• The California Department of Public Health (CDPH) has received reports of increasing numbers of children hospitalized with Mycoplasma pneumoniae (M. pneumoniae) infections from March through June 2025 at hospitals in the San Francisco Bay Area and the San Joaquin Valley. California does not perform routine surveillance for M. pneumoniae; as such, systematic statewide data is not available. 

• National trends of Mycoplasma pneumoniae infections do not currently mirror the increase in cases reported in California counties. 

• The CDPH Viral and Rickettsial Disease laboratory (VRDL) has been partnering with the US Centers for Disease Control and Prevention (CDC) to facilitate azithromycin resistance testing for M. pneumoniae positive respiratory specimens. To date, no increase in azithromycin resistance has been detected in California. 

• Providers should consider testing for M. pneumoniae infection in children with community acquired pneumonia or other respiratory illnesses, especially those who aren't clinically improving on antibiotics ineffective against M. pneumoniae such as amoxicillin or other beta-lactams.  

• If interested, hospitals and local health departments with increased M. pneumoniae admissions can contact the CDPH Viral and Rickettsial Disease Laboratory at vrdl.submittal@cdph.ca.gov​ to coordinate shipment of specimens to CDC for azithromycin resistance testing for surveillance purposes.  These results should NOT be used for diagn​osis, treatment, or assessment of patient health or management.​
  

Increase in Number of Mycoplasma pneumoniae infections 

The California Department of Public Health (CDPH) has recently received reports from some San Francisco Bay Area and San Joaquin Valley local health departments (LHDs) and hospitals about an increased number of Mycoplasma pneumoniae (M. pneumoniae) associated hospitalizations in children. Since there is no California-specific reporting requirement or surveillance for M. pneumoniae, and M. pneumoniae is often not included in diagnostic work ups, California state-wide specific trends in M. pneumoniae infections are not known.  

There is also no national reporting requirement or dedicated surveillance system for M. pneumoniae, however the US Centers for Disease Control and Prevention (CDC) uses syndromic and commercial laboratory data to track national trends. CDC has not observed a national increase in M. pneumoniae infections in 2025. CDC previously reported an increase in M. pneumoniae infections in the late spring/early summer of 2024 and released a bulletin on October 18, 2024. 

The CDPH Viral and Rickettsial Disease laboratory has been collaborating with CDC to facilitate molecular characterization and azithromycin resistance testing for M. pneumoniae for surveillance purposes. To date, no increase in azithromycin resistance has been detected in California.  

Background on M. pneumoniae 

M. pneumoniae is a bacterium that is spread through respiratory droplets. Infection usually causes mild respiratory illness but can also cause more severe disease requiring hospital care, including new or worsening asthma, severe pneumonia, and encephalitis. M. pneumoniae infections can occur at any age, but they most often occur among children ages 5–17 years and young adults. Persons recovering from respiratory illness or who have weakened immune systems are at risk for more severe disease.  

The number of M. pneumoniae infections varies seasonally (more common in summer and early fall) and usually follows a cyclical pattern, peaking every 3 to 7 years. During the COVID-19 pandemic the percentage of M. pneumoniae diagnoses and positive M. pneumoniae test results decreased (likely due to increased use of non-pharmaceutical interventions like masking, improved respiratory hygiene, and social distancing practices) but began increasing again during the fall of 2023 and again in 2024.  

Most people with mild M. pneumoniae infections will recover on their own, but people with more severe illness may require antibiotics. M. pneumoniae bacteria are naturally resistant to certain antibiotics (e.g., beta-lactams such as amoxicillin) that are commonly used to treat other kinds of bacterial pneumonia. Macrolides (e.g., azithromycin) are the first-line antibiotic treatment for M. pneumoniae infection. Macrolide-resistance in M. pneumoniae is estimated to be less than 10%  overall in the United States (and even lower in the West Coast region) but may be higher in certain regions (the southern and eastern United States) or within clusters or outbreaks.