Map of Simcoe Muskoka

Infectious Diseases

West Nile virus

West Nile virus (WNv) is a disease caused by the virus with the same name. It is spread to humans by mosquitoes that have fed on the blood of infected birds. Many people infected with West Nile virus have mild or no symptoms. People with weaker immune systems and people with chronic diseases are at greater risk for serious health effects from WNv such as meningitis, encephalitis and acute flaccid paralysis. For more information, see the health unit’s fact sheet on West Nile virus.

The Simcoe Muskoka District Health Unit conducts surveillance for WNv in our area by testing mosquito larvae and adults. For more information about these activities, see the health unit’s fact sheet on West Nile virus surveillance.

Simcoe Muskoka
Technical Notes

Simcoe Muskoka

West Nile virus is a rare disease both locally and provincially. There have between zero and seven confirmed and probable cases in Simcoe Muskoka every year since 2002. In 2021, no cases of West Nile virus disease was reported in Simcoe Muskoka.



The following graph shows the incidence rate of West Nile virus disease in Simcoe Muskoka and Ontario between 2002 and 2021. The first year of disease activity in Ontario was 2002 with almost 400 cases in the province. The number of cases declined significantly in Ontario in subsequent years but increased once again in 2011 and 2012. In 2021, the incidence rate of West Nile virus in Simcoe Muskoka was zero cases per 100,000 population and the Ontario rate was 0.2 cases per 100,000 population.

The increase observed in 2017 is likely due in part to the warm and wet summer which created favorable conditions for both the virus and mosquitos in the environment.


More detailed data for Ontario and each health unit can be found on Public Health Ontario’s interactive Reportable Disease Trends in Ontario tool.

Technical Notes

There are many factors that influence how many cases are reported to the health unit, as explained in the Technical Notes of the Infectious Diseases page. It is unclear to what extent the global COVID-19 pandemic has influenced the reporting and spread of other infectious diseases – counts and rates of disease since 2020 should be interpreted with caution.

West Nile virus became reportable in the province of Ontario in 2003.

Page last updated August 19, 2022