Mycoplasmas, the smallest self-replicating organisms, include M. pneumoniae (pneumonia), M. genitalium and Ureaplasma urealyticum (urethritis). M. pneumoniae is a common cause of community-acquired pneumonia.

Epidemiology

• Prevalence
  • Responsible for 15-20% of all cases of pneumonia, higher rates among school children and people in closed populations (military recruits)
    • Only 2-5% of patients require hospitalization (15-20% hospitalization rates for other causes of pneumonia)
• Transmission - respiratory droplet

Organism

• M. pneumoniae
  • Flask-shaped bacteria that lack a true cell wall and have very small genomes, making the bacteria depend on the host for nutrients
  • As intracellular parasites (prokaryotes), they depend on host for nutrients
  • Cultivation in vitro is difficult due to fastidious nature

Clinical Presentation

• Often indistinguishable from other viral and atypical bacterial pathogens
• Initial symptoms - malaise, myalgias, sore throat, headache (retro-orbital), ear pain and fever
• In patients who progress to pneumonia - chills, chest pain, nausea, vomiting and diarrhea may occur
• Dry, nonproductive cough occurs 3-5 days after onset of initial nonspecific symptoms
  • Later, cough may produce mucopurulent sputum
  • Patients usually seek medical attention on days 5-7, when cough may become paroxysmal and nocturnal
  • Cough may persist several weeks following resolution of constitutional symptoms
• Extrapulmonary manifestations of M. pneumoniae infections
  • Immunologically-mediated syndromes
    • Skin rashes
    • Erythema nodosum or erythema multiforme
    • Anemia
    • Thrombocytopenia
    • Guillain-Barré syndrome
  • Syndromes caused by the spread of organism
    • Bullous hemorrhagic otitis
    • Arthritis
    • Acute respiratory distress syndrome (ARDS)
    • Myocarditis
    • Encephalitis/meningitis

Diagnosis

• Indications for testing - viral pneumonia presentation with persistent cough
• Laboratory tests 
  • PCR - rapid test to identify M. pneumoniae
    • Increased sensitivity if sputum is the sample tested
  • IgG, IgM by ELISA, EIA - usually requires acute and convalescent samples
  • Culture - inadequate for acute diagnosis, relatively insensitive and requires a long incubation (up to 4 weeks)
• Imaging studies 
  • Chest radiograph findings - patchy, unilateral infiltrates or diffuse, bilateral interstitial process

Differential Diagnosis

• Influenza
• Parainfluenza 
• Legionella
• Chlamydia Pneumoniae
• Respiratory Simplex Virus
• Bordetella 
• Adenovirus
• Metapneumovirus