Stonewall Veterinary Clinic
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West Nile
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About West Nile
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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.
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.
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West Nile Virus in CanadaWNv was first confirmed in Canada in August 2001 in a wild bird in
Ontario. Exposure of horses to WNv was confirmed in 2002 in Manitoba, Saskatchewan, Ontario and Quebec. In 2002, Quebec and
Ontario reported the first human cases in Canada. In 2003, WNv reached as far west as Alberta where cases in mosquito pools,
birds, horses and humans were reported. British Columbia is the only province in Canada to have never detected WNv.
Testing HorsesHorses are incidental hosts and the level of virus in their blood is
very low for a short period of time. Mosquitoes feeding on infected horses are not likely to become infected. There is no
scientific evidence to indicate that WNv can be transmitted directly from horses to other species, including humans. Therefore,
quarantine of affected horses is not necessary. Detection of exposure to WNv in most horses is restricted to a blood test
that identifies antibodies to WNv. Routine testing of horses is not recommended, even if the virus has been confirmed in the
area. Specialized laboratory tests can confirm the presence of WNv in the brain or spinal cord of horses dying or being euthanized
and are available to your veterinarian.
Treatment for HorsesThere is no specific therapy for WNv infection. Veterinarians use supportive
therapy such as intravenous fluids and good nursing care to prevent secondary infections.
Protecting Horses from Infection with WNvAlthough 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 small, 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.
 This was prepared by:
Dr. Gerald Ollis
Chief Provincial
Veterinarian
Food Safety Division
Alberta Agriculture and Food
Source: Agdex 663-45. Revised
June 2007.
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