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Biosecurity and Health Committee:
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| Author: | Biosecurity and Health Committee: Canadian Pari-Mutual Agency; The Horsemen's Benevolent and Protective Association of Ontario; Ontario Harness Horse Association; Ontario Horse Racing Industry Association; Ontario Ministry of Agriculture and Food; Ontario Racing Commission; University of Guelph; Woodbine Entertainment Group. |
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| Creation Date: | 01 September 2003 |
| Last Reviewed: | 01 September 2003 |
West Nile virus (WNv) is a mosquito-borne disease that can cause inflammation of the central nervous system (encephalitis).
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.
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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.)
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.
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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.
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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
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