Legionnaires’ disease outbreak has sickened 40 people in Ontario. Here is what you need to know

This 2009 colorized 8000X electron micrograph image provided by the U.S. Centers for Disease Control and Prevention shows a large grouping of gram-negative Legionella pneumophila bacteria. (THE CANADIAN PRESS/AP-Janice Haney Carr)

A recent outbreak of Legionnaires’ disease in London, Ont. is raising the profile of the bacterial infection, with some wondering whether it can pose a risk to the greater public.

The illness, which is caused by Legionella bacteria, manifests as a severe form of pneumonia. Common symptoms include fever, cough and shortness of breath.

Infectious disease physician Dr. Isaac Bogoch explains the bacterial infection lives in nature, which means it can be found in soil and water. However, he stresses that the illness doesn’t spread from person to person and can only be contracted by breathing it in by way of human-made systems.

“This is usually acquired by inhaling the bacteria,” he tells CTV News. “You get sporadic cases, but oftentimes you can get outbreaks. And the reason you get outbreaks is sometimes the bacteria can get into HVAC systems, water coolers and water towers.”

How common is it?

While rare, there have been cases of the illness spreading through indoor environments like hotels or offices, via HVAC systems, cooling towers or air conditioning.

Data released by Public Health Ontario (PHO) in May showed that there were 363 confirmed cases of the virus in 2024, which was roughly in line with the annual average of 354.

The rates across much of the GTA were all under 1.9 cases per 100,000 people. n 2024, the Middlesex-London Health Unit (MLHU) reported the highest rate of legionellosis at about 7.7 cases per 100,000 people.

How is it treated?

The infection is treated with the same common antibiotics used for pneumonia.

When a case of Legionella is identified, there will be a public health effort to trace where it was potentially acquired. This is to verify if there are other cases and what potential common source led to the outbreak.

“If there are people in the same place like a shopping centre, hotel or church, that are all getting pneumonia, you can start to look for potential contaminated air conditioning, HVAC systems, etcetera,” Bogoch says.

He adds that there currently isn’t an outbreak of Legionnaires disease in Toronto. While the illness appears infrequently, Bogoch says it’s also not uncommon for cases to occur.

The risk of Legionella is especially high for older adults, smokers, those with chronic lung disease or anyone who is immunocompromised.

When does it occur?

Residents can be exposed to the Legionella bacteria at any point but Public Health Ontario says that most cases occur between June and September, with July seeing the largest volume of cases.

PHO says that last July the positivity rate peaked at 6.4 per cent before trending downward. It says that the proportion of confirmed cases that resulted in hospitalization in 2024 was 76.6 per cent. About five per cent of cases resulted in death in 2024, according to PHO.

What happened in London?

The health unit in London, Ont., said this week that more than 40 cases of legionella have been confirmed within a six-kilometre radius, with one case resulting in death.

The Middlesex-London Health Unit has said it doesn’t know the source of the outbreak but still considers the risk to the public to be “low.”

“Most people who are exposed to legionella bacteria will not become infected or develop symptoms. Risk factors for Legionnaire’s disease including being elderly, immunocompromised or having underlying lung conditions,” a spokesperson told CTV News earlier this week.

Why is it called Legionnaires disease?

The name Legionella stems from when the bacterial infection was first characterized in 1976, after many people became ill during a Legion convention in Philadelphia, leading to an epidemiologic investigation.