Early seasonal increase in legionellosis activity in Ontario: clinical vigilance recommended

To: Ottawa Physicians and Nurse Practitioners  
Date: May 20, 2026          
From: Dr. Emil Prikryl, Associate Medical Officer of Health
Subject: Early seasonal increase in legionellosis activity in Ontario: clinical vigilance recommended

Dear Colleagues,

The Ministry of Health has advised public health units that two concurrent clusters of Legionella in Hamilton and Toronto are being actively monitored. These clusters suggest an earlier-than-expected increase in Legionella activity this season. Although legionellosis cases in Ontario typically peak in July, cases can occur year-round. In 2025, 75.7% of confirmed legionellosis cases in Ontario required hospitalization and 7.3% resulted in a fatal outcome.

Given higher-than-usual provincial case counts at this time of year, we recommend a high index of suspicion for legionellosis in patients presenting with pneumonia, particularly among adults over 60 years of age and others at increased risk of severe disease.

Laboratory diagnosis

If legionellosis is suspected, in addition to urine antigen testing, please submit a lower respiratory tract specimen for culture whenever possible. A clinical culture isolate is critical to support genetic linkage to environmental isolates during source investigations. While the urinary antigen test is commonly used, it detects only Legionella pneumophila serogroup 1. It does not detect infections caused by non-serogroup 1 Legionella species or serogroups and cannot support genetic sequencing.

Public Health Ontario has information on diagnostic testing under the letter “L”.

Background on Legionnaires’ disease

Legionnaires’ disease is a severe form of pneumonia caused by the inhalation of aerosolized water containing Legionella bacteria. Following an incubation period of 2–10 days (commonly 5–6 days, but up to 19 days), symptoms can include fever, non-productive cough, shortness of breath, anorexia, malaise, myalgia, headache, confusion, and sometimes diarrhea or abdominal pain. Hospitalization is common with case-fatality rates of 10%. More clinical information on risk factors, diagnosis, and treatment can be found through the Centers for Disease Control and Prevention (CDC) (English only).

Reporting to Ottawa Public Health

If you have a suspected or confirmed case of legionellosis, it should be reported immediately:

• Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224 and leave a detailed, confidential message including your contact information; or fax 613-580-9640. For more details on how to report, please visit Reporting a communicable disease.
• After hours, weekends, or holidays: Call 3-1-1 and ask to speak to Public Health on call.

Thank you for your continued vigilance and partnership in protecting the health of our community.