1 dead, over 40 sick as legionnaires' outbreak spreads in southeast London, Ont.

One person is dead and more than 40 people have become ill in an outbreak of legionnaires' disease in London, Ont.

The Middlesex-London Health Unit, which declared the outbreak, said the cases were reported within the last week. Most of the people with the severe respiratory illness live and/or work within a six-kilometre radius of one another, in the southeast side of the city, officials said. 

On Tuesday, officials said the health unit is working closely with Public Health Ontario and the Public Health Ontario Laboratories to investigate and try to determine the source of the outbreak, adding the investigation may take time as they need to do environmental samples of many types of cooling systems. 

"I know that is what most people are anxious to understand, is where it is coming from, and so are we. Unfortunately, we don't yet have a location," said Dr. Joanne Kearon, the health unit's associate medical officer of health. 

"Our teams go out to actually collect water samples and swabs from cooling systems and then send that to the lab to compare it to clinical samples from people who have become ill to see if we are able to find the source."

The health unit has received 43 reports of Legionella this year, with a few cases reported each month, which Kearon said is expected. However, cases rose to more than 30 last week, prompting officials to declare the outbreak, she added. 

"Given the weather we've been having, it's understandable that it has spiked," she said. We compare it to what we would expect every season, and this exceeded significantly what we have seen previously."

This is the second outbreak of legionnaires' disease in London since last summer. Last year's outbreak was localized in the city's east end and led to two deaths and 30 total reported infections. 

"At this time, it is not known whether it [the current outbreak] is the same location as the legionnella outbreak from last year," the health unit said. "However, this is being considered as part of the ongoing investigation." 

Bacteria found in water sources

Legionnaires' disease is caused by legionella bacteria, naturally found in water sources such as hot tubs, cooling towers, hot water tanks, large plumbing systems or parts of air-conditioning systems. 

Legionella is not transmitted from person to person. If the bacteria are aerosolized or misted into the air (via wind or fans), people may inhale the bacteria and become unwell. Most people exposed to legionella don't get ill, but some may experience Pontiac fever, a mild, flu-like illness that commonly resolves itself.

Individuals who are older, have lung problems or are immunocompromised (have a weak immune system) are at greater risk of serious infection.

While she didn't have specific information about the individuals who contracted the illness, Kearon said there's a wide age range of people who've been affected, and those over 50 are at a higher risk. 

Health unit officials were not able to determine the exact source of the outbreak last year.

The 2024 outbreak came to a natural end by late August as most cooling systems turn off around that time. The health unit predicts that will be the case this year as well. 

How outbreaks start

Legionella bacteria can usually be found in faucets or HVAC units, but ultimately there needs to be an epidemiologic investigation to find the common source, according to Dr. Isaac Bogoch, an infectious disease specialist at Toronto General Hospital.

He noted there can be sporadic cases from the soil, but most outbreaks are associated with contaminated water systems and buildings. The disease is not transmitted through human contact. 

"It's not in the air — it's in water — but you can usually pinpoint it to a common source," he said. "So for example, like an HVAC system in a particular building, you look for where all those people who are infected had a common source of contact."

Health officials are able to detect the disease more easily as diagnostics have improved in recent years and more people are aware of it, but Bogoch said that doesn't necessarily mean legionnaires; disease is on the rise. 

Anyone who develops symptoms such as fever, cough and shortness of breath is encouraged to seek medical attention.

All business owners and property managers should ensure all cooling equipment is regularly maintained and sanitized as per the manufacturer's instructions. 

Although masks can decrease the risk of infection, the health unit is not recommending regular community-wide use of them specifically for the prevention of legionnaire's disease, as the public risk is low.