Legionella - Pontiac Fever and Legionnaires Disease

Last Literature Review: February 2020 Last Update:

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 (eg, immunocompromised individuals). Laboratory testing such as polymerase chain reaction (PCR), culture, and urinary antigen testing is used to diagnosis legionellosis. 

Quick Answers for Clinicians

Which individuals are at increased risk for legionellosis?

People who are ≥50 years old, current or former smokers, or immunocompromised, and those with a chronic lung disease, cancer, or other 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), and people 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 (eg, 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 conditions:

  • Lack of response to outpatient antibiotic treatment for community-acquired pneumonia 
  • 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 current changes in water quality that may lead to 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

There are many laboratory tests available for the diagnosis of Legionella. All tests have advantages and disadvantages. Generally, PCR, urinary antigen testing, and culture are preferred, although other methodologies may be useful in certain situations. The table below compares the various testing strategies.

Legionella Testing Strategies
Test Method Advantages Disadvantages

Can be performed on wide range of sample types, including sputum and lung tissue (although lower respiratory samples are preferred)

Detects all clinically relevant species and serogroups

Fast turnaround time (<2 days)

High specificity and sensitivity

Assays may vary by laboratory

False-positive results are possible because PCR can detect nonviable Legionella

Urinary antigena


Not affected by antibiotic 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


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 (>5 days)

Variable sensitivity

Unreliable after initiation of antibiotic treatment


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

Sensitivity is highly dependent on patient population and specimen type

Not recommended for initial diagnosis of legionellosis

Serology Detects multiple species and serogroups

Requires paired sera collections at onset of symptoms and after 3-6 wks for accurate analysis

Long turnaround time (due to need for paired sera collection)

aPositive result is considered confirmatory.

DFA, direct fluorescent antibody

Sources: CDC, 2020 ; Pierre, 2017 

ARUP Laboratory Tests


Medical Experts



David R. Hillyard, MD
Adjunct Associate Professor of Pathology, University of Utah
Medical Director, Molecular Infectious Diseases, ARUP Laboratories