MLHU - Health Status Resource

West Nile Virus

West Nile Virus

Key Findings: 

Ontario public health units routinely monitor sites within their jurisdictions for the presence of mosquitos infected with West Nile virus, as one of several measures to reduce the risk of this vector-borne illness being transmitted to humans. Between 2005 and 2018, the number of mosquito pools that were positive for West Nile virus was relatively low in the Middlesex-London region, with no sustained increasing or decreasing pattern. At the same time, the burden of human illness due to West Nile virus infections was also low, with fewer than 35 cases reported among Middlesex-London residents across the entire 14-year time frame.

The low occurrence of West Nile virus positive mosquito pools and human cases may be due in part to mosquito control efforts that are undertaken by the Middlesex-London Health Unit each year. As weather patterns evolve each year, and as climate changes over time, ongoing public health prevention efforts are needed locally and across the province to continue to mitigate the risk of West Nile virus infections.

West Nile virus vector and human case surveillance

View more information about West Nile virus mosquito surveillance and control or West Nile virus infection.

From 2005 to 2018, the number of positive mosquito pools in the Middlesex-London region fluctuated, with some years where no mosquito pools tested positive for West Nile virus. The greatest number of positive mosquito pools was 17, reported in 2012. Overall, there was no sustained pattern of increase or decrease in the number of positive mosquito pools reported across the 14-year time frame (Figure 14.4.1).

Between 2005 and 2018, the number of West Nile virus infections reported among Middlesex-London residents was low, with a total of 34 confirmed and probable cases. The annual number of West Nile virus infections varied, with no cases reported in some years, and peaking in 2018 at 12 cases (Figure 14.4.1).

2012 and 2018 were the years in which both the number of West Nile virus positive mosquito pools and reported human cases were highest in Middlesex-London. However, across the 14-year time period there did not appear to be a strong association between positive mosquito pools and human cases, as there were several years in which West Nile virus cases were reported among Middlesex-London residents but no positive mosquito pools identified, and vice versa (Figure 14.4.1).

Interpretive Notes

In addition to surveillance for human cases of and mosquitos carrying West Nile virus, the Middlesex-London Health Unit vector-borne disease program also conducts routine surveillance for black-legged ticks infected with Lyme disease. The program also monitors the occurrence of mosquitos causing Eastern Equine Encephalitis and Zika virus, although no locally-acquired human cases of either disease have been reported in Ontario or Canada.

The number of human cases reported in these analyses likely underestimates the true occurrence of West Nile virus infections. This is because most people infected with West Nile virus do not experience symptoms, and those who do may experience only mild symptoms1 and not seek care from a health care professional.

Since 2002, provincial and national analyses of West Nile virus infections have included both confirmed and probable cases. The analyses shown here align to the provincial and national reporting standards and include infections that meet either the provincial confirmed or probable case definition.

How this Indicator was Calculated: 
Ontario Public Health Standard: 

Ontario Public Health Standards: Requirements for Programs, Services, and Accountability – Infectious and Communicable Diseases Prevention and Control (pages 42-47)
Healthy Environments and Climate Change Guideline
Population Health Assessment and Surveillance Protocol, 2018


1. Government of Canada [Internet]. Ottawa (ON): Government of Canada; [modified 2019 Nov 4]. Symptoms of West Nile virus; [modified 2015 Jun 26; cited 2019 Nov 4]; [about 2 screens]. Available from:

Last modified on: November 19, 2019

Jargon Explained

Mosquito pool
A mosquito pool is a group of female mosquitos of the same species. A mosquito pool can range in number from one to 50 mosquitos. Mosquito pools are identified in traps that are set by Ontario public health units within their jurisdictions.

Positive pool
A positive pool is a mosquito pool that has tested positive for West Nile virus.