Post-pandemic surge of Mycoplasma pneumoniae in Ontario, 2024: molecular surveillance and resistance trends relative to 2018-2023.

Mycoplasma pneumoniae (Mpn) is a common respiratory tract pathogen known to cause cyclic epidemics every 3-4 years. For 4 years (2020-2023), a low positivity rate (0.64%) for Mpn laboratory testing at Public Health Ontario suggested minimal circulation of Mpn in Ontario, Canada, likely owing, in part, to COVID-19-related non-pharmaceutical interventions. In 2024, we observed a dramatic re-emergence of Mpn, characterized by large increases in the number of laboratory-confirmed cases (n = 1,122) and the positivity rate (19.1%) of laboratory detections. Patient age distribution of Mpn infections in 2024 did not differ from 2018 to 2019; however, analysis of patient setting at the time of specimen collection as a proxy for disease severity saw a significant increase in the proportion of Mpn-positive specimens collected from patients in hospital emergency departments (ED) (P < 0.001), and a decrease in the proportion from patients in inpatient wards (P = 0.001). Macrolide-resistant rates were statistically equivalent in 2024 (14.6%) compared to 2018-2019 (12.4%), as was the ratio of p1-1 and p1-2 adhesin subtypes. Combined 2018-2024 data show that a significantly greater proportion of Mpn-positive pediatric specimens occurred in community clinics/private practices (P = 0.001), compared with adults, where a greater proportion were in EDs (P = 0.012), or intensive/critical care units (P < 0.001). Our findings suggest that the magnitude of Mpn infections in 2024 was not due to a novel strain, but rather due to the increased transmission of strains previously circulating in Ontario following the cessation of COVID-19 pandemic-related non-pharmaceutical interventions, and exacerbated by population immunity debt and the cyclic nature of Mpn.IMPORTANCEMycoplasma pneumoniae (Mpn) is a common cause of community-acquired respiratory infection and is known for causing "atypical" pneumonia. Infections show seasonal and cyclical patterns with larger epidemics every 3-4 years. Because Mpn is transmitted via respiratory droplets, incidence was very low during the COVID-19 pandemic when masking and physical distancing were widely utilized. In late 2023 through 2024, many jurisdictions worldwide reported a surge in Mpn cases. This increase, along with reports of emerging resistance to first-line treatment macrolide antibiotics, raised concerns about the management of severe infections. Here, we describe the post-COVID-19 pandemic re-emergence in Mpn in Ontario, Canada. Despite high case numbers detected in 2024, infections were not more severe than in pre-pandemic years (2018-2019). Mpn genotype ratios and macrolide resistance rates were statistically comparable between 2018/2019 and 2024, suggesting re-emergence of previously circulating strains rather than the introduction of a novel hypervirulent strain.