Causes of West Nile virus

Explore the cause of West Nile virus, how it is spread and where it is found.

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What causes West Nile virus?

A bite from an infected mosquito can cause infection with West Nile virus. Infected mosquitoes can spread the virus to humans and other animals.

How is West Nile virus spread?

West Nile virus is usually spread by the bite of a mosquito that has fed on an infected bird.  In very rare cases, West Nile virus can be spread through:

  • blood transfusion
  • organ or tissue transplants
  • pregnancy (from mothers to unborn babies)
  • breast milk
  • exposure of laboratory workers to infected medical specimens

You cannot get West Nile virus by:

  • donating blood
  • touching or kissing a person with the virus
  • touching nurses or others who care for someone with the virus

Canadian Blood Services and Héma-Québec routinely test blood donations in Canada for the presence of West Nile virus. This is done to avoid accidental infection through blood transfusion.

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