Legionellosis

Legionellosis is an infection caused by Legionella pneumophila, a gram-negative rod that thrives in warm aqueous environments such as drinking-water systems, hot tubs, and air-conditioning units. Transmission occurs by inhaling contaminated, aerosolized water droplets. Legionellosis is a common nosocomial infection and outbreaks are typical. Notable risk factors include smoking, chronic lung disease, advanced age, and immunosuppressive conditions. There are two forms of legionellosis: Legionnaires' disease and Pontiac fever. Patients with Legionnaires' disease present with atypical pneumonia (shortness of breath, cough), commonly in combination with various other symptoms, including gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion). Laboratory abnormalities are common, especially hyponatremia. Pontiac fever is a milder, self-limiting, flu-like illness. A urine antigen test is used to confirm infection with L. pneumophila. Fluoroquinolones are the treatment of choice for Legionnaires' disease. In the US, legionellosis is a notifiable disease and steps should be taken to eliminate contaminated sources and to prevent future outbreaks.

• Frequency: : occurs rarely in infants, almost solely in adults (of any age) and typically in outbreaks
• High-risk groups ^[1][2]
  • Elderly individuals
  • Individuals with chronic diseases (e.g., COPD, emphysema, diabetes, CKD)
  • Immunocompromised individuals
  • Smokers

Epidemiological data refers to the US, unless otherwise specified.

Causative organism

• Legionella pneumophila (gram-negative, obligate aerobic, facultative intracellular rod) causes over 90% of Legionnaires' disease outbreaks.
• The optimal water temperature for Legionella is 25–42°C (77–108°F). ^[3][4]
• Pontiac fever is generally due to lesser-known types of Legionella.

Path of infection

• Inhalation of contaminated aerosols
  • Cold and hot water systems (e.g., those found in hotels, hospitals, and retirement homes)
  • Whirlpools/hot tubs, swimming pools, showers
  • Air-conditioning systems with contaminated condensed water
• Person-to-person transmission is uncommon.

Since transmission from person to person is uncommon, isolation is unnecessary.

Locations at particular risk of outbreak

• Nursing homes
• Hospitals
• Confined travel accommodations (e.g., cruise ships, hotels, resorts)

References:^[2][5]

Legionnaires' disease

• Incubation period: 2–10 days
• Clinical features
  • Fever, chills, headache
  • Severe pneumonia
    • Unilateral lobar pneumonia
    • Atypical pneumonia: dry cough which can become productive, shortness of breath, bilateral crackles
  • Relative bradycardia (uncommon)
  • Diarrhea
  • Neurological features, especially confusion, agitation, and stupor
  • Failure to respond to beta-lactam monotherapy

Legionellosis should always be considered in patients with signs of atypical pneumonia and diarrhea in combination with possible exposure (e.g., cruise ship travel, use of a whirlpool).

Pontiac fever

• Incubation period: 1–3 days
• Mild, self-limiting course of legionellosis without pneumonia.
• Flu-like symptoms (e.g. fever, headache, and muscle ache)

References:^[6]

See “Diagnosis of pneumonia”.

Laboratory findings

• Blood
  • Hyponatremia (serum sodium < 130 mEq/L) and hypophosphatemia are common.
  • Aminotransferases and creatinine may be elevated.
  • Possible thrombocytopenia and leukocytosis
• Urine
  • Legionella urinary antigen test: most important diagnostic tool ; rapid test, but only detects serogroup 1 ^[7]
  • Hematuria and proteinuria are common. ^[6]
• Respiratory secretions
  • Gram stain of respiratory secretions shows many neutrophils but, usually, no organisms.
  • Visualization of Legionella requires silver stain.
  • Legionella culture: slow; requires buffered (iron and cysteine) charcoal yeast extract agar (results after 3–5 days) ^[8]
• PCR: high sensitivity, high specificity ^[7]
• Serology: : A four-fold rise in antibody titer confirms legionellosis. However, the antibody titers have low specificity and sensitivity, and seroconversion can take up to 12 weeks. Therefore, more rapid tests, such as the urinary antigen test or PCR, are more often used.

The legionaries drew their iron swords, donned their silver helmets, and jumped off the ship to burn the town to coal: legionellosis, iron buffered medium, silver stain, history of cruise ship travel, charcoal yeast extract agar.

Imaging ^[9]

• Chest x-ray: Diffuse reticular opacities are commonly seen (especially in atypical pneumonia).
• Chest CT
  • Bilateral or unilateral consolidative changes and/or ground-glass opacities
  • Air bronchograms may also be seen.

Legionnaires' disease should be treated early with antibiotics, especially because of its high mortality rate of 10%. ^[10]

• If atypical pneumonia is suspected but not yet verified: see “Medical treatment of pneumonia”.
• If legionellosis is verified:
  • Drug of choice: fluoroquinolones (preferably levofloxacin, alternatively moxifloxacin) for 7–10 days
  • Initial parenteral treatment is recommended for all patients to avoid possibly poor gastrointestinal absorption ^[11]
  • Second-line treatment: macrolides (e.g., erythromycin or azithromycin) for 3 weeks (IV at first, later orally)
• If patients are unresponsive to monotherapy, consider adding rifampin or tigecycline. ^[12]

In any case of atypical pneumonia, antibiotic treatment needs to cover Legionella.

• Legionellosis is a notifiable disease.
• Course of action when contaminated water sources are detected in medical facilities
  • Contaminated water systems should be disinfected .
  • Use terminal tap water filters, especially for high-risk patients (e.g., immunocompromised or the elderly).

References:^[5]