Legionella Clusters, Early Season Case Increases and Resource Reminders

Article

May 22, 2026

Currently, there are two unrelated Legionella clusters in Ontario, one in Hamilton (external link) and one in Toronto (external link). These clusters are occurring earlier in the year than expected, as cases are typically highest in the summer months, although they can occur year-round.
The Legionella bacteria that causes Legionellosis is commonly found in the environment. The disease is not spread person to person, but occurs through the inhalation of droplets containing the bacterium.
Infection can range from mild flu-like symptoms (i.e., Pontiac fever) to more serious lung infection or pneumonia (i.e., Legionnaires’ disease), which can require hospitalization and may be fatal.

Legionellosis symptoms include high fever, chills, shortness of breath and a cough. Medical examination and testing are needed to determine whether someone has pneumonia caused by the bacteria.
In 2025, 75.7% of confirmed legionellosis cases in Ontario required hospitalization and 7.3% resulted in a fatal outcome.
Clinicians should maintain a high index of suspicion for legionellosis in patients presenting with pneumonia, particularly among older adults (aged 50 years and older), and (but not limited to (external PDF)) individuals with weakened immune systems such as organ transplant patients, lung conditions, cancer, diabetes and a smoking history (current or former).
First-line treatment for Legionnaires’ disease in the community setting (external link) is a macrolide or a respiratory fluoroquinolone. Pontiac fever, on the other hand, is a self-limited illness that lasts about a week and does not benefit from antibiotic treatment.
Promptly report legionellosis cases to Halton Region Public Health by calling 311, no later than the next business day.

PHOL testing options for Legionnaires’ disease include PCR and culture (external link) (sputum) and Urine Antigen Test (UAT) (external link) using a urine specimen.
While commonly used, UAT only detects L. pneumophila serogroup 1 and does not detect infection caused by non-serogroup 1 or allow for sequencing.
Submitting a sputum culture is critical, as clinical culture isolates are required to make genetic linkages to environmental cultures during source investigations.
For hospitalized patients, specimens should be collected during the acute phase of illness, as close to admission as possible. Antibiotic treatment should not be delayed to collect specimens.