“It can be extremely dangerous”: A microbiologist on the rise of invasive Strep A

Microbiologists are warning of a global spike in aggressive, sometimes life-threatening infections related to the bacteria that causes strep throat. In Canada, 327 cases of invasive group A streptococcal, or iGAS, were reported between August 2022 and February 11, 2023—nearly 100 more than average. These infections happen when the bacteria enters the skin, muscle tissue or blood stream and proliferates to cause dangerous conditions like toxic shock syndrome or necrotizing fasciitis. In some cases, they can fly under the radar until it’s too late: since October, over eight per cent of cases involving children and youth in Ontario have been fatal. 

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John McCormick, a molecular microbiologist at the Lawson Health Research Institute in London, Ontario, studies such infections. Here, he explains what he thinks is behind this surge, whether Canadians should be worried about the rise in cases, and how we can protect ourselves against infection.

Let’s start with brass tacks: what are iGAS infections, and how do they differ from strep throat?

Invasive group A streptococcal infections, which people also call Strep A infections, are caused by the same bacteria that causes strep throat. People are familiar with the latter: it often affects children, causes a fever, swollen tonsils and lymph nodes in the neck, and can be cured pretty easily with antibiotics like amoxicillin or penicillin. We do not think of the bacteria as particularly menacing. Ten per cent of children carry it in their throat at all times, but never develop symptoms. 

In rare cases, however, that same bacteria can creep through skin lesions or sores into a site  in the body with no microorganisms to combat it—like underneath the skin, in the blood, or inside an organ—and develop into an invasive infection. Then, it can become extremely dangerous. 

What happens when someone develops iGAS, and how would they know whether their infection is serious? 

The symptoms can progress rapidly in the first 12 to 24 hours. Infected people might first notice an extremely painful bruise forming under the skin or in a muscle, at the site of infection. The bruise might shift—I have heard of cases where physicians can take a sharpie, mark around the bruise and watch it move in the span of an hour. While I am not a clinician, I tell people that if someone sees a bruise on themselves that’s red, swollen, extremely painful and moving rapidly, they should go to the emergency room immediately.

What’s the usual prognosis? Are there any long-term effects?

These infections can cause necrotizing fasciitis, which basically kills your tissues. There are cases where a surgeon had to come in and remove tissue or even a limb to stop those infections because they can be so aggressive and can even attack the organs. IGAS can also lead to streptococcal toxic shock syndrome—a hyper-inflammatory reaction that can be fatal—and even meningitis in rare cases. 

Is it treatable? 

Yes. Penicillin usually works well to treat the infection, though it appears that the death rate of people with these infections in Canada hovers around 10 per cent. It is also my understanding that people who recover from it can sometimes walk away with lasting complications.

Are some people more at risk than others?

Studies show that children who’ve had chicken pox are 50 times more likely to develop invasive streptococcal infections—that is a huge risk factor. A recent study from the Netherlands showed that roughly half of children who developed an iGAS infection had had an episode of chicken pox prior to it. The reason for the link might be that chicken pox leaves small lesions on the skin, which provide an entry point for the infection. Other vulnerable populations include older adults, people who inject drugs, and transplant or chemotherapy patients with weakened immune systems. 

These infections have recently been spiking in Canada: we’ve seen double the cases in 2024 than we’ve seen over the last five years. Do we know why? 

There are two leading theories as to what is going on. First, there is this idea that, during the pandemic, kids gained less exposure to the bacteria because they spent much of their time at home. They never developed immunity to it. Now, they are more vulnerable to infections. 

The second idea is that the strains of this bacteria have mutated to become more aggressive over the last few years. Some strains that appeared in the U.K. seem to produce more of a toxin that increases the chance of invasive infection or toxic shock syndrome. Recent viral seasons like the flu and COVID-19 are probably contributing to the spike as well.

Have we seen these mutated strains in Canada? And are they contagious?

We have found mutated strains in Canada and are studying them in the lab to learn how to neutralize them. But it is still unclear how contagious they are, and how contagious plain strep throat is, for that matter. There have been studies done in classrooms that show that active strep throat infections are somewhat contagious. But, as I said, some people carry streptococcal bacteria at all times, and do not appear to transmit it to other people. On the plus side, we have found that streptococcal infections are only transmitted from human to human—it does not seem like one could contract the bacteria from animals, food, plants, or water.

Have we seen a spike like this one before? 

Yes, it’s important to note that this spike is not a complete anomaly. Case numbers were down in the pandemic years, probably because of our reduced social contact, but we saw a similar surge in 2019. In fact, we have consistently observed slight increases in cases during winters since the 1980s—which is likely when the strains mutated. 

What’s your level of concern here?

In Canada, a spike is not a cause for widespread panic: only one in 100,000 people become infected, and we have antibiotics to fight those infections. I am more worried about other parts of the world with antibiotic shortages like India, China, Australia, and New Zealand. Over there, people more commonly develop disorders from untreated streptococcal infections, like rheumatic heart disease that damages the heart valves. Altogether, invasive streptococcal infections are responsible for 300,000 deaths per year worldwide; that number would be lower if everyone had access to antibiotics.

Canada does have an amoxicillin shortage. Should we be concerned for years to come? 

I am more concerned about what people call the other pandemic—the antibiotic resistance crisis. Bacteria evolve over time to become resistant to the drugs we use to kill them. In our lab, we saw strands of bacteria trying to become resistant to penicillin, but they still have not succeeded. That is great news—if these bacteria become immune to our drugs, it will be a disaster. But I have not seen that happen yet. There is also good news: humans evolve, too, and over generations, we may also develop immunity to these infections. 

For now, what should people do if they think they have strep throat? 

If you have a very sore throat that lingers for a few days, see your physician, because it might be strep throat and should be treated with an antibiotic. Being proactive is important because if you have an active infection and are not taking antibiotics to treat it, you are  more likely to spread it to other people. 

Is there anything else we can do to dodge infection?  

There is no vaccine for invasive streptococcal disease; several labs, including ours, are working on developing one. If those become a reality, they will probably not go to Canadians; they will be sent to the places in the world that need them most. For now, I encourage people to vaccinate their children for chicken pox to decrease their chances of contracting a more severe infection. It is also helpful to practice good hygiene: refrain from putting your hands in your eyes or mouth at public places, wash your hands when you come home, and keep your child away from school or the workplace if he or she has strep throat.