Why is it so hard to find the source of a legionnaires’ outbreak? – CBC

From sampling to testing, the bacteria behind the illness presents many challenges

Sep 03, 2025

It took months of mapping cases, dozens of locations sampled and weeks of lab sequencing to determine the likely culprit behind a recent outbreak of legionnaires’ disease in London, Ont. — one that killed four people and infected about 100 others.

It’s just the latest example of why it’s so difficult to pinpoint where the illness is spreading from.

Last week, health officials with the Middlesex-London Health Unit (MLHU) said the likely source of the outbreak was the cooling towers of a local meat-processing plant in the city’s east end. Health officials also determined the facility was likely linked to another outbreak last year that left two people dead and 30 others sick.

And in New York’s Harlem neighbourhood this summer, it also took weeks of meticulous searching to identify the cause of a legionnaires’ outbreak there, now linked to cooling towers from a hospital and a nearby construction site.

Read more: https://www.cbc.ca/news/health/legionnaires-outbreak-london-source-1.7621817