U.S. children’s hospitals and health agencies are monitoring a possible rise in cases of invasive group A strep infections in children, which can cause severe illness and be deadly.

It’s a dangerous but rare disease that leads to around 1,500 to 2,300 deaths in the United States annually, according to the Centers for Disease Control and Prevention. The agency says between 14,000 and 25,000 cases usually occur each year. There’s been a lull in cases during the pandemic, according to the CDC.

Now, hospitals in Colorado, Texas and Arizona say they’re seeing more cases of invasive group A strep (iGAS) in children than are typical. Two children in the Denver area have died from the infection, state health officials said.

The infections occur when strep A bacteria, which typically causes mild infections like a sore throat, spreads to other parts of the body like the bloodstream or spinal fluid. That can cause “flesh-eating” skin infections, pneumonia, or toxic shock syndrome.

In the United Kingdom, more than a dozen children have died with invasive group A strep infections since early fall. The World Health Organization said in a statement Thursday that France, the Netherlands, Sweden and Ireland are also reporting unusual increases in cases.

The CDC said it is looking into reports of a possible increase in infections in the United States.

“It’s possible that there may be increases in iGAS infections this year, as is being observed for other infectious diseases that spread person to person,” the CDC said in a statement.

The increase in invasive strep A infections might be linked to the ongoing surge in respiratory viruses, said Wassim Ballan, the division chief of infectious diseases at Phoenix Children’s Hospital. Some people carry strep A bacteria in their throat without it causing an infection. But if they’re sick from a virus, the bacteria can invade through the mucous membranes in the nose and throat.

“That can cause the type of severe disease we describe as invasive group A strep infection,” Ballan said.

What is invasive group A strep?

Normally, group A strep isn’t a major cause for concern. It’s a bacteria that can cause illness in both adults and children but those illnesses are typically mild. The CDC estimates that several million cases of non-invasive group A strep illnesses occur each year.

Those cases include infections like strep throat, which typically includes sore throat, fever and painful swallowing, and scarlet fever, which causes a red, sandpaper-like rash. They’re both treatable with antibiotics.

In rare cases, strep A can spread to areas of the body that normally don’t have any bacteria — like the blood, spinal fluid and joint fluid. That causes the more serious invasive group A strep infections.

People with invasive group A strep can develop “flesh-eating” skin infections, blood infections, toxic shock syndrome and pneumonia. The condition is usually treated in the hospital with IV antibiotics.

Are U.S. children’s hospitals seeing a surge?

Doctors are seeing varying trends nationwide. Some hospitals — like Boston Children’s Hospital and West Virginia University Medicine Children’s Hospital — told ABC News they are not seeing an increase in cases.

Other states are tracking an uptick. There have been 11 cases of invasive group A strep in children in Colorado since Nov. 1, the Colorado Department of Public Health and Environment said in a statement to ABC News, including two children who have died from the infection. Typically, the state only has one or two cases per month in children, the department said.

Texas is seeing a similar increase in cases. Texas Children’s Hospital treated over 60 cases during October and November, Jim Versalovic, the hospital’s pathologist in chief, told ABC News. That’s over four times the number of cases seen during the same time period in 2021, he said.

Phoenix Children’s is also treating more patients with invasive group A strep than usual, Ballan said.

Hospitals and health departments are not required to report cases of invasive group A strep to the CDC, so there isn’t a nationwide picture of how the disease might be spreading.

The agency told ABC News in a statement that it’s talking with hospitals around the country to get a fuller understanding of any trends.

What can families do to stay safe?

Doctors tell ABC News that all cases of strep should be seen by a doctor, severe or not. Parents should be on the lookout for fever, sore throat, trouble swallowing, or kids not acting like themselves. If your child is prescribed antibiotics, it is essential that they take all the medication — even if they start to feel better.

Parents should also keep an eye out for signs of toxic shock syndrome and “flesh-eating” skin infections, which can be a sign that a strep infection is invasive. Symptoms of toxic shock include fever, chills, muscle aches, nausea and vomiting. Early signs of a serious skin infection include a fast-spreading swollen area of skin, severe pain and fever. Later on it might look like blisters, changes in skin color, or pus at the infected area.

Strep spreads through coughs and sneezes. It can also spread by drinking from the same glass as someone who is sick, or touching a surface with strep and then touching the nose or mouth. Practicing good hygiene — like washing hands, surfaces and plates or glasses — can keep it from spreading.

Because the infections are appearing along with and after respiratory virus infections, parents should also make sure children are up-to-date on flu and COVID-19 vaccinations, Ballan said.

“Knowing that a lot of those infections are preceded by a viral infection, the focus should be on trying to prevent the ones that we can prevent,” he said.

Nicole McLean MD, MPH, is a member of the ABC News Medical Unit and a resident physician in pediatrics at Columbia University/New York-Presbyterian.

Additional reporting from ABC News’ Emma Egan and Eric Strauss.