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

Table of Contents

1. Disease Information
2. Human 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 central nervous system (encephalitis).

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 encephalitis with death occurring in a small percentage of patients. People at greater risk generally are the elderly, and those who are immune compromised. Transmission of WNv is mainly via bites from infected mosquitoes. There is also evidence of WNv transmission through blood transfusions and organ transplants. Infected horses are NOT a source of infection for humans, unless nervous tissues (brain, spinal cord) or cerebrospinal fluid are handled.

| Top of Page |

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, grinding teeth and end in either humane euthanasia or death. The disease can progress from an animal being normal, and working at its peak of performance, to recumbent within 12 hours. Approximately 30% of cases die or are euthanized. Horses of all ages, breeds, and health status are susceptible to WNv. Infected horses do not pose a threat to other horses or to human health. (See above.)

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 is present in mosquito pools and birds in every county of the province of Ontario.

| Top of Page |

Section 5: Prevention

• Ensure your horse is properly protected by using the West Nile virus vaccine. The initial vaccination for WNv requires two injections 3 - 6 weeks apart. A booster just prior to the peak risk period of August-October will further enhance immunity. Vaccinations should begin 2 months before the start of the mosquito season. Incomplete vaccination during the season may not be protective.
• 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

The Ministry of Health and Long Term Care is the lead agency in Ontario for WNv in humans. West Nile virus in animals has now been named a 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 Veterinary Teaching Hospital and the Animal Health Laboratory, the Ontario Ministry of Agriculture and Food will collect and analyze 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.

| Top of Page |

Committee's Recommendations

1. Horse owners should ensure that their horses have completed a vaccination program (initial and booster shots) for WNv at least 1 month prior to mosquito season.
2. Track 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

West Nile Virus - Ontario Ministry of Agriculture, Food and Rural Affairs

Pesticides and Mosquito Control - Ontario Ministry of the Environment

Health Canada - Pest Management Regulatory Agency (PMRA)
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

| Top of Page |