Legionella pneumonia

Last revised by Dr Sonam Vadera on 28 Sep 2022

Legionella pneumonia, also known as Legionnaires' disease, refers to pulmonary infection primarily with the organism Legionella pneumophila. It is sometimes classified as an 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 5.

The commonest form of transmission is thought to be from inhalation of contaminated aerosols produced in association with water sprays, jets or mists.

Aspiration of contaminated water or ice, particularly in susceptible hospital patients it also thought to be an important source.

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 aerobic gram-negative bacillus Legionella pneumophila (thought be responsible for around 80% of cases) and, to a lesser extent, by organisms such as

  • Tatlockia (Legionella) micdadei
  • Legionella bozemanii
  • Legionella longbeachae

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 5.

Pleural effusions can be common and are occasionally seen even in the absence of lung field infiltrates 1. Resolution of infiltrates may be slow, and the tendency for delayed clearing should be considered before initiating any further invasive diagnostic investigation.

Occasionally cases may give a bulging fissure sign.

Multi-lobar or multi-segmental changes have been reported to be typical on CT scans 3. Described features include:

  • bilateral or unilateral single or multifocal consolidative changes (most common 3) and/or ground glass opacities 2; pure ground glass changes can also uncommonly occur
  • those with extensively consolidated lesions can have associated cavitation 3
  • non segmental distribution may be more common than segmental distribution 12
  • can have a subpleural or peribronchovascular pattern 2,8
  • pleural effusions can be common but not always present 13

Management is typically with antibiotics such as azithromycin 9.

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 10. The bacteria Legionella pneumophila was isolated six months later by American microbiologist Joseph McDade 10.

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