Deadly legionnaires' outbreaks in London, Ont., bring role climate change may play to the forefront

There's heightened emphasis on the role climate change may play in the spread of legionnaires' disease as London, Ont., deals with its second deadly outbreak since the summer of 2024.

Some scientists point to the Earth's warming temperatures and changing weather patterns — some of the signals of climate change — as playing a big role in accelerating the growth and spread of legionella, the bacteria that cause the serious respiratory condition.

The Public Health Agency of Canada (PHAC) says legionella infection rates have increased dramatically, particularly between 2004 and the present. Experts also say most infections are never reported.

"We're seeing these increases in cases, but is that because we're doing better at finding the cases or is there something happening out there that's causing more increases in growth or dispersion of the bacteria?" said Juliette O'Keeffe, a Vancouver-based senior scientist with the National Collaborating Centre for Environmental Health.

In the case of the London outbreaks that have led to the deaths of six people, an industrial cooling tower in the city's east has been identified by the Middlesex-London Health Unit (MLHU) as the likely source.

Legionella are waterborne bacteria that thrive in warm, moist and stagnant conditions. When aerosolized, these bacteria can be inhaled.

Inhalation sometimes causes legionellosis, which ranges from a mild illness commonly called Pontiac fever to legionnaires' disease.

O'Keeffe believes her research has made some progress in identifying potential explanations for the rapid spread of legionella.

"Temperature is definitely a driver. If you have more warm days, you're going to have more conditions that are favourable for legionella growth," she said, pointing to between 25 and 45 C as ideal.

Warm temperatures can also promote the spread of the bacteria, Okeeffe said, noting most outbreaks are linked to cooling systems.

Higher temperatures mean more hours spent running cooling systems, and needing to build more of them.

Other climate factors also come into play when it comes to the spread of legionella, including humidity — more cases are reported in hot and humid areas of Canada, such as southwestern Ontario and the East Coast.

"Legionella is dispersed from cooling towers in aerosols — tiny water bubbles that are dispersed over an area. When you have a humid environment, they persist better," O'Keeffe said.

A Health Canada report released in 2022 lists legionella multiple times as having strong potential to be affected by climate change.

There's also research that has established a link between some of the signals of climate change (like the warming climate, precipitation and humidity) and spread of the bacteria.

For its part, the MLHU mentioned the changing environment as a concern during a recent interview with CBC News, when the health unit reported the link between London's legionnaires' outbreaks and the cooling tower of Sofina Foods Inc.

Legionnaires' disease and the spread of legionella also are a concern in the United States, said Joan Rose, the Homer Nowlin Chair in water research and the director of the Water Alliance at Michigan State University.

Legionella is "one of the most important waterborne pathogens we're dealing with now," said Rose, who has been studying water microbiology for some 40 years and noted concerns in line with O'Keeffe's.

"Outbreaks have been increasing, but more importantly, cases [in general] have been increasing," she said. "Of course, we're getting better at diagnostics, but we believe this is a true increase in the incidence of the disease."

Rose and O'Keeffe both said more frequent rainfall could play a role.

Research shows that rain events are correlated with legionellosis outbreaks in some areas, said O'Keeffe. She said while the reasons for that aren't clear, she suspects it's related to the movement of water.

"Legionalla is naturally occurring in groundwater and surface water. When you have these events, it mobilizes the bacteria, stimulates them to start growing."

To better control growth and spread of legionella, Rose and O'Keeffe suggested more research on effective disinfection practices and tighter regulations.

Quebec, New Brunswick, Hamilton and Vancouver, for instance, have strict registration and safety requirements for cooling tower operators, while Ontario is among jurisdictions that do not.

"We don't have a lot of regulations that look at how we monitor, then how we report the information on this bacteria," Rose said, echoing sentiments other experts have shared with CBC.

"That's one of the main things that we can do," said O'Keeffe. "This isn't really all that new. We know that cooling towers are a source. We know that maintaining them well is a way to reduce exposures and to keep that the water clean."