The Middlesex-London Health Unit is declaring an outbreak of Legionnaires’ Disease in the City of London.
Within 2025, mostly over the last week, 43 cases have been reported to the Health Unit, including one fatality. Most of the cases live and/or work in a 6 km radius of one another in the southeast side of the city.
What is Legionnaires’ Disease?
Legionnaires’ disease is a serious respiratory illness caused by Legionella bacteria. Symptoms typically develop two to ten days after exposure and may include:
- High fever
- Chills
- Dry cough
- Shortness of breath
The infection can lead to pneumonia and, in severe cases, may be life-threatening.
Legionella bacteria are naturally found in water sources such as hot tubs, cooling towers, hot water tanks, large plumbing systems or parts of air-conditioning systems. If the bacteria are then aerosolized or misted into the air (via wind or fans), people may inhale the bacteria and become unwell. Most people exposed to Legionella do not become ill, though some may experience a milder illness called Pontiac Fever, which resolves itself. Individuals who are older, have lung problems or are immunocompromised are at greater risk for serious infection. Legionella is not transmitted from person-to-person.
What is Being Done?
The Health Unit is working in close partnership with Public Health Ontario and the Public Health Ontario Laboratories to investigate and identify the source of the outbreak. Environmental sampling of potential sources of Legionella is ongoing. Since Legionella can be found in many types of cooling systems, the investigation may take time. Sometimes in investigations like this one the source of the bacteria is never definitively found, though the outbreak may naturally end.
Recommendations
If anyone develops symptoms (fever, cough, shortness of breath),they are encouraged to seek medical attention as needed.
The Health Unit asks all business owners and property managers to ensure that all cooling equipment is regularly maintained and sanitized as per the manufacturer’s instructions. This is the best way to reduce the risk of further legionella outbreaks.
If you’re a cooling tower operator, learn about prevention here: www.healthunit.com/health-hazards#legionella.
For guidance on cleaning potential sources of legionella in residential settings, please visit: www.healthunit.com/uploads/legionnaire_info_sheetv2.pdf
Frequently Asked Questions
1. Will masks work (Surgical vs. N95)?
While face masks can decrease your risk of infection with respiratory illnesses, including Legionella, the public risk related to legionella is low. Therefore, the Health Unit is not recommending regular community-wide face mask use specifically for the prevention of Legionnaire’s disease.
2. Why are we giving a general area for the outbreak and not a direct location?
Legionella is able to aerosolize and travel significant distance. The source of the Legionella spread has not yet been found.
3. Is it the same location as last year?
At this time, it is not known whether it is the same location as the Legionella outbreak from last year. However, this is being considered as part of the ongoing investigation.
For more information about Legionnaires’ Disease, visit: https://www.healthunit.com/legionnaires-disease.