West Nile virus (WNV) is a relatively new disease in Manitoba. The virus is transmitted by mosquitoes. Most people who are bitten by an infected mosquito do not become ill and for those who do, the symptoms are usually mild. In some cases, the virus causes serious illness and sometimes death.
Human cases of West Nile virus were first detected in (southern) Manitoba in summer 2003.
The number of serious WNV cases in Manitoba has ranged from 1 to 72 per year since 2003. The number of WNV-related deaths per year has ranged from 0-4.
Most mosquitoes do not carry WNV. In Manitoba,the main carrier of the virus is the Culex mosquito, which is usually less common than other mosquito types.
The risk of WNV varies from year to year. The risk is influenced by precipitation, temperature, mosquito populations and many other factors. Manitobans are at highest risk of being bitten by a WNV-infected mosquito in July, August and early September, but exposures have occurred in June. Mosquito traps are monitored throughout the summer to help provide regular, ongoing information on the risk of West Nile virus.
West Nile virus was first identified in Africa in 1937 and after that, in Europe. It was reported in North America in New York City in 1999. Since then it has been identified in most of the U.S. and many parts of Canada. In Manitoba, WNV was first detected in birds of the crow family, horses and mosquitoes in July 2002.
Usually, people infected by West Nile virus have no symptoms and do not become ill. Of those who do become ill, most will develop West Nile Virus Non-Neurological Syndrome, an illness with symptoms such as fever, headache, fatigue and body aches.
Less frequently, the virus can cause more serious illness (West Nile neurological syndrome), including encephalitis, an inflammation of the brain. People with pre-existing medical conditions and older adults are more at risk for severe illness. However, illness has occurred in all age groups. Encephalitis can have serious complications. These complications may include weakness, paralysis, confusion, coma or death.
There is no vaccine or specific treatment for West Nile virus. Milder symptoms of West Nile Virus Non-Neurological Syndrome usually improve without medical care. Anyone experiencing severe symptoms (ex: persistent high fever, muscle weakness, headache) should seek medical attention promptly for diagnosis and care.
For specific advice regarding medical care, contact your health care provider. Or, you can call Health Links-Info Santé at 788-8200 (in Winnipeg) or toll-free 1-888-315-9257.
People can become infected if they are bitten by mosquitoes that have previously bitten infected birds. Even when WNV has been identified in an area, most mosquitoes do not carry the virus.
There are other less common ways that WNV maybe spread. Cases from blood transfusions and tissue transplants have been observed, but are rare. All donated blood is tested for the presence of WNV.
There is also evidence to suggest that pregnant women can pass the virus to their unborn babies, and that it may also be passed on through breast milk. There is some evidence that poultry workers exposed to a WNV outbreak among turkeys may have become infected with WNV.
You can protect yourself by minimizing your exposure to mosquito bites.
You can help reduce mosquito numbers by getting rid of mosquito habitats around your home.
You can help detect West Nile virus in your area by assisting with bird surveillance efforts
Reduce the time you spend outdoors between dusk and dawn. The peak mosquito hours are around dusk and dawn, but Culex mosquitoes will also bite during the night.
Wear light-coloured, loose-fitting, long-sleeved tops and long pants when outside, especially between dusk and dawn.
Apply an appropriate insect repellent, according to Health Canada Guidelines and label instructions. For more information, see the fact sheet Insect Repellents: It’s Your Health.
The fewer Culex mosquitoes that hatch, the lower the risk of WNV. Culex mosquitoes lay their eggs on standing water. Even small amounts of water allowed to stand for a week or more may produce adult mosquitoes.
To reduce the places around your home where Culex mosquitoes may lay their eggs, eliminate unnecessary standing water:
Adult mosquitoes like to rest in long grass and sheltered shady areas. You may reduce your exposure to mosquitoes by regularly maintaining these areas around your home. Steps you can take include:
For more information, see the fact sheet Reducing Mosquito Numbers Around Your Home.
Birds from the crow family (corvids) are not the main source of WNV in Manitoba. However, these birds are very susceptible to the virus. Many die soon after becoming infected. Testing dead crows and other corvids provides an early indication of the presence of infected mosquitoes in an area.
In Manitoba, dead corvids are no longer required as an early indicator of WNV because information collected in past years has confirmed when and where the virus will appear. Manitoba Health will continue to use other indicators, such as mosquito surveillance, to determine the human health risk of WNV in the province.
If you need to dispose of a dead bird, take the follow precautions:
For more information on how to dispose a dead bird, see the fact sheet Disposing of Dead Birds and Small Animals.
People who spend a lot of time outdoors for recreation (ex: camping, golfing) or work (ex: farming, construction) can follow the same basic preventive measures as anyone else (see How Can I Protect Myself?). These precautions are especially important in back country areas where there are large amounts of shallow, standing water.
Horses bitten by infected mosquitoes can become seriously ill and die. Younger domestic birds have also been affected by WNV.
Disease due to WNV has not been reported among cattle. Research has shown that weaning pigs may be susceptible to the virus. Cases of infected dogs and cats have been reported rarely.
For more detailed information on the West Nile virus and livestock operations, contact the Veterinary Services Branch of Manitoba Agriculture and Food in Winnipeg at 204-945-7652.
West Nile virus is an emerging disease. New information about the virus, and how to protect and control against it, continues to evolve.
Ongoing efforts to detect the presence of the virus in mosquitoes and people, combined with information and research from other jurisdictions, will help guide public health recommendations for mosquito control and other actions. For information about specific mosquito control activities in your area, contact your local municipality. For general information about mosquito control, call HealthLinks-Info Santé (see below).
Summer in Manitoba is an opportunity to enjoy all kinds of healthy outdoor activities. To protect yourself and your family from exposure to mosquito bites, you can follow the basic preventive measures outlined on this site when planning outdoor activities.
For more information on West Nile virus, please contact:
Health Links - Info Santé