Biosecurity and Health Committee
Protocol for the Management of West Nile Virus in Horses

Table of Contents

  1. Disease Information
  2. Human Health Risk Data
  3. Horse Health Risk Data
  4. Ecology Information
  5. Prevention
  6. Regulatory Information
  7. Committee's Recommendations
  8. More Information

Section 1: Disease Information

West Nile virus (WNV) is a mosquito-borne disease that can cause inflammation of the brain and spinal cord (encephalomyelitis).

West Nile Virus Cases in Ontario Horses, 2001 - November, 2008

Total Number of WNV
Cases Confirmed or Probable
Number per County/Region

4 - Brant
3 - Chatham-Kent
9 - Haldimand-Norfolk
4 - Halton
3 - Hamilton-Wentworth
6 - Lambton
1 - Middlesex
1 - Muskoka-Parry Sound
12 - Niagara
2 - Northwestern
1 - Ottawa
3 - Oxford
1 - Perth
6 - Toronto
5 - Waterloo
1 - Wellington
41 - Windsor-Essex
4 - York


1 - Durham
1 - Eastern Ontario
2 - Essex
1 - Halton
2 - Niagara
1 - Peel
1 - Perth
1 - Peterborough


1 - Brant
1 - Durham
1 - Elgin
1 - Lambton
1 - Muskoka
3 - Nipissing
1 - Sudbury District


1 - Essex
2 - Lambton
1 - Niagara
1 - Waterloo


1 - Niagara
1 - Nipissing
1 - Waterloo


1- Leeds and Grenville
1- Nipissing

Section 2: Human Health Risk Data

In humans, WNV often causes asymptomatic infection or mild febrile disease, sometimes with a rash. However, it can also cause severe illness in the form of encephalomyelitis with death occurring in a small percentage of patients.

Infected horses are NOT a source of infection for humans, unless nervous tissues (brain, spinal cord) or cerebrospinal fluid are handled. Caution should be exercised to prevent viral contact with open wounds or mucous membranes when performing examination or necropsy on a suspect horse.

Section 3: Horse Health Risk Data

Horses affected with WNV can display a variety of signs. Often, infected horses are initially noted to have a fever and muscle fasciculations (twitches or tremors). They frequently become ataxic (unsteady on their feet) and, in some cases, can not walk. Other signs include: knuckling over, head tilt, inability to stand, circling, weakness or paralysis of limbs, apparent blindness, lip droop and grinding teeth. The disease can progress from an animal being normal, and working at its peak of performance, to recumbent within 12 hours. Approximately 30% of infected horses die or are euthanized. Horses of all ages, breeds, and health status are susceptible to WNV.

Section 4: Ecology Information

Birds are the natural hosts for this virus. The virus has over-wintered in the wild bird and mosquito populations in Canada. The life cycle of the virus starts with a mosquito taking a blood meal from birds infected with WNV. Birds amplify the virus. Infected mosquitoes then transmit WNV to humans and animals when taking a blood meal. Vertical transmission in the mosquito (adult to eggs to larvae to adult) of the WNV has been demonstrated. The incubation period of a WNV infection in horses is usually five to fifteen days. The virus has been detected in mosquito pools and birds in every county of the province of Ontario.

Section 5: Prevention

  • There are three licensed vaccines currently available: an inactivated whole virus vaccine, a recombinant canary pox vector vaccine and a modified live chimera vaccine.
    • Ensure your horse is properly protected by using the West Nile virus vaccine.
    • For the inactivated and canary pox vector vaccines, initial vaccination for WNV entails two injections 4 - 6 weeks apart. A booster just prior to the peak risk period of August-October may further enhance immunity. The modified live chimera vaccine is labeled as a single dose administered to horses greater than 5 months of age. Vaccinations should begin before the start of the mosquito season. Incomplete vaccination during the season may not be protective. Vaccination semi-annually or more frequently may be required, depending on risk.
    • Using the inactivated vaccine, foals should receive their first vaccination at 4 to 6 months of age, followed by a booster 4 to 6 weeks later and a third at 10 to 12 months of age, followed by an annual booster prior to the expected risk period.
    • Due to the complexity and variability of the three WNV vaccines, consult the manufacturer's data sheet for each vaccine.
  • Do, or have a professional do, a site evaluation and eliminate all mosquito breeding grounds (stagnant water), where possible. Evaluate the breeding areas that can not be eliminated and choose the most appropriate method to prevent mosquitoes from breeding.
  • If you choose to use a pesticide, read the label carefully and observe all label precautions in order to protect people, pets and livestock.

Section 6: Regulatory Information

West Nile virus in animals has now been named an immediately reportable disease under the Health of Animals Act, which is administered by the Canadian Food Inspection Agency. The tracking of positive WNV equine cases is an important part of the public health surveillance program. This information is of great value to the Public Health Units. With the help of the University of Guelph Veterinary Teaching Hospital and the Animal Health Laboratory, the Ontario Ministry of Agriculture, Food and Rural Affairs collects and analyzes the information. The township location of infected horses is required to determine if and where additional prevention efforts may be needed. This information is also an invaluable part of the ongoing research into the epidemiology of WNV and its impact on the Ontario horse industries.

Committee's Recommendations

  • Horse owners should ensure that their horses have completed a vaccination program (initial and booster shots) for WNV prior to the mosquito season.
  • Track/barn owners should:
    • Hire a professional to do a site evaluation and eliminate all mosquito breeding grounds (stagnant water), where possible;
    • Evaluate the breeding areas that can not be eliminated and choose the most appropriate method to prevent mosquitoes from breeding.

More Information

Guidelines for Vaccination of Horses - American Association of Equine Practitioners

Animal Health Monitoring & Surveillance - United States Department of Agriculture

Equine Neurological Disease - Ontario Ministry of Agriculture, Food and Rural Affairs

Pesticides and Mosquito Control - Ontario Ministry of the Environment

Mosquito Control - Health Canada
Pest Management Information Service 1-800-267-6315

Human Health

Ontario Ministry of Health and Long-Term Care
Toll-free hot line - 1-877-234-4343

Health Canada

For more information:
Toll Free: 1-877-424-1300
Author: The Biosecurity and Health Committee includes members from: Canadian Pari-Mutuel Agency; The Horsemen's Benevolent and Protective Association of Ontario; Ontario Equestrian Federation; Ontario Harness Horse Association; Ontario Horse Racing Industry Association; Ontario Ministry of Agriculture, Food and Rural Affairs; Ontario Racing Commission; University of Guelph; Woodbine Entertainment Group.
Creation Date: 01 September 2003
Last Reviewed: 28 January 2016