Health Professionals Update – Vol. 19, No. 16
From:
Dr. Nicholas Brandon, MD, MA, MSc, CCFP, FRCPC, Acting Medical Officer of Health
In this issue
- Legionellosis activity and situational update
- Legionellosis clinical considerations
- Testing and reporting
- Prevention and facility actions
Key messages
- Increase suspicion for legionellosis in patients presenting with pneumonia.
- Use appropriate laboratory testing, including respiratory specimens when possible.
- Report cases promptly to support investigation and control.
- Review and maintain building water systems ahead of peak season.
Legionellosis activity and situational update
The Ontario Ministry of Health is currently monitoring Legionella clusters in Hamilton and Toronto, indicating an early start to seasonal activity, although cases occur year-round and typically peak in summer. Provincially, incidence averages at approximately 2.1 to 2.6 cases per 100,000 population annually (about 350 cases per year), with most cases occurring sporadically. In 2025, 75.7% of cases required hospitalization and 7.3% resulted in death, highlighting the potential severity of illness.
In Peel region, 31 cases (1.85 cases per 100,000) were reported in 2025, with trends consistent with provincial patterns.
Legionellosis clinical considerations
Legionellosis is acquired through inhalation of contaminated aerosolized water from sources such as showers, cooling towers, hot tubs, fountains, humidifiers and other aerosol-generating water systems, and is not transmitted person-to-person. Incubation period is typically 2 to 10 days (up to 19 days).
Clinical presentations include:
- Legionnaires’ disease: pneumonia with fever (> 39 C), cough, shortness of breath, and possible gastrointestinal or neurologic symptoms.
- Pontiac fever: milder, self-limited influenza-like illness without pneumonia.
Higher risk groups include:
- Adults 60 years and older.
- Individuals who smoke.
- Those with COPD, diabetes, renal disease, malignancy, or immuno-compromising conditions.
- Those who work around large building water systems like cooling towers.
Testing
Maintain a high index of suspicion for legionellosis in patients presenting with community-acquired pneumonia, particularly in those with compatible risk factors or severe illness.
The optimal diagnostic approach includes urinary antigen testing (UAT) for rapid detection of L. pneumophila serogroup 1 and collection of lower respiratory specimens for PCR/culture, where possible. UAT detects the most commonly identified serogroup but does not detect less common serogroups or Legionella species. Collection of respiratory specimens for culture is critical, as culture allows for identification of additional species and serogroups and enables linking of clinical isolates to environmental samples during outbreak investigations. Refer to Public Health Ontario (PHO) Laboratory Services for additional information.
Reporting
Legionellosis is a reportable disease. Report confirmed and suspected cases to Peel Public Health (905‑799‑7700, fax: 905‑565‑1456) and include a detailed exposure history for the 14 days prior to symptom onset to support timely public health investigation and follow-up.
Prevention and facility actions
Healthcare providers and organizations are encouraged to be aware of Legionella risks and ensure appropriate preventive measures are in place. This includes working with facility management and infection prevention and control teams, where applicable, to support the maintenance and monitoring of building water systems ahead of peak season.
Facilities should follow recognized standards, including:
- ASHRAE Standard 188-2018 (Risk Management for Building Water Systems)
- ASHRAE Guideline 12-2023 (Managing the Risk of Legionellosis Associated with Building Water Systems)
To support awareness in health care settings, consider sharing the following resources with IPAC and facility teams:
- PHO rounds: Legionella Risk Management and Investigations in Health Care Facilities
- PHO rounds: Legionella in Healthcare Settings: When Risk Becomes Reality
Contact us
The Health Professionals Update (HPU) is distributed to physician offices in Peel region via email. Share this update with other health professionals in your clinic.
More information:
For questions about this update, email healthprofessionals@peelregion.ca.