Legionnaires' outbreak in London over but source is still a mystery

The Middlesex-London Health Unit (MLHU) has declared the end of the legionnaires' disease outbreak that killed three people and infected 70 people, landing many in hospital.

It's the second year in a row that the legionella bacteria has made people sick in London, and again, the source of the

This year's outbreak, which was declared on July 8, spanned a six-kilometre radius across the city and included the region's largest hospital. In mid-July, testing of Victoria Hospital's cooling towers came back positive.

In addition to those tests, the MLHU said it tested 26 other cooling tower sites, but could not find a match.

Legionnaires' disease is caused by legionella bacteria, naturally found in water sources. If conditions are right, it can contaminate 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.

Last summer, the MLHU declared an outbreak that killed two people and infected 30 others.

The Canadian government says the average number of reported cases of legionnaires' disease in this country is generally under 100 per year, but the actual number of cases may be much higher, as many people with pneumonia may not be tested for legionella.

This year, the MLHU said 91 per cent of cases, or 64 people, were hospitalized, with one person still being treated.

The health unit said Wednesday that the search for the source of the bacteria will continue.