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HALIFAX - Following a sharp rise in cases of walking pneumonia among Canadian children, rates of the infection seem to be dropping — but doctors are now warning families about flu and respiratory syncytial virus, or RSV.

Dr. Kirstin Weerdenburg, a pediatric emergency physician at Halifax’s IWK Health Centre, said she’s diagnosed more cases of walking pneumonia — called mycoplasma pneumoniae — in the past six months than she has in her more than 15-year career. The hospital recorded 11 cases of the infection in 2023, compared to 163 confirmed cases in the first 10 months of 2024.

"But recently, in the last month or so, I've started to see a bit less of it and I'm starting to see a lot more of the seasonal viruses … respiratory syncytial virus and influenza," she said in a recent interview.

Dr. Jesse Papenburg, a pediatric infectious disease specialist and medical microbiologist at the Montreal Children's Hospital, said he’s seeing the same thing. "As this mycoplasma, or walking pneumonia, outbreak is starting to trail off, we're now going to have other problems with influenza."

While many cases of flu are mild, Papenburg said, the illness presents a higher risk to young children, those over 75 and those with certain chronic medical conditions. The virus is responsible for the hospitalization of more than 1,000 children in Canada a year, according to data from the Public Health Agency of Canada.

"It's a major cause of hospitalization in Canada, especially among vulnerable populations," he said in an interview, adding that it's not too late to receive a flu vaccine.

Typically, symptoms of flu infections begin within one to four days after exposure to the virus, with many people experiencing a sudden fever, cough or muscle aches. Other common symptoms are chills, fatigue, headache, sore throat or a runny nose. Some people, and especially children, may deal with nausea, vomiting or diarrhea.

Papenburg said families should also be prepared to look out for RSV, which often presents within two to eight days after exposure to the virus with some combination of symptoms like coughing, sneezing, wheezing, decrease in appetite and energy, fever or runny nose. In infants, the symptoms also include irritability and difficulty breathing. 

"We are in our RSV season right now in Quebec, with positivity rates over 20 per cent," Papenburg said. 

Most children will have experienced an RSV infection by the time they turn two, and the illness is a common cause of bronchiolitis — a type of lung infection — and pneumonia, the Public Health Agency of Canada says.

Papenburg lauded decisions by the Quebec and Ontario governments to introduce this fall free immunization against RSV for young infants.

"RSV is the leading cause of hospitalizations in children under one year of age, and this is the first time we have something available that can prevent severe RSV disease," he said of the vaccine, which is expected to reduce emergency department visits and hospitalizations.

"This is as close to a game-changer as we've had in pediatrics recently, at least from an infection perspective," Papenburg said. 

This report by Ïã¸ÛÁùºÏ²Ê¹ÒÅÆ×ÊÁÏ was first published Jan. 7, 2025.