A deadly outbreak of streptococcal infections that's killed 10 people in northern Ontario is likely to spread but doesn't pose a widespread threat because it appears to be clustered around distinct groups of people, the province's chief medical officer of health said Thursday.

"It doesn't seem to be showing and spreading throughout the community there but spreading within a very specific subgroup of the community," Dr. David Williams said a day after Thunder Bay's public health unit announced the deaths.

"It seems to be a very limited part of the community... Some related with homeless, some with an intravenous drug user group, some related with some urban populations we're trying to follow-up on.

"For the province at wide, I would say no, it's not a big concern."

The city's health officials also said they've treated another 75 people with invasive Group A streptococcal infections over the past year.

"We are obviously interested - I am, as the chief medical officer of health - of how well they're doing up there on this one, and with the uniqueness of the situation, with the unique community issues," Williams said. "I'll be interested to hear how they'll approach it as a community action."

Cases of Group A strep began surfacing in the Thunder Bay area in late summer 2007, which wasn't particularly unusual for an infectious disease that's known as the cause of strep throat.

But cases continued to rise, growing beyond the normal six cases and one death per year.

When the local health unit recognized an influx of more severe cases in February, officials began their probe and found about half of the cases belonged to a strain never seen before in the province, said health unit epidemiologist Lee Sieswerda.

Called emm-59, the strain was already known in the western provinces. Starting in 2006, a smattering of victims across the West became infected with the bug that's now linked to more than 300 cases in the region, said Greg Tyrrell, director of Edmonton's National Centre for Streptococcus.

"This is a type we really haven't seen much in Canada in the past. We don't know why," Tyrrell said, adding the centre is conducting studies to learn more.

At its worse, the Group A streptococci bacterium causes life-threatening illnesses such as necrotizing fasciitis - commonly known as flesh-eating disease - and streptococcal toxic shock syndrome.

In Thunder Bay, the infected group ranges in age from the mid-20s to the mid-50s and is comprised of about 60 per cent men. About 38 per cent are classified as intravenous drug users and another 15 per cent are "underhoused," Sieswerda said. Although he didn't know the breakdown of the dead, he said they ranged in age from 39 to 77.

"The risk to the general public is very low," Sieswerda said, adding he stands by the health unit's decision to not notify the public until this week.

Health officials have worked to contain the outbreak by providing intensive followups, tracing the closest contacts of the infected people and increasing street outreach, he said.

But Sieswerda, Williams and a prominent microbiologist all agree there's a strong possibility the infection will continue to spread.

"We've seen it in Western Canada, we're seeing it move east," said Dr. Donald Low, chief microbiologist at Toronto's Mount Sinai Hospital.

Low said he wouldn't be surprised if the infection migrated by the end of October or November.

"There's a lot of movement of individuals from city to city, so there may well be an opportunity for this thing to hitchhike along with somebody and be introduced into a new community of individuals that are susceptible," he said.

Health Ministry officials were made aware of the outbreak in May.

NDP Leader Howard Hampton called the provincial government "negligent" for not informing the public of the outbreak.

"What it demonstrates is a failure for the McGuinty government to take public health and public health protection seriously," he said.

"You do no harm when you inform the public, especially when you inform the public and then provide the public with information about what precautions should be taken."

Hampton has called for a review of whether the Health Ministry met its obligations regarding the outbreak under the Health Protection and Promotion Act.