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Legionella Infections

(Legionnaires' Disease)


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Reviewed/Revised Apr 2022 | Modified Sep 2022
Topic Resources
  • The infection is often acquired by inhaling contaminated water droplets, as may be sprayed from shower heads or air-conditioning systems.

  • People have fever, chills, and muscle aches, and breathing may be difficult and painful.

  • Doctors identify the infection by analyzing samples of sputum, fluids from the lungs, or urine.

  • Antibiotics, such as fluoroquinolones, can effectively treat legionnaires' disease.

Infection with Legionella bacteria was first identified in 1976 when there was a large outbreak of fatal pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. The most common symptom of... read more Overview of Pneumonia at a convention of the American Legion in Philadelphia, Pennsylvania. Thus, the infection was called legionnaires’ disease.

The following increase the risk of getting a Legionella infection:

Legionella bacteria are often present in soil and fresh water. Fresh water containing these bacteria may enter a building's plumbing system. Thus, a Legionella outbreak often begins in a building’s water supply. In such cases, people usually get the infection by inhaling contaminated water droplets that have been sprayed from shower heads, misters, decorative fountains, whirlpool baths, or water cooling towers for air-conditioning.

Legionnaires' disease is not spread from person to person.

Legionella bacteria usually infect the lungs, causing legionnaires' disease.

Sometimes the bacteria affect only the upper respiratory tract and do not cause pneumonia. This infection is called Pontiac fever and is milder than legionnaires' disease.

Rarely, these bacteria can also infect other areas of the body, mainly in people with a weakened immune system or a serious illness. The heart is most commonly affected, but the brain and spinal cord, liver, spleen, lymph nodes, and intestines can also be affected.

Symptoms of Legionella Infections

Symptoms of legionnaires’ disease resemble the flu. People have fever, chills, a general feeling of illness, muscle aches, headache, and confusion. Other symptoms include nausea, loose stools or watery diarrhea, abdominal pain, cough, and joint pain. People may have difficulty breathing, and breathing may be painful. They may cough up blood.

With treatment, most otherwise healthy people recover. However, certain characteristics increase the risk of death:

  • Acquiring the infection in a hospital (up to half of infected people die)

  • Being older

  • Having a weakened immune system

Without treatment, about 5% of people with legionnaires' disease die, but this rate is higher (up to 40%) in people who are older, have a weakened immune system, or have acquired the infection in a hospital.

People with Pontiac fever have fever, headache, and muscle aches but no cough or other respiratory symptoms. Symptoms resolve on their own in about a week.

Diagnosis of Legionella Infections

  • Culture and analysis of a sample of sputum or fluid from the lungs

  • Sometimes urine tests

To diagnose the infection, doctors take samples of sputum or fluid taken from the lungs and send them to a laboratory to grow (culture) and identify the bacteria. Polymerase chain reaction (PCR) testing may be done. It increases the amount of the bacteria's DNA and thus makes the bacteria easier to identify.

A sample of urine may be checked for specific substances produced by the bacteria (antigens Antigen tests Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more ). These tests cannot detect some types of Legionella bacteria, but they can detect the type that causes most infections.

A chest x-ray is taken to check for pneumonia.

Treatment of Legionella Infections

  • An antibiotic

People with legionnaires' disease should be given an antibiotic. Usually, a fluoroquinolone, such as levofloxacin or moxifloxacin, is given intravenously or by mouth for 7 to 14 days and, if people have a greatly weakened immune system, sometimes up to 3 weeks. Other effective antibiotics include azithromycin, clarithromycin, and doxycycline.

People with Pontiac fever do not require antibiotics.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

  • Centers for Disease Control and Prevention (CDC): Legionella: A resource providing information about Legionella, including outbreaks and prevention

Drugs Mentioned In This Article

Generic Name Select Brand Names
Iquix, Levaquin, Levaquin Leva-Pak, Quixin
Avelox, Avelox ABC Pack, Avelox I.V., MOXEZA, Vigamox
Azasite, Zithromax, Zithromax Powder, Zithromax Single-Dose , Zithromax Tri-Pak, Zithromax Z-Pak, Zmax, Zmax Pediatric
Biaxin, Biaxin XL
Acticlate, Adoxa, Adoxa Pak, Avidoxy, Doryx, Doxal, Doxy 100, LYMEPAK, Mondoxyne NL, Monodox, Morgidox 1x, Morgidox 2x , Okebo, Oracea, Oraxyl, Periostat, TARGADOX, Vibramycin, Vibra-Tabs
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