West Nile virus

West Nile virus is a mosquito-borne infection. Most people who get West Nile virus experience no symptoms or only mild flu-like symptoms. But it can cause serious illness and there are few medical options to treat or prevent it. So it is important to learn more about it, what the risks are, and how you can reduce these risks.

The first human case of West Nile virus infection in Canada was reported in Ontario in 2002. Since then, cases have been reported so far in:

  • British Columbia
  • Alberta
  • Saskatchewan
  • Manitoba
  • Ontario
  • Quebec

Outside of these provinces, any reported cases have been linked to travel.

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What is West Nile virus?

West Nile virus is an infection that is carried by mosquitoes. It is found in many countries, including Canada and the USA.

The virus was first discovered in Africa in 1937. The first human case of West Nile virus disease in North America was reported in the USA in 1999.The first case was reported in Canada in 2002. Since then, it has continued to occur in various regions of Canada during the late spring, summer and early fall months.

How is West Nile virus spread? What are the risks?

Most people get West Nile virus when an infected mosquito bites them. Mosquitoes are infected with the virus when they bite infected birds. In most parts of Canada, the risk of becoming infected with West Nile virus is greatest between mid-April and the first hard frost in late September or October. Most human infections occur between July and early September.

In a very few cases in the past, people got West Nile virus from blood transfusions, with organ transplantation, or through a mother-to-child transfer. If you need a blood transfusion you can be assured that the risk of getting West Nile virus in this way is extremely low. Canadian Blood Services and Héma-Québec screen blood donations in case the donor is carrying the virus unknowingly.

You cannot catch West Nile virus by:

  • touching or kissing a person with the virus
  • touching nurses or caregivers who treat someone with the virus
  • giving blood

What are the symptoms of West Nile virus infection?

Most people who are infected with West Nile virus (70%-80%) do not notice or report any symptoms. Of the rest, most have mild symptoms that usually appear 2 to 15 days after infection. These include:

  • fever
  • headache
  • mild rash
  • body aches

Very few people (fewer than 10% of those with symptoms) have severe symptoms that affect the central nervous system (nerve tissues in the brain and spinal cord). These may include:

  • very bad headache
  • fever
  • stiff neck (you might have trouble moving your neck side-to-side)
  • nausea or vomiting
  • difficulty swallowing
  • drowsiness
  • blurred vision or worsening eyesight
  • confusion
  • muscle weakness and reduced coordination

Those at higher risk for serious health effects from West Nile virus include:

  • people over the age of 50
  • people with chronic diseases, such as cancer, diabetes, alcoholism or heart disease
  • people who require medical treatment that may weaken their immune system, such as those who have received transplants or are taking chemotherapy for cancer

How is West Nile virus treated?

There is no special treatment for West Nile virus infection. Patients are treated for their symptoms.

Mild symptoms

If you have mild symptoms (such as a low fever and body aches), you will likely get better in a few days. In some cases of mild illness, recovery times can be somewhat longer (weeks or months). Over-the-counter pain relievers can be used to reduce your fever and discomfort.

Severe symptoms

If you have more serious symptoms (such as high fever or vomiting), you should see a health care provider right away. A blood test can confirm whether West Nile virus is present.

Serious cases are treated with supportive therapies including fluids, breathing support, and prevention of complications. Hospitalization or nursing care may be necessary.

West Nile virus is a relatively new disease in North America. Its long-term effects are not fully understood. Most people, even those with serious symptoms and health effects recover completely, while others experience ongoing health problems. These problems may include:

  • physical effects, such as long-term muscle weakness and paralysis, fatigue and headaches
  • mental effects, such as confusion, depression, problems with concentration and memory loss
  • functional effects, such as difficulty with preparing meals, going out, shopping

Scientists do not yet know why some people recover while others continue to have varying degrees of health problems.

How can I prevent West Nile virus infection?

The easiest way to prevent West Nile virus infection is to avoid mosquito bites. Remember that mosquitoes are often most active at dawn (first light) and dusk (just before dark).

Cover exposed skin

When you are outside wear:

  • long pants and loose-fitting shirts with long sleeves
  • socks and a hat (try a mosquito net over your hat to protect your head, face and neck)
  • light coloured clothing (mosquitoes are attracted to dark colours)

Use insect repellent

When you are outside, you should use insect repellents (bug sprays and lotions) that contain the chemicals DEET or Icaridin. DEET (diethyltoluamide) is sold as a colourless oily liquid preparation that keeps away mosquitoes and ticks. Icaridin is a newer repellent that was approved by the Canadian Pest Management Regulatory Agency in 2012.

When using bug spray, you should always read the entire label and follow the directions on the container. To avoid getting insect repellant in your mouth or eyes, try spraying it into your hands first. Then rub it onto your face.

Be especially careful when you put bug spray on children. Always spray your own hands first, then apply it to their skin.

Get rid of mosquito breeding areas

Mosquitoes breed in standing water (water that doesn't move or flow). Get rid of standing water around your home by following these tips:

  • drain or dry off water in old tires (even tire swings), rainwater barrels, children's toys, flowerpots and wading pools
  • clean eavestroughs regularly to prevent clogs that trap water
  • tip fishing boats and gear onto their sides to drain
  • replace the water in outdoor pet dishes and bird baths at least two times a week

Keep mosquitoes out of your home

Put screens on your windows and doors to keep mosquitoes out of your home.

How can I know where West Nile virus infections occur?

Surveillance means checking to see if West Nile virus has been found in people, birds and mosquitoes. During the West Nile virus season (mid-April to October) the Public Health Agency of Canada conducts ongoing surveillance across Canada. The Agency produces weekly Maps & Stats: West Nile MONITOR.

Report dead birds

Dead birds in your community may be a sign of the presence of West Nile virus. If you find a dead bird, do not handle the body with your bare hands. Always wear rubber gloves when touching any dead bird or animal. Contact the Canadian Cooperative Wildlife Health Centre to report dead birds that you suspect have West Nile virus.

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