The increase in diagnoses among young children is “remarkable,” health officials said, because the bacterium M. pneumoniae is not typically considered a leading cause of pneumonia in this age group.

The CDC did not give a reason why the surge is occurring particularly among young children, although doctors have cautioned that some children may not have developed immunity to the bacteria after staying home rather than going to daycare or daycare during the pandemic lockdowns to go to school.

“We had a number of children who were not exposed to viruses or larger bacteria as usual every year,” said Dr. Vandana Madhavan, clinical director of pediatric infectious diseases at Mass General for Children. “Now we have a larger group of children who have never seen some of these infections before.”

The “changing infection patterns” have been observed in all types of respiratory diseases since the pandemic, Madhavan said. But unlike viruses like whooping cough, where rising diagnoses are linked to falling vaccination rates, it's still unclear why M. pneumoniae infections in particular have skyrocketed.

While no major “clusters” of the bacterial infection have been reported in Massachusetts, M. pneumoniae infections in the state have increased at a rate similar to that seen nationally, according to a spokesman for the state Department of Health.

M. pneumoniae infections, which most commonly occur in older children and young adults, are generally mild and often manifest as a chest cold, according to the CDC. When the infection progresses to pneumonia, it appears to be less serious than other forms of the disease, hence the term “walking pneumonia.”

However, younger children may experience more serious infection symptoms such as diarrhea, wheezing or vomiting, officials said.

The infections are usually diagnosed in the inpatient setting, but Madhavan pointed out that many people with “walking pneumonia” are only treated on an outpatient basis, if they seek treatment at all. Therefore, the actual number of cases is likely to be significantly higher than reported, she said.

While the disease is often mild in adults, it can lead to serious complications and even neurological problems in children, Madhavan said.

The CDC recommends that parents monitor for M. pneumoniae symptoms such as fever, headache, and a slowly worsening cough. Children with asthma or reactive airway disease are at higher risk of serious infection.

Parents of children with worsening symptoms, such as wheezing or difficulty breathing, should seek medical attention, officials said.

Madhavan said it's important for parents to watch for persistent or unusual symptoms, although walking pneumonia behaves differently in each case.

“It’s not a specific thing,” she said. “But if the cough seems to be getting worse, not better; if dehydration is present; If the kids have been missing a lot of school – these are all reasons to think… “Do we need someone to measure oxygen levels, listen to the lungs and do a full exam?” To see if anything is going on that requires antibiotic treatment requires?”

Doctors recommend similar measures to prevent the spread of walking pneumonia and other respiratory illnesses: good hygiene, masks and physical distancing around other people indoors.

Camilo Fonseca can be reached at [email protected]. Follow him on X @fonseca_esq and on Instagram @camilo_fonseca.reports.