Animals Infected with WNv

Over 140 species of wild and domestic birds can be infected with WNv. As well, a wide range of wild and domestic animals can also be infected with WNv, including bears, mountain sheep and goats, horses, mules, donkeys, cattle, alpaca, dogs and cats. However, it is important to understand that disease is rare in the majority of these species. Only domestic geese, horses, mules and donkeys appear to be severely affected by WNv, and may develop clinical disease.

Symptoms in Horses

Most horses bitten by a mosquito infected with WNv will not develop clinical disease. They develop an asymptomatic infection, eliminate the virus and are none the worse for it. Symptoms in those horses that do become sick can include listlessness, a change in demeanor, drooping lips, muscle twitching, a lack of co-ordination, weakness in the limbs, partial paralysis or an inability to get up. A fever is not always present. A veterinarian should examine infected horses because these clinical signs are similar to those caused by Western Equine Encephalitis, Eastern Equine Encephalitis and Rabies.

To prevent handlers from being hurt, caution must be exercised when handling horses affected by nervous disorders such as WNv.

There is no specific treatment for horses affected with WNv. Up to 35 per cent of horses showing clinical disease may die or have to be euthanized because of complications of the disease. Some recovered horses may exhibit permanent neurological deficits.

WNv - A Reportable Disease

WNv infection in horses is a Reportable Disease under Alberta’s Livestock Diseases Act. This legislation requires anyone suspecting or knowing of a horse infected with WNv to report that fact to the Chief Provincial Veterinarian’s office at 780-427-3448. The federal government has made WNv in any species of animal or bird an Immediately Notifiable Disease under Canada’s Health of Animals Act. This requires diagnostic laboratories to report the diagnosis of WNv to the Canadian Food Inspection Agency within 48 hours.

Protecting Horses from Infection with WNv

Although the risk of disease in any individual horse is very low, the consequences for some affected horses can be severe. Preventive measures should be discussed with your local veterinarian. These measures include minimizing exposure to Culex tarsalis mosquitoes. This species of mosquito breeds in warm, still puddles of water. These puddles of water include those found in poorly drained eavestroughs, bird baths, discarded rubber tires and even hoof prints formed in mud. Consideration must be given to providing screened housing and avoiding outdoor activities during peak times of mosquito feeding, such as dawn and dusk. Using topical insect repellents and/or smudges may also be useful. Reduce potential mosquito breeding sites by eliminating standing water, cleaning water troughs weekly and keeping grass levels short around buildings.

Vaccines against WNv are licensed in Canada for use in horses and are available from veterinarians. Horse owners should contact their veterinarian for information about the vaccines available and recommendations about their use as part of a comprehensive disease prevention program. Vaccinated horses intended for export to the European Union or Japan will require certification of vaccination. Consult with the nearest district office of the Canadian Food Inspection Agency for up-to-date export requirements.

For more information
Alberta Agriculture and Food’s Ropin’ the Web
Office of the Chief Provincial Veterinarian
Canadian Food Inspection Agency 
Public Health Agency of Canada
Alberta Health and Wellness - Fight the Bite Tips for Animals

You may also contact Alberta Agriculture and Food’s Ag-Info Centre toll free at 1-866-882-7677.