Pneumonia, Mycoplasma

BASICS

Definition: Bronchopulmonary infection caused by Mycoplasma species, most commonly Mycoplasma pneumoniae
Synonyms: Primary atypical pneumonia, Eaton agent pneumonia, cold agglutinin-positive pneumonia, walking pneumonia
Typical population: Children/young adults, but can occur in all ages; epidemics in close communities (dormitories, barracks, skilled nursing facilities)
Incubation: 1–4 weeks (average 2–3 weeks)
Course: Usually acute; often begins as upper respiratory infection and may progress to pneumonia

US incidence: ~1 million cases/year (annual infection rate 1%)
Peak age: 5–20 years (rare <5 years)
Season: Autumn/winter
Responsible for: Up to 15–20% of all community-acquired pneumonia (CAP) cases yearly
Predominant sex: Male = Female
Epidemics: Close living quarters (schools, military, nursing homes)

Pathogen: M. pneumoniae (short-rod, no cell wall, not seen on Gram stain)
Transmission: Aerosol droplets; highly contagious
Pathogenicity: Attaches to respiratory epithelium → ciliary dysfunction → persistent hacking cough
Other: May worsen asthma or COPD; can cause extrapulmonary manifestations (neurologic, dermatologic, cardiac)

Asthma exacerbation (children)
COPD flare
Extrapulmonary: rashes, arthritis, cardiac conduction defects, neurologic symptoms

Often unremarkable early
Fever, hacking/pertussis-like cough, lassitude
Later: rhonchi, rales, wheezes, mild pharyngeal erythema, sometimes pleural friction rub
Rare: skin rashes (erythema multiforme, Stevens-Johnson syndrome)

Viral/bacterial/fungal pneumonia
Tuberculosis
Other atypical pneumonia: Chlamydia, Legionella, Coxiella, Francisella, Pneumocystis
COVID-19 (recommend testing to rule out)

Clinical diagnosis is common; treat empirically if suspicious
PCR for M. pneumoniae DNA (nasopharyngeal/throat swab, BAL) is most sensitive/specific if needed
WBC: normal or elevated; ESR: may be high
Procalcitonin: not recommended
Cold agglutinins: positive in 50%, but neither sensitive nor specific
CXR: reticulonodular or patchy lower lobe infiltrates; 10–15% have small pleural effusions
CT: patchy "tree-in-bud" opacities, segmental ground glass
Gram stain/culture: Not helpful (organism lacks cell wall, slow growth)

Doxycycline (preferred for CAP in adults and children >8 yrs)
Adults: 100 mg PO BID × 14 days
Children >8 yrs: 2–4 mg/kg/day (max 200 mg/day) divided BID
Azithromycin
Adults: 500 mg PO × 1 day, then 250 mg daily × 4 days
Children >3 months: 10 mg/kg × 1 day (max 500 mg), then 5 mg/kg (max 250 mg) days 2–5
Clarithromycin
Adults: 500 mg PO BID × 14–21 days
Children >6 months: 15 mg/kg/day divided BID × 10–14 days
Erythromycin
Adults: 500 mg PO q6h × 14–21 days
Children: 20–50 mg/kg/day divided q6–8h × 10–14 days

Pregnancy: Azithromycin preferred (Category B); avoid doxycycline and fluoroquinolones.
Children <8 years: Macrolides preferred.

Severe CAP risk class IV/V
Elderly or significant comorbidities
Inability to maintain oxygen saturation
Altered mental status, hemodynamic instability, severe neurologic or extrapulmonary findings

Follow-up CXR: In patients >50 years and all smokers (6–8 weeks post-recovery)
Monitor for complications: Rash, neurologic symptoms, worsening respiratory status
Antibiotic prophylaxis for close contacts: Not routinely recommended

Most common: Reactive airway disease, hemolytic anemia, erythema multiforme
Rare: Meningoencephalitis, GBS, myocarditis, pericarditis, Stevens-Johnson syndrome, acute respiratory distress syndrome, nephritis

Mycoplasma pneumoniae is a leading cause of atypical ("walking") pneumonia, especially in school-aged children and young adults.
Diagnosis is usually clinical; PCR confirms.
Responds well to empiric doxycycline or macrolide therapy.
Outbreaks in dorms, barracks, nursing homes.
May exacerbate asthma/COPD.