Legionnaires' disease

Legionnaires' disease
Other names	Legionellosis,[1] legion fever
Chest X-ray of a severe case of legionellosis upon admission to the emergency department
Specialty	Infectious disease, pulmonology
Symptoms	Cough, shortness of breath, fever, muscle pains, headaches[2]
Usual onset	2–10 days after exposure[2]
Causes	Bacteria of the Legionella type (spread by contaminated mist)[3][4]
Risk factors	Older age, history of smoking, chronic lung disease, poor immune function[5]
Diagnostic method	Urinary antigen test, sputum culture[6]
Prevention	Good maintenance of water systems[7]
Treatment	Antibiotics[8]
Prognosis	10% risk of death[8]
Frequency	~13,000 severe cases a year (US)[9]

Legionnaires' disease is a form of atypical pneumonia caused by any species of Legionella bacteria,^[3] quite often Legionella pneumophila. Signs and symptoms include cough, shortness of breath, high fever, muscle pains, and headaches.^[2] Nausea, vomiting, and diarrhea may also occur.^[1] This often begins 2–10 days after exposure.^[2]

A legionellosis is any disease caused by Legionella, including Legionnaires' disease (a pneumonia) and Pontiac fever (a related upper respiratory tract infection),^[10] but Legionnaires' disease is the most common, so mentions of legionellosis often refer to Legionnaires' disease.

The bacterium is found naturally in fresh water.^[4] It can contaminate hot water tanks, hot tubs, and cooling towers of large air conditioners.^[4] It is usually spread by breathing in mist that contains the bacteria.^[4] It can also occur when contaminated water is aspirated.^[4] It typically does not spread directly between people, and most people who are exposed do not become infected.^[4] Risk factors for infection include older age, a history of smoking, chronic lung disease, and poor immune function.^[5][11] Those with severe pneumonia and those with pneumonia and a recent travel history should be tested for the disease.^[12] Diagnosis is by a urinary antigen test and sputum culture.^[6]

No vaccine is available.^[7] Prevention depends on good maintenance of water systems.^[7] Treatment of Legionnaires' disease is commonly conducted with antibiotics.^[8] Recommended agents include fluoroquinolones, azithromycin, or doxycycline.^[13] Hospitalization is often required.^[12] The fatality rate is around 10% for healthy persons and 25% for those with underlying conditions.^[8]

The number of cases that occur globally is not known.^[1] Legionnaires' disease is the cause of an estimated 2–9% of pneumonia cases that are acquired outside of a hospital.^[1] An estimated 8,000 to 18,000 cases a year in the United States require hospitalization.^[9] Outbreaks of disease account for a minority of cases.^[1][14] While it can occur any time of the year, it is more common in the summer and autumn.^[9] The disease is named after the outbreak where it was first identified, at a 1976 American Legion convention in Philadelphia.^[15]