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Legionella - Legionnaires Disease and Pontiac Fever

Medical Experts

Contributor

Hillyard

David R. Hillyard, MD

Professor of Pathology, University of Utah

Medical Director, Molecular Infectious Diseases, ARUP Laboratories

Legionellosis is a bacterial disease caused by Legionella that can present as either legionnaires disease or Pontiac fever. Legionnaires disease is a severe form of pneumonia that often requires hospitalization. Pontiac fever is a milder disease that is often self-limiting. Legionella is naturally found in freshwater environments but can become a health concern when it grows and spreads in human-made water systems such as hot tubs and large plumbing systems. Inhalation of Legionella-containing water droplets can cause infection, especially in people at increased risk (e.g., immunocompromised individuals). Legionellosis is diagnosed through the combination of urinary antigen laboratory testing and a culture or polymerase chain reaction (PCR) test.

Quick Answers for Clinicians

Which individuals are at increased risk for legionellosis?

People who are 50 years or older, current or former smokers, or immunocompromised, and those with chronic lung disease, cancer, or another underlying illness such as diabetes or kidney failure are at increased risk for legionellosis. Additionally, people who have recently traveled with an overnight stay outside of their homes (including a stay in a healthcare facility), those who have used respiratory therapy equipment, and individuals who have had recent exposure to hot tubs are at increased risk for legionellosis.

Should laboratory testing for Legionella be performed on adults with community-acquired pneumonia?

The American Thoracic Society (ATS) and the Infectious Diseases Society of America (IDSA) do not recommend routinely testing for Legionella in adults with community-acquired pneumonia (CAP), except in cases indicated by epidemiologic factors (e.g., recent outbreak or travel) or in cases of severe CAP. In adults with severe CAP, a combination of urinary antigen testing and culture or nucleic acid-based testing such as polymerase chain reaction (PCR) is recommended.

Indications for Testing

Legionnaires Disease

Laboratory testing for legionnaires disease should be considered in patients who have pneumonia (often initially confirmed with a chest X-ray) and meet one or more of the following criteria:

Lack of response to outpatient antibiotic treatment for community-acquired pneumonia (CAP)
Severe pneumonia (particularly if intensive care is required)
Immunocompromised status
History of recent travel (within 10 days of disease onset)
Risk for legionnaires disease with healthcare-associated pneumonia
Exposure to a setting with a legionnaires disease outbreak

Testing for healthcare-associated legionnaires disease is especially important in settings with a confirmed diagnosis within the last 12 months, positive environmental Legionella testing in the past 2 months, or recent changes in water quality that could result in Legionella growth.

Pontiac Fever

Testing for Pontiac fever is generally not indicated because the disease is mild and resolves without treatment. However, if there is a possibility that the patient has been exposed to Legionella at the same time or place as another patient with laboratory-confirmed legionellosis, testing can be considered.

Laboratory Testing

Legionellosis is diagnosed with the combination of a urinary antigen test that detects Legionella pneumophila serogroup 1 and a culture or molecular test that can detect clinically relevant species and serogroups. There are many laboratory tests available for the diagnosis of Legionella. The following table compares the various testing methodologies.

*Legionella* Testing Methodologies
Testing Methodology	Advantages	Disadvantages
PCR^a	Can be performed on a wide range of sample types, including sputum and lung tissue (although lower respiratory samples are preferred) Detects clinically relevant species and serogroups Fast turnaround time High specificity and sensitivity	Assays may vary by laboratory
Urinary antigen^a	Rapid Detects Legionella infection for days to weeks after treatment Provides retrospective evidence of Legionella infection	Does not detect all species or serogroups, so false-negative results are possible Does not enable comparison between clinical and environmental isolates
Culture^a	Detects all species and serogroups of Legionella Enables comparison of clinical and environmental isolates to help determine the source of an outbreak	Technically difficult, requires specialty media Long turnaround time Variable sensitivity Unreliable after initiation of antibiotic treatment
DFA	Can be performed on lung tissue in addition to respiratory samples Detects multiple species and serogroups Can be used as a confirmatory test for suspected Legionella colonies isolated from culture	Technically difficult Test availability may be limited
Serology	Detects multiple species and serogroups	Requires paired sera, the first collected within 2 wks of symptom onset and the second collected 3-6 wks later, for accurate analysis Long turnaround time (due to need for paired sera collection)
^aPositive result is considered confirmatory. DFA, direct fluorescent antibody Sources: CDC, 2025