Invasive Group A Streptococcus Infections Are Surging

“Overall, the theme of this US invasive GAS [group A streptococcus] disease update looks to be a backward slide in the social determinants of invasive GAS, most evident in, but not limited to, disadvantaged and marginalized populations in the US.”

Joshua Osowicki, MBBS, PhD, Theresa L. Lamagni, MSc, PhD, Journal American Medical Association, editorial, April 7, 2025

“The increasing invasive GAS burden, particularly among groups that have been economically and socially marginalized, requires urgent attention.”

Christopher J. Gregory, MD, MPH, et al., Centers for Disease Control and Prevention

Group A Streptococcus (aka Streptococcus pyogenes) infections are incredibly common, e.g., strep throat and impetigo, and, fortunately, most are relatively mild. But invasive group A Streptococcus (iGAS) infections are a different story. These infections are highly destructive, often lethal, and, although normally rare, on the rise in the U.S. (See, “Invasive Group A Streptococcal Infections in 10 US States,” Journal of the American Medical Association, April 7, 2025). In today’s post, I describe iGAS infections and discuss what’s behind their recent increase.

What is group A Streptococcus? Working on wound infections in 1874, the German surgeon Theodore Billroth discoveredStreptococcus pyogenes (S. pyogenes), a gram-positive “chain-forming” bacterium. S. pyogenes is a virulent bacterium and has always been considered an important human pathogen. In the 1940s and 1950s, S. pyogenes caused waves of strep infections, like rheumatic fever and scarlet fever in children. These diseases have now all but disappeared from industrialized countries. But S. pyogenes has not. Over the past several decades, streptococcal infections caused by highly invasive strains of S. pyogenes have captured increased attention.

What is invasive group A Streptococcus (iGAS)? The Centers for Disease Control and Prevention (CDC) defines iGAS as an infection caused by S. pyogenes that has invaded a normally sterile part of the body. (The British added “with a severe clinical presentation” to this definition.) This means these bacteria are found in places that are typically free of germs like the blood, pleural fluid, joint fluid, or deep tissues. These streptococci bacteria are the cause of serious infections like bacteremia (bloodstream), pneumonia (lungs), cellulitis (skin), and meningitis (brain and spinal cord). Those at greater risk of acquiring an iGAS infection are people who have: (1) a chronic illness such as diabetes or cancer; (2) a weakened immune system; or (3) skin lesions.

iGas infections progress rapidly and can be life-threatening. You may have heard of two of the particularly severe forms of iGAS infection: streptococcal toxic shock syndrome (STSS); and necrotizing fasciitis (NF)—aka “flesh-eating disease.”

STSS symptoms include fever, dizziness, lightheadedness, rashes or peeling of the skin, muscle and abdominal pain and diarrhea, nausea or vomiting. This infection progresses quickly causing shock and potential organ failure. NF symptoms include redness, blisters, swelling or skin pain, fatigue, fever, nausea and low blood pressure. Streptococci causing NF rapidly destroy muscles, fat, and skin and can lead to tissue loss and organ damage. If you experience symptoms of either STSS or NF, you should seek immediate medical attention. Early diagnosis and treatment are important for improving iGAS outcomes, especially with STSS or NF.

How are iGAS infections treated? Group A streptococcal infections are treated with antibiotics. To me, one of the many mysteries of group A Streptococcus is that it remains uniformly susceptible to penicillin. One might think that after decades-long exposure to penicillin the bacterium would evolve a mechanism to resist penicillin’s killing capacity. This has yet to happen.

The same also holds true for other beta-lactam antibiotics that are similar to penicillin, such as ampicillin and cefotaxime.Group A streptococcal infections remain uniformly susceptible to these antibiotics. But despite their enduring susceptibility to these beta-lactam antibiotics, 228 isolates (1.3%) identified in the recent JAMA study became resistant to other antibiotics (macrolides and clindamycin) over time.

Given the severity of iGAS infections, patients usually require hospitalization and, not uncommonly, are cared for in an intensive care unit. Depending upon the type of underlying infection, multidisciplinary teams of physicians are involved in the management of the patient’s iGAS infection. In the case of necrotizing fasciitis, the “antibiotic of choice” is the scalpel. Therefore, a general surgeon is usually in charge as debridement of gangrenous tissue is critical.

How to prevent iGAS infections. There is no vaccine to prevent iGAS infections. For decades, many brilliant microbiologists and vaccinologists have worked on developing a vaccine, yet none exists. Therefore, as with so many infections, your best means of prevention is washing your hands with soap and water frequently and keeping your skin clean.

The CDC provides guidance for managing iGAS infections by preventing transmission and addressing potential risk factors, particularly among household contacts. (See, also, “UK guidelines for the management of contacts of invasive group A streptococcus [iGAS] infection in community settings,” UK Health Security Agency, March 2023).  To prevent the spread of iGAS infections, the CDC emphasizes hand hygiene, covering coughs and sneezes and proper washing of all eating utensils and drinking glasses. The CDC also recommends that all household members be up-to-date on flu and chickenpox vaccines as these infections can increase the risk of iGAS.

Why are iGAS infection increasing? The CDC-led surveillance study of iGAS infections cited earlier was based on a review of 21,312 patients with iGAS, 1981 of whom died. The cases were reported from 10 US states from 2013-2022. During this time interval, the overall incidence of iGAS nearly doubled, increasing from 3.6 to 6.2 cases per 100,000 persons.

Many risk factors appeared to contribute to the increasing incidence of iGAS infections in the CDC study. These included: older age (65 years or older), living in long-term care facilities, American Indian or Alaska Native heritage, people experiencing homelessness, and IV drug use.

In previous Germ Gems posts, I identified the social determinants of health (SDOH) as risk factors for many serious infections. The SDOH are shaped by the distribution of money, power, and resources at global, national, and local levels.The American College of Physicians highlights that SDOH include nonmedical factors that can affect a person's overall health and health outcomes, such as economic stability, education, social and community context, health care access and quality. As is evident from the CDC study, the SDOH play a significant role in the rise in iGAS infections.

The path forward. As iGAS infections continue to rise, addressing the SDOH becomes increasingly important. Work toward reversing this trend and protecting those most at risk from these devastating infections would include focusing on prevention strategies, improving access to healthcare, and reducing socioeconomic disparities. But, given the current politicalization of public health in the U.S., good luck with that!