Legionnaires' outbreak grows in London with the source still a mystery

Officials with the Middlesex-London Health Unit (MLHU) are still working to identify the source of a legionnaires' disease outbreak as the number of people getting sick continues to grow.

One person has died since the outbreak was declared on July 26, and 23 others have contracted the respiratory illness that can lead to fevers, chills, a dry cough, shortness of breath and pneumonia. Four people are currently in hospital, the health unit said Wednesday. 

It explained that it's sampled 10 cooling towers at larger industrial buildings in southeast London and found evidence of the legionella bacteria in three of them, said Dr. Joanne Kearon, acting medical officer of health.

She would not give any further specifics around location and says all systems where the bacteria was found have been sanitized. 

"In outbreaks such as what we're seeing right now, given the spread across a distance, typically we are looking at larger systems for where that legionella may be sourced," she said. "Very often, it's linked to cooling towers that provide cooling systems for larger buildings. It's not related to an indoor exposure, we are looking at larger industrial outdoor systems."

The bacteria is usually found in water environments like hot tubs, cooling towers, large plumbing systems or parts of an air-conditioning system. If it's misted into the air through wind or fans, people can breathe it and become unwell. It's not transmitted between people and symptoms usually present within two to 10 days after encountering the bacteria.

Large radius to analyze

MLHU maps where patients who have contracted the bacteria have travelled, including their home and work addresses.

In this outbreak, everyone was moving around within five kilometres of one another in southeast London. 

"This allows us to narrow down the field of where we are going to be testing for legionella. However, a five-kilometre radius is still quite a large geographic area and so that does mean that the investigation may take some time," she said.

"Legionella is common in the environment, both natural and man made systems. Most cases of legionella over time are not actually linked to an outbreak and are what we call sporadic cases. We typically receive a handful of [sporadic] cases here in London every year."

Keaon said she was unable further narrow down where these cases were found, citing an ongoing investigation. She added that providing a specific source wouldn't help people understand their personal risk, given how the airborne disease spreads.  

The screened samples taken from the cooling towers have been sent to Public Health Ontario labs, where they will be cultured or grown to see if there's a match with human samples. Cultures can take ten days to produce and it's possible the source may never be found, Kearon said. 

The health unit will provide a further update on the outbreak on Monday.