Clinical Features

Legionellosis is a bacterial disease caused by Legionella that can present as either Legionnaires’ disease or Pontiac fever. Legionnaires’ disease causes severe pneumonia requiring hospitalization, while Pontiac fever generally resolves on its own. Although extremely rare, Legionella can also cause extrapulmonary infections, such as endocarditis or wound infections.

The following table summarizes key clinical differences between Legionnaires’ disease and Pontiac fever.

Table summarizes key clinical differences between Legionnaires’ disease and Pontiac fever
Legionnaires' disease Pontiac fever
Clinical features Illness with pneumonia. Clinical symptoms of pneumonia may vary but include acute onset of lower respiratory illness with fever and/or cough (according to the CSTE* case definition)
Additional symptoms (e.g., myalgia, shortness of breath, headache, malaise, chest discomfort, confusion, nausea, diarrhea, or abdominal pain) may be present
A milder, self-limiting illness without pneumonia (according to the CSTE* case definition)
Symptoms include fever, chills, myalgia, malaise, headaches, fatigue, nausea and/or vomiting
Pneumonia (clinical or radiographic)** Yes No
Pathogenesis Replication of organism Possibly an inflammatory response to endotoxin
Incubation period 2 to 14*** days after exposure A few hours to 3 days after exposure
Percent of people who become ill, when exposed to the source of Legionella Less than 5%2 May be greater than 90%3
Treatment Antibiotics Supportive care (because illness is self-limited)
Isolation of the organism Possible Never demonstrated
Outcome Hospitalization common
Case-fatality rate: 10% (25% for healthcare-associated)
Hospitalization uncommon
Case fatality rate: extremely low
* CSTE: Council of State and Territorial Epidemiologists
** 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
*** The incubation period for Legionnaires’ disease is most commonly 5 to 6 days from the time of exposure to symptom onset, with a range of 2 to 14 days1, but public health officials have reported incubation periods up to 26 days under rare circumstances.2 For surveillance purposes, public health officials collect exposure histories for the 14 days before date of symptom onset.
  1. Egan JR, Hall IM, Lemon DJ, Leach S. Modeling Legionnaires’ disease outbreaks: Estimating the timing of an aerosolized release using symptom-onset datesexternal iconexternal iconEpidemiology. 2011;22(2):188–98
  2. Fraser DW, Tsai TR, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumoniaexternal iconexternal iconN Engl J Med. 1977;297(22):1189–97.
  3. Glick TH, Gregg MB, Berman B, et al. Pontiac fever. An epidemic of unknown etiology in a health department: I. Clinical and epidemiologic aspectsexternal iconexternal iconAm J Epidemiol. 1978;107(2):149–60.