The increase in diagnoses in toddlers is “noteworthy,” health officials said, because M. pneumoniae bacteria is not typically considered a major cause of pneumonia for that age group.

The CDC did not provide a reason for why the increase is mainly among young children, although doctors have suggested that some children may not have developed immunity to the bacteria after staying home during pandemic lockdowns instead of going to daycare or school.

“We had a whole reservoir of children who were not getting the usual annual exposure to viruses or important bacteria,” says Dr. Vandana Madhavan, clinical director for pediatric infectious diseases at Mass General for Children. “Now we have a larger group of children who haven’t seen some of these infections before.”

The “shifting infection patterns” have been observed in all kinds of respiratory diseases since the pandemic, Madhavan said. But unlike viruses such as whooping cough – where increasing diagnoses are linked to declining vaccination rates – it is still unclear why M. pneumoniae infections in particular have increased.

Although no major “clusters” of the bacterial infection have yet been reported in Massachusetts, M. pneumoniae infections have increased in the state at a similar rate as nationally, a Department of Health spokesperson said.

M. pneumoniae infections, which occur most often in older children and young adults, are generally mild and often present in the form of a chest cold, according to the CDC. When the infection progresses to pneumonia, it may appear less severe than other forms of the condition. Hence the term ‘walking pneumonia’.

For younger children, however, more serious symptoms of infection may include diarrhea, wheezing or vomiting, officials said.

The infections are usually diagnosed in an inpatient setting, but Madhavan noted that many people with “walking pneumonia” only receive outpatient treatment, if they seek treatment at all. Consequently, the actual number of cases is likely to be significantly higher than what has been reported, she said.

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

The CDC recommends that parents be aware of M. pneumoniae symptoms such as fever, headache, and a slowly worsening cough. Children with asthma or reactive airway diseases are at greater risk for serious infections.

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 pay close attention to any persistent or unusual symptoms, even though walking pneumonia presents differently in each case.

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

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

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