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Legionellosis and legionnaires’ disease 

Legionellosis and legionnaires’ disease

Legionellosis and legionnaires’ disease

J.T. Macfarlane

and T.C. Boswell



Minor amendments to aetiology, epidemiology, and diagnosis sections.

Updated on 31 May 2012. The previous version of this content can be found here.
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date: 30 March 2017

Legionellaceae are Gram-negative bacilli, of which Legionella pneumophila is the principal cause of human infections. Their natural habitats are freshwater streams, lakes, thermal springs, moist soil and mud, but the principal source for large outbreaks of legionellosis is cooling systems used for air conditioning and other cooling equipment, with infection transmitted by contaminated water aerosols. Middle-aged men, smokers, regular alcohol drinkers, and those with comorbidity are most at risk.

Clinical features and diagnosis—(1) Legionnaires’ disease (pneumonia)—typically presents with high fever, shivers, headache, and muscle pains; respiratory symptoms are sometimes minimal; confusion and diarrhoea may dominate the clinical picture. (2) ‘Pontiac fever’—an acute non-pneumonic form that presents as a self-limiting, influenza-like illness. Detection of urinary antigen has become the mainstay for diagnosis.

Treatment, prognosis and prevention—aside from supportive care, the first choice antibiotics are macrolides (e.g. erythromycin, clarithromycin) and/or fluoroquinolones (especially levofloxacin). Case fatality is 5 to 15% in previously well adults, but much higher in those who are immunocompromised or develop respiratory failure. Prevention is by the correct design, maintenance, and monitoring of water systems. Notification of a case allows a public health investigation into the likely source and the detection, prompt treatment, and/or prevention of additional cases.

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