Why deadly, invasive strep A infections are surging in Canada and beyond

This story is part of CBC Health's Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven't subscribed yet, you can do that by clicking here.

When Colin Hunter got a sore throat and a fever last spring, he didn't think much of it.

A week later, after packing and unpacking cardboard boxes, the middle finger of his right hand started to feel a bit irritated. By mid-afternoon, it was swollen. And by early evening, just hours later, it was clear something was very, very wrong.

"It went from nothing to, by about 5 p.m., a big plump grape on the end of my finger, turning black, swollen around the fingernail on all sides, and throbbing with each heartbeat," Hunter recalled. "That's when I went to the emergency room."

Physicians at his local hospital feared it was a case of necrotizing fasciitis, also known as flesh-eating disease. So they cut open his finger, ran some tests and came back with a surprising diagnosis: He had a strep A infection.

Hunter had never heard of such a thing in someone's hand. Then he realized his prior throat pain and fever might have been an undiagnosed case of strep throat — and that the bacteria could have gotten inside his finger through something as small as a paper cut.

The 47-year-old Guelph, Ont., resident ended up spending five days on intravenous antibiotics. During his hospital stay, physicians said the infection was likely spreading through his bloodstream, marking the route along Hunter's arm with a black marker.

He fully recovered after treatment in the hospital and another round of antibiotics at home. But what haunts Hunter now is what would have happened if he hadn't sought medical help as soon as he did.

"I might not have kept my finger," he told CBC News. "Or my life."

Fast-spreading and potentially deadly strep infections — known as invasive group A strep, or iGAS — are on the rise in Canada, hitting a new high in 2023, as CBC News first reported last week.

Provinces have also reported dozens of deaths in just the last few months, including at least 10 children across Ontario and B.C. Other countries, such as the U.K. and Japan, have also reported notable surges.

The big question is: Why? Why are we seeing a spike in these serious infections, both in Canada and abroad?

Scientists say there's no single smoking gun, but rather a complex set of factors, some of which surfaced even before the COVID-19 pandemic.

"We don't fully understand that, at this point," said Dr. Jennifer Guthrie, an assistant professor of microbiology and immunology at Western University in London, Ont. "And it's probably become more pressing to understand that recently with the rise."

Rates were quietly rising for years

Group A streptococcus is a family of bacteria that typically causes milder illnesses, such as painful strep throat, skin infections or scarlet fever. In those cases, antibiotics usually clear things up.

The invasive form occurs when strep bacteria enter the soft tissues or the bloodstream. "It's the highway to every single point in your body," said Michael Johnson, an associate professor in the University of Arizona's department of immunobiology.

Though relatively rare, those iGAS infections often progress rapidly, leading to a range of serious illnesses ranging from toxic shock syndrome to sepsis to brain inflammation. Among the various types of bacteria that can make humans sick, this family probably causes more kinds of diseases than any other, Johnson noted.

"It does not play well with others."

Most estimates suggest around one in 10 people die from iGAS, at times within 12 to 24 hours of being infected.

WATCH | Severe strep infections on the rise:

Severe strep A infections are on the rise

10 days ago

Duration 2:51

Doctors say they’re seeing significantly more invasive strep A infections, caused by a bacteria that usually leads to milder illnesses. But in some cases it can enter the bloodstream and quickly become deadly.

And rates of those infections were quietly rising for years, research suggests. One Canadian study, published in September 2022, used a national disease surveillance database to look at rates of iGAS between 2009 and 2019.

The team, including University of Toronto researcher and infectious diseases specialist Dr. Allison McGeer, found the incidence of those infections went from four cases per 100,000 people to a little more than eight — doubling in just a decade.

The latest tally of 4,600 iGAS samples processed for 2023, provided to CBC News by the Public Health Agency of Canada, suggests the rate could now be as high as 12 cases per 100,000 people.

McGeer told CBC News there are multiple theories about the increase.

The family of bacteria behind strep-related illnesses often thrives in congregate settings, including long-term care facilities and the shelter system, since it spreads through close contact, including open wounds and airborne droplets from someone coughing, talking or sneezing.

So there's speculation that rising levels of homelessness and intravenous drug use in Canada could mean more people living in situations "where you have more breaks in the skin and issues with accessing good hygiene," leaving those populations vulnerable to strep infections, McGeer said.

Dr. Ivan Ying, an infectious diseases physician with Mackenzie Health, a hospital network north of Toronto, said global travel may be an additional factor, given the massive spike in international tourism observed in the 20 years leading up to the pandemic.

Group A strep is also a diverse family of bacteria, with hundreds of subtypes. The most common ones circulating in any given season tend to ebb and flow, scientists say, and they're linked to different types of disease.

"I think travel, and close settings, and different strains crossing the border might have something to do with [the rise]," Ying said.

The U.K., for instance, observed a new lineage of group A strep prior to the pandemic that "does appear to be associated with some increased severity," McGeer said. (That lineage has also been reported in Canada.)

"It was on its way up," McGeer said. "Then we got the pandemic."

LISTEN | Senior health and medical reporter Lauren Pelley on the rise of invasive strep A:

Front Burner19:53Invasive strep A: what you need to know

'Perfect storm' of respiratory, bacterial infections

That's when the strep situation got even more complex.

Public health restrictions put in place to curb transmission of SARS-CoV-2, the virus behind COVID-19, had a ripple effect on other pathogens, too. Influenza disappeared for more than a year. Multiple countries reported reduced rates of pneumococcal disease.

And group A strep "was down by 50 or 60 per cent for the next year-and-a-half," McGeer said.

Then it started to re-emerge, along with a variety of other infectious diseases. At the same time, Guthrie noted, vaccination rates for both COVID and flu petered out.

Several scientists agreed it may be a sheer numbers game, where higher rates of other illnesses — including old staples like flu and respiratory syncytial virus (RSV), as well as COVID-19 — are providing strep bacteria with a massive pool of people already weakened by other infections.

"It's a perfect storm of having a lot of these viruses also co-circulating that also leave people vulnerable to strep A," said Guthrie.

WATCH: Warnings about invasive strep A infections across Canada:

Health authorities across Canada are warning about a sharp increase in cases of invasive streptococcus A

9 days ago

Duration 4:44

Last year, 4,600 cases of invasive group A strep (iGAS) infections were recorded in Canada — a record number. Alberta Health says rates of iGAS have been increasing since 2017, with 768 confirmed cases reported in 2023.

In B.C., for instance, the province's four recent strep-related deaths among children involved two cases where group A strep was identified alongside influenza, and another two where it was identified alongside another viral respiratory illness, human metapneumovirus.

One piece of the puzzle may be the fact that run-of-the-mill strep infections can damage the cells of the upper respiratory tract, noted McGeer.

Another may lie in changes to the immune system during an acute viral infection.

"There is no question that having increases in viral infections are associated with increases in complicated bacterial infections, to some degree," she added. "Any two infections tend to be worse than any one infection."

No strep A vaccine yet

Underpinning all those factors? The lack of a strep A vaccine.

"Group A strep is something we should, God damn it, have a vaccine for," said McGeer. "Because it's on the list of severe diseases where you can get really sick, really quickly — even if you're perfectly healthy — and you can't do anything about it."

Researchers around the world have been trying to make a successful vaccine against strep A for decades, but there are major challenges to getting one to market.

One Canadian-Australian team that includes researchers at the University of Alberta is in the midst of early clinical trials for its vaccine candidate, and clinical lead Dr. Vanessa Meier-Stephenson was upfront about the complexities at play.

A major issue, she explained, is that researchers need to identify and track all the strains that cause severe disease — which could number upwards of 230 — while also watching for fluctuations in the strains that are most common in the population.

WATCH: Spike in invasive strep A infections reported in 2023:

Concerning rise in cases of invasive form of strep A

9 months ago

Duration 2:22

Doctors are seeing an increase in the number of patients experiencing an invasive form of group A strep. The illness is common but can be serious when bacteria spreads to the blood or deep tissue, and needs to be treated quickly.

On top of that, the arrival of strep A bacteria can sometimes spark a faulty immune response,where the body produces antibodies that, instead of targeting the virus, attack the body's own tissues. That inflammatory reaction leads to autoimmune conditions such as rheumatic fever, which can cause joint pain, swelling and damage to the heart valves.

Thanks to antibiotic use that cuts short most strep infections, those impacts are now much more rare in countries like Canada. But there's concern strep vaccines could mimic that mechanism and lead to a similar autoimmune response in some recipients.

"There's a huge safety component that gets associated with that," Meier-Stephenson said.

Still, she and other scientists are hopeful that a strep A vaccine will eventually reach Canadians — if there's enough public interest and pharmaceutical support.

In the meantime, multiple clinicians say they expect strep A infection rates will slow down eventually and that invasive forms of disease will remain relatively rare.

"It'll probably rise a bit, but at a certain point stabilize," said Guthrie. "But I'd love to know for sure."

The next issue of CBC Health's Second Opinion will soon be in your inbox.



Discover all CBC newsletters in the Subscription Centre.