In the lungs of mice, this results in granulomatous pneumonia. Clinical signs in immunocompromised rodents are generally more severe. In addition, infection has been shown to lower thyroxine levels in plasma. Natural primary or opportunistic infection of laboratory mice and rats would interfere with such studies. M. pulmonis lacks a cell wall and has membrane-associated hemolytic activity.
Prevalence rates can be high within animal facilities.
Transmission is primarily intrauterine and by aerosol. The organism readily establishes infection by colonizing the nasopharynx and middle ears. Infection is usually asymptomatic, causing some researchers to consider M. pulmonis a commensal under ideal conditions. Its pathologic effects vary, depending on a variety of host, organismal, and environmental, including concurrent infection with copathogens. Levels of susceptibility differ according to host stock and strain.