Strep infections are most well known for causing strep throat, which can be treated with antibiotics. But this type of bacteria can also cause more serious illnesses—even flesh-eating infections.
The bacteria responsible for the more serious infections are on the rise. In a report published April 7 in JAMA, researchers led by those at the U.S. Centers for Disease Control and Prevention (CDC) found that cases of invasive group A Streptococcus have been steadily increasing in the U.S. over the last decade, from 2013 to 2022. The data also showed that the bacteria are resistant to some commonly used antibiotics.
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Here’s what to know about the rise in strep infections.
What is group A strep?
Group A Streptococcus bacteria can cause illnesses from the common strep throat to sepsis, toxic shock syndrome, and necrotizing fasciitis (more commonly known as flesh-eating bacterial infections). Cases of strep throat aren’t considered invasive, but if the bacteria leave the throat and venture into more sterile areas of the body, such as the blood and the lungs, then more serious diseases can occur. “Not all group A strep are the same,” says Dr. Robert Frenck, director of the Vaccine Research Center at Cincinnati Children’s Hospital and chair of the section on infectious diseases at the American Academy of Pediatrics, who was not involved in the study.
The study found that most of the invasive cases of strep were caused by seven to eight types of group A strep. “The different types circulate at different times,” says Frenck. “You can go for years without certain types circulating, and then all of a sudden, they will come back.” For reasons that aren’t fully understood, “a lot of it is cyclical.”
How common is strep A?
In another recent study, Frenck and his team found that 15% to 20% of children had the bacteria in their throat, and most didn’t have any symptoms. If people are colonized—the medical term for carrying bacteria without being sick—and then develop skin lesions or wounds, their chances of developing an invasive case of strep could increase.
“If you are someone who has breaks in their skin, or skin sores, and you have the strep bug in your throat, that provides an opportunity for the bacteria in your throat to get in your skin if you touch your mouth, and then through [the skin] into the bloodstream,” says Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center and a co-author of the new JAMA study.
Why are strep A infections increasing?
To explain why a given infection is rising in a population, health experts generally look to two factors, says Schaffner. First, they study whether the bacteria are changing in any way to become more severe and better at causing human illness. (That research is currently underway, and it’s not clear yet whether this is the case.)
Second, they look at who is being infected to understand what risk factors might be contributing to infections. The study, which included data from more than 34 million people in 10 states, found that most of the infections were in certain groups of people. These included seniors, people who live in long-term care facilities, the homeless population, and IV drug users. All of these people might have weaker immune systems or are more prone to skin sores or lesions—a main risk factor for invasive strep.
Underlying health conditions such as diabetes and heart disease can also increase the risk that a common strep infection can become invasive.
Invasive cases aren’t necessarily caused by stronger bacteria. More commonly, the bacteria will start infecting more vulnerable areas of the body. Older age and weakening immune responses can contribute to this, says Frenck.
Can invasive strep cause flesh-eating bacteria?
Necrotizing fasciitis, in which bacteria can destroy skin, is also caused by strep A bacteria. These infections mostly happen when the bacteria—often picked up in bodies of water like lakes—enter open wounds in the skin.
Can you test for group A strep?
Yes, but the gold standard testing for bacteria requires taking a sample (via swab, if it’s in the throat) and culturing the bacteria to see if it will grow. Doctors often prefer to start antibiotic treatments as quickly as possible and send patients home with antibiotic prescriptions without waiting for the culture results, which can take a few days. Rapid tests that detect the antigens the body makes against the bacteria can provide results in 10 to 20 minutes, but sometimes lead to false negative results.
“One of the things we are trying to do for pediatricians is to be more focused in culturing,” says Frenck. Because sore throats can be caused by more than just strep bacteria, he says doctors should be more selective by assessing people’s symptoms first before deciding whether to perform a throat culture. “If there is pus on the tonsils, and lymph nodes under the jaw are tender and the they have a fever, then it’s more reasonable to do a culture,” he says. “But if they have a runny nose and cough—neither of which are not associated with group A strep infection—we would say hold off on the culture, because kids may end up with antibiotics who don’t need them, and other bacteria can develop resistance.”
How are strep A infections treated?
Because strep A infections are caused by bacteria, antibiotics are the best treatment. Fortunately, says Frenck, the oldest antibiotic, penicillin, remains an effective first-line treatment for group A strep infections.
For people with invasive strep and a high load of bacteria, doctors may add other antibiotics such as clindamycin, but the study also found that more bacteria are becoming resistant to these supplemental antibiotics. During the study period, for example, the percentage of bacteria from infected patients that were resistant to the most-used classes of macrolides and clindamycin antibiotics increased 12.7% to 33.1%.
“In the past, any number of classes of antibiotics could treat group A strep infections very, very effectively,” says Schaffner. “Now we have to be much more careful; you have to know what antibiotic profile of strep are circulating in your neck of the woods.”
Is there a vaccine against strep A?
Not yet, although researchers have been working on one for decades. Because different types of strep circulate at different times, one challenge for vaccine makers has been finding conserved regions of the bacteria to target in a shot. “For group A strep, that’s been hard,” says Frenck.
The goal of a strep vaccine would be to prevent some of the longer term consequences of strep infection, such as rheumatic heart disease, which are triggered by the immune system responding to the infection. “There are downstream immunological consequences of strep infections, and the last thing you want to do in making a vaccine is create one that produces rheumatic heart disease or glomerular nephritis,” says Schaffner.
How can you protect yourself from invasive strep A?
As with so many infections, the best prevention is washing your hands frequently and keeping skin clean—along with being extra careful about exposure to potentially contaminated bodies of water if you have fresh cuts or wounds.