Canada: Confirmed Legionnaires’ Disease Cases Reported in the Middlesex-London Region in July; Outbreak Source Remains Unknown

On July 8, the Middlesex-London Health Unit (MLHU) in Ontario declared an outbreak of Legionnaires' disease in the City of London. As of July 29, 70 confirmed cases have been reported, with most affected individuals living or working within a six-kilometer radius of the city's southeast side. In the current outbreak, 91 percent of cases (64 cases) required hospitalization.

The outbreak's source remains unknown, but officials suspect a common source like a contaminated water system. On July 23, Legionella bacteria were detected in Victoria Hospital's cooling towers, which lie within the six-kilometer outbreak zone. Testing is underway to determine if the strain matches the outbreak strain; however, officials stress this does not confirm the hospital as the source of the community outbreak.  What is Legionnaires’ Disease?

Legionnaires' disease is a severe form of pneumonia caused by Legionella bacteria, which spread through inhaling aerosolized water droplets. The bacteria occur naturally in soil and warm water. They can thrive in water systems such as hot water tanks, cooling towers, air conditioning systems, and spas.  

The disease is not transmitted from person-to-person. Most healthy people exposed show no symptoms or only mild, flu-like illness, but the risk of severe illness increases for those aged over 50, smokers, and people with pre-existing health conditions.  

Symptoms typically appear 2-14 days after exposure, including cough, fever, headaches, muscle aches, and shortness of breath. Due to the ubiquitous nature of Legionella bacteria, not all outbreaks are traceable. The source is not found in about 50 percent of Legionnaires' disease outbreaks.  

Canada has seen several Legionnaires' disease outbreaks often tied to contaminated cooling towers with notable outbreaks in Toronto in 2005, Quebec City in 2012, and Orillia in 2022. These events underscore the risks associated with aging infrastructure and insufficient water system monitoring. In response, some provinces have introduced measures such as cooling tower registries and stricter regulations to prevent future outbreaks.  

While the risk to the general public remains low, the current situation in the MLHU highlights the need for continued vigilance in public health surveillance and environmental management, especially as warmer temperatures and changing climate conditions may increase the likelihood of Legionella proliferation.  

This represents the most complete data available as of Aug. 1. 

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