Legionella pneumonia

Legionella pneumonia, also known as Legionnaires' disease, refers to pulmonary infection primarily with the organism Legionella pneumophila. It is sometimes classified as atypical pneumonia.


Legionella pneumonia tends to be more prevalent among immunocompromised patients. Legionella pneumophila is found in ~15% (range 2-25%) of adults hospitalized for pneumonia .

Clinical presentation

Apart from the respiratory and inflammatory related symptoms, hyponatremia is a recognized feature due to inappropriate ADH secretion.



The infection is primarily caused by the bacteria Legionella pneumophila and, to a lesser extent, by organisms such as Tatlockia (Legionella) micdadei, Legionella bozemanii or Legionella longbeachae.

Radiographic features

Radiographic appearances often lag behind the clinical picture there can be deterioration on imaging despite clinical improvement.

Imaging features can be variable although changes are often described as multifocal and bilateral. There may be a middle and lower zone predominance .

Pleural effusions can be common and are occasionally seen even in the absence of lung field infiltrates . Resolution of infiltrates may be slow, and the tendency for delayed clearing should be considered before initiating any further invasive diagnostic investigation.


Multilobar or multisegmental changes have been reported to be typical on CT scans . Described features include:

  • bilateral or unilateral single or multifocal consolidative changes (most common ) and/or ground glass opacities ; pure ground glass changes can also uncommonly occur
  • those with extensively consolidated lesions can have associated cavitation
  • can have a subpleural or peribronchovascular pattern

Treatment and prognosis

Management is typically with antibiotics such as azithromycin .

History and etymology

The disease was discovered and named after an outbreak in 1976 during the American Legion convention in Philadelphia, resulting in the deaths of 29 Legionnaires . The bacteria Legionella pneumophila was isolated six months later by American microbiologist Joseph McDade .

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