WEST NILE VIRUS: Background Information

Click here for a PDF “print-friendly” version of this fact sheet

What is West Nile Virus?

West Nile Virus (WNV) is a mosquito-transmitted disease. The virus can infect humans, birds, mosquitoes and other mammals. Once infected with West Nile Virus, symptoms can progress to encephalitis and meningitis.

• WNV was first isolated in West Nile, Uganda, Africa in 1937.

• An outbreak of WNV in New York, during the summer of 1999, signalled the arrival of the virus in North America. The 1999 WNV outbreak in New York caused seven human deaths.

• WNV was first detected in Canada in the summer of 2001. Positive birds and mosquito pools were found in southern Ontario and the West Nile Virus Surveillance Program was initiated.

• In the summer of 2002, the first human cases were detected in Canada. In total, 340 human cases were confirmed, 319 of which were in Ontario.

• Since its arrival in 2001, WNV activity has continued in Ontario and has spread across most of Canada.