As health officials in London, Ont., try to understand what's behind an outbreak of invasive group A streptococcal infections that began more than 18 months ago, the Public Health Agency of Canada (PHAC) says "most Canadian jurisdictions" have seen increased cases in recent years.

But most are not considered "outbreaks," as those generally occur within a specific population and usually involve people who have had close contact with one another.

On Monday, the Middlesex-London Health Unit said it had received reports of more than 132 cases of invasive group A streptococcus infection since April 1, 2016, and nine people had died.

Almost a third of patients had two of the most serious infections associated with streptococcus: streptococcal toxic shock syndrome and necrotizing fasciitis, also known as "flesh-eating" disease.

The situation in London is serious, said Dr. Isaac Bogoch, an infectious disease specialist at University Health Network in Toronto, but it appears to be a local outbreak.

It's also important to understand that group A streptococcus is a common bacteria that lives in the backs of people's noses and mouths or on skin wounds, Bogoch said. Many of us have the bacteria on our skin or in our bodies and we never even know it.

When it does make us sick, it's often in the form of strep throat — a relatively mild infection, he said.

So what's happening in London?

"The issue here is not all group A streptococcus is created equally," Bogoch said. "Some strains of the bacteria are more invasive than others. This appears to be a more invasive strain."

Invasive group A streptococcus can go deeper in the body — including the bloodstream, muscles and joints — and cause life-threatening infections like flesh-eating disease.

Both Bogoch and PHAC emphasize that in the majority of cases, group A streptococcus does not cause severe illness.

Infectious disease specialist Dr. Isaac Bogoch says group A streptococcus is a common bacteria that lives among us all the time. (CBC)

In London, about half of the people affected by the outbreak have been living in vulnerable situations, including "injection drug users and/or those without access to stable housing," Dr. Gayane Hovhannisyan, associate medical officer of health, said in a statement.

"Yet there has also been an increase in ... infections among people who have no connection, and are not related, to the outbreak in people who inject drugs or who are under-housed," she said. "We need a better understanding of what's happening, which is why we've issued this alert."

Vulnerable populations

According to PHAC, there have been "several" outbreaks of invasive group A streptococcal infections in various parts of the country since early 2016, and they seem to be unrelated to one another.

Most outbreaks don't "typically" happen in the general population, the agency said in a statement to CBC News, but in "high-risk populations such as those living in crowded housing."

Toronto Public Health identified a group A streptococcus outbreak at Seaton House, a men's shelter, in early 2016. It was officially declared over in October 2017, "after many months of effort at controlling the spread," Toronto associate medical officer of health Dr. Michael Finkelstein said in an email.

During that outbreak, there were 67 confirmed cases — but unlike in London, only nine of them were the "invasive" strain, he said.

There are no current outbreaks of invasive group A streptococcal infections in Toronto, Finkelstein said.

People who live in shelters or crowded housing or who are homeless are at higher risk of getting sick, Bogoch said, because group A streptococcus is transmitted through direct contact with the bacteria — including coughing, sneezing or touching the wound of someone who is infected.

Vulnerable populations may also be less likely to seek or have access to medical care promptly when they do get infected — a critical factor, because the bacterial infection is actually not hard to diagnose or treat, he said.

"People can get really sick really fast with invasive group A streptococcal infections, and they should be seen right away," Bogoch said.

The standard treatment is penicillin, he said — or other antibiotics if the patient is allergic. In severe infections, like flesh-eating disease, surgery might be necessary.

What should people do to protect themselves?

PHAC says "there is no simple explanation" for the increase in invasive group A streptococcal infections in recent years.

But there's no reason to panic, Bogoch said, and the steps to prevent and treat infection are simple.

One of the most important factors is awareness — and that's why London officials issued the alert, he said. If health-care professionals know that a patient showing signs of infection has been in an area where there is an outbreak, they can keep that in mind while determining a diagnosis. In addition, they can offer antibiotics to people who have been in close contact with patients who have been diagnosed with invasive group A streptococcus to prevent them from becoming infected too, Bogoch said.

Similarly, if people living in an area where an outbreak has been declared have signs or symptoms of infection, they know they should seek medical attention immediately.

Bogoch also recommends the following for all Canadians: