Quebec City's director of public health, François Desbiens, says there's a chance his team won't be able to solve the mystery of where the outbreak came from. (Radio-Canada)

Quebec City's large outbreak of legionnaires' disease outbreak appears to be under control, the city's public health director said Friday.

Dr. François Desbiens said measures the city's public health department has taken to ensure cooling towers in the outbreak zone have been cleaned and disinfected appear to be bearing fruit.

But it is too soon to say the outbreak is over, he insisted in an interview.

"At this time, what we say is the outbreak is under control, because we don't have new cases with the beginning of the disease after Aug. 29," Desbiens said.

"But as there is an incubation period from two to 10 days and some people could have their disease [set in] 14 days or 20 days later — but this is rare — we prefer to wait... till we're there to be sure scientifically that the outbreak is completely terminated."

His office reported one additional case on Friday, bringing the total to date to 176 cases. Since the outbreak began in July, 11 people have died from the disease.

There have only been three cases this week, a significant slowing of the case count from earlier in the outbreak. And none of the people who have become ill started to show symptoms after Aug. 29, which is why Desbiens hopes the outbreak is on the wane.

Hope to pinpoint source

His office is also making some progress in their efforts to find the source of contagion, though that work is painfully slow. For that they can blame the microbe, which grows poorly and slowly in a laboratory setting.

Legionnaires' disease is caused by legionella bacteria. There are 40 species in the genus, but a subtype called Legionella pneumophila is generally responsible for legionnaires' infections. And it appears to be the cause of the Quebec City outbreak as well.

Samples from nine patients show the exact same bacteria — Legionella pneumophila serogroup 1, pulsovar A, Desbiens said.

Desbiens said water samples were taken from all of the 129 cooling towers in the outbreak zone in the Old Quebec area. Some have grown legionella bacteria that can be compared against the human samples. A provincial microbiology lab in Montreal is currently working to type those environmental samples.

There remains, however, a possibility that the investigators won't be able to match the bacteria from patients to bacteria from a cooling tower.

Many of the samples taken from towers have not produced bacteria. That could mean some weren't contaminated in the first place. But in some cases, it might be because the lab couldn't coax the bacteria to grow.

Desbiens said while he hopes his team will be able to solve the mystery, the most important thing is to stop the spread of the disease.

"I'm happy to say that the outbreak is under control," he said. "I hope we'll be able to find the place, to explain the situation. But what is most important for me, it's that the outbreak ceases."

Thrives in cooling towers

Legionnaires' disease manifests as a flu-like pneumonia that usually only sickens people with lung problems or weak immune systems. The legionella bacteria thrive in warm temperatures and can breed in the stagnant water of cooling towers before spreading in the mist released by them.

Many larger buildings use cooling towers as part of their air-freshening and ventilation systems.

The most fatal Canadian legionnaires' outbreak in the last 20 years was in Toronto, where a 2005 outbreak at a nursing home caused 21 deaths and made 127 people sick.

This summer's Quebec outbreak was all the more tragic because a report 15 years ago suggested ways to prevent it but was ignored, Quebec City Mayor Régis Labeaume has said.

With files from CBC News