Dr. Francois Desbiens said measures the city's public health department have 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 with The Canadian Press.
"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.
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 bug, which grows poorly and slowly in a laboratory setting.
Legionnaires' disease is caused by Legionella bacteria. There are 40 species of Legionella but a subtype called Legionella pneumophila is generally responsible for Legionnaires' infections. Specifically, Legionella pneumophila serogroup 1 is the most common cause of the disease.
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.
"We have nine of the same pulsovars, so maybe we'll able to find the same pulsovar in one of our cooling towers," he said. "We're still waiting for the results to come to us from the lab."
(A sample from a 10th patient revealed a different version of the bacteria, but Desbiens said that can happen. "We can have an isolated case of Legionella inside a major outbreak.")
Desbiens said water samples were taken from all of the 129 cooling towers in the outbreak zone; some have grown Legionella bacteria that can be compared against the human samples. Quebec's 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."
"I have to protect the health of the people from an infectious disease and it has worked. Now let's see if the lab will be able to show me where it was. It would be good to know it. But it's not essential to have it to (bring) the outbreak under control."