08/28/2012 06:51 EDT | Updated 10/28/2012 05:12 EDT

Finding Legionnaires' outbreak source like 'looking for a needle in a haystack'

TORONTO - Public health investigators trying to find the source of Quebec City's legionnaires' outbreak have a tough job on their hands, experts say.

Imagine trying to home in on a source of an outbreak when the bug you are seeking is found all over — even in puddles of rain water on city streets. When it comes from a large family of bacteria, you need to distinguish the outbreak type from all its relatives.

Add to that the fact that building owners and maintenance companies may be scurrying to clean up cooling towers to ensure they aren't spewing bacteria-laced water droplets. So by the time investigators arrive to do an inspection the evidence of contamination may be gone.

These are some of the challenges that may be facing the Quebec City team as they struggle to pinpoint how a total of 126 people, as of Tuesday, have become sick in the centre of the historic city. To date, eight people have died.

Legionnaires' outbreaks like this one — in a community rather than in a confined space like a hospital or a hotel — can be particularly difficult to crack, suggested a Legionella expert from the U.S. Centers for Disease Control.

And Dr. Lauri Hicks, who works in the CDC's respiratory diseases branch, said sometimes an answer can't be found.

"In a community outbreak, the number of potential sources can be too numerous to count. It can be like looking for the proverbial needle in the haystack," Hicks said in an interview Tuesday from Atlanta.

She recalled a recent U.S. outbreak where the final answer evaded the investigating team, which included experts from the CDC. Hicks wouldn't disclose where the outbreak occurred and would only say it happened in the past couple of years.

"There are certainly community outbreaks where we've been involved where the source hasn't been identified — or there have been multiple potential sources identified and it's just not clear which is the most likely one," she said.

Cooling towers were thought to be the source of the bacteria in the outbreak Hicks referred to. And public health officials in Quebec City have also fingered cooling towers as the likely culprit in this case.

As of Monday, they had inspected more than 100 in the identified zone of infection, a large swath covering most of Quebec City's lower town, near the provincial legislature. They plan to revisit 30 later this week.

They are looking for Legionella bacteria. Legionnaires' disease is caused by Legionella, generally — though not always — Legionella pneumophila.

In fact, there are 40 species of Legionella bacteria, said Cyril Guyard, a molecular biologist with Public Health Ontario who is an expert in this field. And within those species there are serogroups. Think of them as families within clans.

The bacteria are found in water sources all over. But outbreak investigators aren't looking for just any Legionella bacteria; they are looking for the type behind this outbreak.

"When you go looking and you cast a wide net — which is probably the right thing to do — you can find a lot of positives because it is so common in the environment," Hicks explained.

"So just because you find it in the environment doesn't necessarily mean that that is the source for the outbreak."

Sifting through all the findings of the bacteria involves a lot of testing and a lot of time, Guyard said.

"You need to find Legionella. Then you need to find the right species. Then you need to find the right serogroup. And then you need to find the right fingerprint. So that can explain why it's so challenging to find the one that matches to the patient," he said.

"I'm not saying it's not doable. ... But it's technically challenging."

Any hunt like this is further complicated by the fact that Legionella bacteria aren't easy to isolate from patients. Without samples from patients, public health laboratories can't be sure which of the bacteria they find in the environment are to blame for the outbreak.

Even if labs do get samples that will grow, the bug grows slowly. It can take 10 days before specimen cultures produce bacteria to test.

"These are not easy investigations to perform. It requires a lot of time, a lot of resources and co-ordinated investigation on both the part of the environmental health folks as well as the epidemiologists," Hicks said.

And while the investigators look and the labs attempt to grow and type and compare bacterial samples, the environment isn't static.

When there's a legionnaires' outbreak, it's common for building owners or managers to disinfect cooling towers, both Hicks and Guyard said.

If an outbreak stops because of that type of response, public health officials can hardly complain. But Guyard admitted that, as a scientist, it can be frustrating because the chance to find the source and add to the knowledge about Legionella may be lost.

"Sometimes answers are not found," he said.

Note to readers: This is a corrected story. An earlier version said 128 people had become sick in Quebec City.