Mycoplasma Pneumoniae

Mycoplasma are the smallest free-living, self-replicating organisms known. These bacteria are contained by a trilayered cell membrane and do not contain a cell wall. They are therefore resistant to beta-lactam antibiotics and do not stain by Gram stain. They are difficult to grow in culture media and their growth is slow.

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http://femsre.oxfordjournals.org/content/32/6/956

 

Epidemiology

  1. Mycoplasma pneumoniae (MP) causes up to 40% of community-acquired pneumonia in children and up to 18% of cases requiring hospitalization. The incidence of MP pneumonia is greatest among school-age children and declines after adolescence.
  2. MP infection most often causes tracheobronchitis rather than pneumonia. Infection is usually mild and may be asymptomatic in adults with a history of previous infection.
  3. The incubation period is 1-3 weeks.
  4. The bacteria are spread by large particles by aerosol to close contacts. The spread is increased within closed settings. 

 

Pathogenesis

  1. MP are inhaled by the host and attach to cells in the respiratory tract. The bacteria produce a P1 adhesin protein that allows attachment to a receptor on the respiratory tract epithelial cells.
  2. Mycoplasma will induce damage to respiratory epithelium from the trachea to bronchioles. Alveoli are usually spared.
  3. Manifestations of illness are mostly confined to the respiratory system, but some cases are associated with extrapulmonary involvement. 
  4. Cold agglutinins are auto-antibodies believed to be the result of antigenic aleration of erythrocytes induced by MP. They may be detected in about 50% of cases and appear about 2 weeks after infection and may last for 6-8 weeks. The height of the titer may correlate with the seriousness of the illness. 

 

Clinical Presentation

  1. The symptoms of MP infection develop gradually over a period of several days and can persist for weeks to months.
  2. MP infections may involve the upper respiratory tract, the lower respiratory tract, or both.
  3. The symptoms are often similar to those caused by other respiratory pathogens.
  4. The most common symptoms include fever, cough, malaise, headache, and sore throat. Chest auscultation often reveals rales, rhonchi, and expiratory wheezes.
  5. Progression to bronchopneumonia is more common in older children (aged 5-15 years) and is relatively rare in younger children (under 5 years).
  6. Radiographic findings
    1. Most frequently: interstitial infiltrates with predilection for lower lobes, commonly with hilar adenopathy
    2. Occasionally presents with a lobar pneumonia
    3. Pleural effusions are not rare
    4. The xray classically looks worse than the clinical picture. 
  7. MP infections are frequent triggers of reactive airway disease (asthma). There is recent speculation regarding a possible role of MP in the development of asthma.
  8. Extrapulmonary symptoms occur in almost 20% of patients with MP infection requiring hospitalization. They are thought to be autoimmune-mediated and include rashes, Stevens-Johnson Syndrome, meningoencephalitis, hemolytic anemia, arthritis, and gastrointestinal symptoms.

 

Differential Diagnosis

  • Viral infections- adenoviruses, parainfluennza, influenza
  • Chlamydia pneumonia
  • Legionnaire's disease
  • Bacterial pneumonias

 

Diagnosis

  • Diagnostic tests are of limited use. There is no test that allows rapid, reliable diagnosis of MP infection.
  • White blood cell count may be normal or slightly elevated.
  • Cold agglutinin titers >1/64 are found in adolescents and adults in approximately 50% of cases. This test is not reliable in children < 12 years of age. Difficult test to perform and should be limited to diagnose older patients. Other atypical pneumonias may induce low titers of cold agglutinins. 
  • CF titers- Difficult to perform and must demonstrate rising and falling titers.
  • IgM- must wait until 8-10 days of illness before detected, therefore not clinically helpful in most situations.
  • PCR- not readily available
  • Presence of polys in sputum without organism is suggestive of Mycoplasma infection.

 

Treatment

  1. Treat with a macrolide (e.g. Azithromycin x5 days). Will also be effective against other community acquired infections such as pneumococcal pneumonia.
  2. Tetracyclines can be used in patients > 10 years old.
  3. Fluoroquinolones are not routinely used in patients <18 years.
  4. May isolate organism for months after treatment.

 

References

  1. Cimolai N. Mycoplasma pneumoniae Respiratory Infection. Pediatrics in Review 1998
  2. Wubbel L. et al. Etiology and treatment of community-acquired pneumonia in ambulatory children. The Ped. Infectious Dis. Journal 1999
  3. Kim C.K. et al. Late Abnormal Findings on High-Resolution Computed Tomography after Mycoplasma Pneumonia. Pediatrics. 2000
  4. Waites K.B. New concepts of Mycoplasma pneumoniae infections in children. Pediatric Pulmonology 2003