Legionellosis Cluster Investigation

Key Messages

• Hamilton Public Health (HPH) is investigating a cluster of legionellosis cases in East Hamilton/Stoney Creek.
• PHS is recommending health care providers have heightened awareness for persons with symptoms of legionellosis, particularly those at higher risk.
• If legionellosis is suspected, HPH is requesting collection of both a lower respiratory tract specimen for culture testing and a urine specimen for urinary antigen testing if possible, particularly for hospitalized patients.

Context

As of May 11, 2026, 19 cases of legionellosis have been reported to HPH since May 4, 2026, residing in East Hamilton/Stoney Creek. The cases presented with symptoms of community acquired pneumonia such as fever, chills, malaise, cough, shortness of breath and confusion and has resulted in 19 hospitalizations. HPH continues to assess possible sources of exposure, however no common source has been confirmed.

Recommendations

HPH recommends health care providers have heightened awareness of the risk of legionellosis among those living in or who have visited East Hamilton/Stoney Creek with the following risk factors: aged 50 and over, weakened immune systems such as organ transplant patients, individuals with lung conditions, cancer, diabetes, and those who smoke.

Best practice for laboratory diagnosis of legionellosis includes the collection of both lower respiratory specimen(s) and urine antigen test. Urine antigen testing allows for the rapid diagnosis of L. pneumophila Serogroup 1, while respiratory specimens are essential, as culture supports linking cases and identifying potential environmental sources.
 

Where possible, particularly for hospitalized patients suspected of legionellosis, please collect:

• Legionella respiratory PCR and culture: collect a lower respiratory tract specimen such as bronchoalveolar lavage (BAL), bronchial wash, sputum, pleural fluid, or lung tissue during the acute phase of illness.
  • Where admitted to hospital these specimens should be collected as close to admission as possible, as well as before or as close to initiation of antibiotics to increase test sensitivity. Antibiotic treatment should not be delayed to collect specimens.
• Legionella urinary antigen testing: collect urine specimens (2.0 ml) during acute phase of illness.

Specimens should be sent in a sterile container and include symptoms and onset date on the Public Health Ontario lab “General Test Requisition Form”.

Additional Resources

• Legionella – Respiratory PCR and Culture | Public Health Ontario 

Contact

Report reportable diseases by calling Hamilton Public Health, Infectious Disease Program at 905-546-2063, Monday through Friday 8:30 am to 4:30 pm. After hours, on holidays and on weekends please call 905-546-2063 and ask to speak to the on-call ID Public Health Nurse