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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 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 10, 2010
ID: 09-ID-45
Committee: Infectious Diseases
Title: Public Health Reporting and National Notification for Legionellosis
Council of State and Territorial Epidemiologists (CSTE)

The 2005 case definition [4 pages] 05-ID-01 was re-published in the 2009 CSTE position statement [10 pages] 09-ID-45, approved by the CSTE membership in 2010. The case definition in both of these documents is identical. See also: National Notifiable Disease Surveillance System case definition page.

 

Case Definition for Case Classification

The following are descriptions of criteria to determine whether to classify a legionellosis case as confirmed or suspected.

Clinical Description

Legionellosis is associated with two clinically and epidemiologically distinct illnesses: Legionnaires’ disease, which is typically characterized by fever, myalgia, cough, and clinical or radiographic pneumonia; and Pontiac fever, a milder illness without pneumonia.

Laboratory Criteria for Diagnosis

Confirmed:

  • By culture: isolation of any Legionella organism from respiratory secretions, lung tissue, pleural fluid, or other normally sterile site
  • By detection of Legionella pneumophila serogroup 1 antigen in urine using validated reagents
  • By seroconversion: fourfold or greater rise in specific serum antibody titer to L. pneumophila serogroup 1 using validated reagents on specimens collected 3–6 weeks apart

Exposure period considerations

The incubation period for Legionnaires’ disease is most commonly 2 to 10 days from the time of exposure to symptom onset, with an average of 5 to 6 days, but public health officials have reported incubation periods up to 19 days under rare circumstances.

The incubation period for Pontiac fever is shorter, 24 to 72 hours after exposure.

For surveillance purposes, public health officials collect exposure histories for the 10 days before date of symptom onset. However, in outbreak settings where it is important to consider a wide range of possible sources, use of a 14-day range may be desirable. 

That being said, for definite healthcare-associated Legionnaires’ disease, using a longer period (i.e. 14 days instead of 10) could lead to exclusion of certain cases unintentionally, for example, cases with at least 10 but fewer than 14 days of continuous exposure to a single healthcare facility. For this reason, CDC recommends using a 10-day period to define definite healthcare-associated Legionnaires’ disease.

If the date of symptom onset is difficult to define (e.g., in patients with chronic underlying illness), date determination is deferred to the judgment of the clinicians providing care and the public health officials performing the investigation.

Suspect:

  • By seroconversion: 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) using validated reagents on specimens collected 3–6 weeks apart
  • By seroconversion: fourfold or greater rise in antibody titer to multiple species of Legionella using pooled antigen and validated reagents on specimens collected 3–6 weeks apart
  • By the detection of specific Legionella antigen or staining of the organism in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody (DFA) staining
  • By the detection of specific Legionella antigen or staining of the organism in respiratory secretions, lung tissue, or pleural fluid by immunohistochemistry (IHC)
  • By detection of Legionella species by a validated nucleic acid assay (e.g., polymerase chain reaction or PCR); this technology is evolving, so feel free to contact CDC if you have questions

Case Classification

Confirmed: a clinically compatible case that meets at least one of the confirmatory laboratory criteria

Suspect: a clinically compatible case that meets at least one of the presumptive (suspect) laboratory criteria

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