Case Definitions

Legionellosis vs Legionnaires’ disease

For surveillance purposes, CDC has used the term “legionellosis” historically to ensure that all forms of clinical disease due to Legionella are included.

Elsewhere on this website, though, CDC refers to “Legionnaires’ disease” instead of “legionellosis” when describing cases and outbreaks. Reasons for this include:

  1. 98% of legionellosis cases reported to the Supplemental Legionnaires Disease Surveillance System (SLDSS) are Legionnaires’ disease and not Pontiac fever
  2. Because Legionnaires’ disease can be associated with substantial mortality (while Pontiac fever is self-limited), prevention efforts are often designed with Legionnaires’ disease in mind

Nevertheless, Pontiac fever deserves attention, as outbreaks of Pontiac fever can be large and can place burden on the medical system. Furthermore, Pontiac fever can signal the presence of conditions that support Legionella growth and transmission, and environmental sources that lead to cases of Pontiac fever are often also associated with cases of Legionnaires’ disease.

Although extremely rare, Legionella can also cause extrapulmonary infections, such as endocarditis or wound infections.

CSTE Position Statement
Approval date: June 6, 2019
ID: 19-ID-04
Title: Revision to the Case Definition for National Legionellosis Surveillance [17 pages]

See also: National Notifiable Diseases Surveillance System case definition page.

Case Definition for Case Classification

The following are descriptions of criteria to determine how a case of legionellosis should be classified.

Clinical Description

Legionellosis is associated with three clinically and epidemiologically distinct illnesses: Legionnaires’ disease, Pontiac fever, or extrapulmonary legionellosis.

Legionnaires’ disease: Legionnaires’ disease presents as pneumonia, diagnosed clinically and/or radiographically. Evidence of clinically compatible disease can be determined several ways: a) a clinical or radiographic diagnosis of pneumonia in the medical record OR b) if “pneumonia” is not recorded explicitly, a description of clinical symptoms that are consistent with a diagnosis of pneumonia.

Pontiac fever: Pontiac fever is a milder illness. While symptoms of Pontiac fever could appear similar to those described for Legionnaires’ disease, there are distinguishing clinical features. Pontiac fever does not present as pneumonia. It is less severe than Legionnaires’ disease, rarely requiring hospitalization. Pontiac fever is self-limited, meaning it resolves without antibiotic treatment.

Extrapulmonary legionellosis: Legionella can cause disease at sites outside the lungs (for example, associated with endocarditis, wound infection, joint infection, graft infection). A diagnosis of extrapulmonary legionellosis is made when there is clinical evidence of disease at an extrapulmonary site and diagnostic testing indicates evidence of Legionella at that site.

Laboratory Criteria for Diagnosis


  • Isolation of any Legionella organism from lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site
  • Detection of any Legionella species from lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site by a validated nucleic acid amplification test
  • Detection of Legionella pneumophila serogroup 1 antigen in urine using validated reagents
  • Fourfold or greater rise in specific serum antibody titer to Legionella pneumophila serogroup 1 using validated reagents


  • Fourfold or greater rise in antibody titer to specific species or serogroups of Legionella other than L. pneumophila serogroup 1 (e.g., L. micdadei, L. pneumophila serogroup 6)
  • Fourfold or greater rise in antibody titer to multiple species of Legionella using pooled antigens
  • Detection of specific Legionella antigen or staining of the organism in lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site associated with clinical disease by direct fluorescent antibody (DFA) staining, immunohistochemistry (IHC), or other similar method, using validated reagents
Epidemiologic Linkage Criteria for a Probable Case

Epidemiologic link to a setting with a confirmed source of Legionella (e.g., positive environmental sampling result associated with a cruise ship, public accommodation, cooling tower, etc.).


Epidemiologic link to a setting with a suspected source of Legionella that is associated with at least one confirmed case.

Case Classification

Confirmed: A clinically compatible case with confirmatory laboratory evidence for Legionella.

Probable*: A clinically compatible case with an epidemiologic link during the 14 days before onset of symptoms.

Suspect: A clinically compatible case with supportive laboratory evidence for Legionella.

*The probable case definition has traditionally been used as part of outbreak investigations to aid in the identification of potential sources of exposure. Recent updates to the CSTE Position Statement [17 pages] include new language to help public health investigators better categorize these types of legionellosis cases.