Education Materials

Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.
Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.

Frequently Asked Questions

» What is West Nile Virus?

West Nile virus is spread by the bite of an infected mosquito, that have feasted on the blood of infected birds, and can infect people, horses and some other animals. Symptoms usually appear 10 to 14 days after someone has been bitten by a mosquito that has the virus. Most people who become infected with West Nile virus will have either no symptoms or only mild ones. However, West Nile virus infection can sometimes result in severe and fatal illnesses. Although most people fight off the virus, about one in 150 suffer serious illness, primarily meningitis (swelling of the lining of the brain) or encephalitis (swelling of the brain).

» If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than one per cent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one-mosquito bite are small.

» Who is at risk of being infected?

In general, wild birds are the largest population infected with the West Nile virus. Birds do not normally show signs of infection – they merely act as a host for the virus, however, humans can also become infected with the virus. People living in an area where there are virus carrying mosquitoes are at risk of getting bitten. However, not everyone who is bitten by an infected mosquito will develop West Nile-most cases are very mild; with flu-like symptoms or no symptoms at all.

At risk are:

  • the very young
  • individuals >50 years of age
  • individuals with weakened immune systems
  • individuals with other medical conditions, including diabetes, heart disease, or individuals on dialysis.

» What can I do to reduce my risk of becoming infected with West Nile virus?

Please read our FAQ on "How to Avoid Mosquito Bites" for information on how to reduce your risk of getting West Nile virus.

» Can West Nile virus be transmitted through blood transfusions and organ transplants?

Yes, there have been confirmed cases in Canada and the United States of West Nile virus being spread through blood transfusions and organ transplants. The risk of getting West Nile virus through these procedures is very low, and many believe that the benefits of these procedures outweigh the risk of becoming infected with West Nile Virus. If you are scheduled to receive a blood transfusion or organ transplant, you may wish to discuss your concerns with your doctor or surgeon. [Source: Health Canada]

» Can West Nile virus be transmitted by donating blood?

No. Donating blood is a safe procedure and people cannot become infected when they volunteer to donate blood. If you are experiencing symptoms of West Nile virus infection (or another illness), you will be temporarily deferred from donating blood in order to enhance the safety of the blood supply. Canadian Blood Services is grateful to those who donate blood in order to save the lives of their fellow Canadians.

» Can a pregnant woman pass West Nile virus to her unborn baby?

There has been one known case in the United States of a mother passing West Nile virus to her unborn baby. In this case, the baby was born with serious medical problems. However, it is not known whether the problems were caused by West Nile virus or by other factors. Since evidence shows it is possible to pass West Nile virus to an unborn child, pregnant women should take immediate steps to reduce their risk of mosquito bites if they are in an area where West Nile virus is active. If they develop symptoms of West Nile virus infection, they should call their doctor or health care provider right away. [Source: Health Canada]

» Can West Nile virus be passed to a child through breast milk?

There is evidence that this is possible. In one case in the United States, a woman was infected with West Nile virus after the birth of her child. She breast-fed her baby, and the baby later tested positive for West Nile virus infection. Laboratory tests determined that West Nile virus was present in the breask milk. Investigators looking into the case believe it is unlikely that the baby was infected by a mosquito bite. They concluded that break milk was the most likely source of infection. Despite being infected with West Nile virus, the child had no symptoms and remained healthy. Women who are breast-feeding their children should contact their doctors for advice if they have symptoms of any illness, including West Nile virus. [Source: Health Canada]

This website has been made possible through an unrestricted educational grant from Pfizer Canada Inc.
Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.