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Clinical Features

Legionellosis is bacterial respiratory disease caused by Legionella that can present as either Legionnaires’ disease or Pontiac fever. Legionnaires’ disease is a common cause of severe pneumonia requiring hospitalization, while Pontiac fever generally resolves on its own.

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

Legionnaires’ disease Pontiac fever
Clinical features Fever, myalgia (body aches), and cough are included in the CSTE case definition. These symptoms are typical but may not always be present in all patients and are considered optional for meeting the CSTE case definition. Additional symptoms may be present, such as shortness of breath, headache, confusion, nausea, or diarrhea. A milder illness without pneumonia is included in the CSTE case definition, often a flu-like illness (fever, chills, myalgia, malaise).
Pneumonia (clinical or radiographic) Yes No
Pathogenesis Replication of organism Possibly an inflammatory response to endotoxin
Incubation period 2 to 10* days after exposure 24 to 72 hours after exposure
Percent of persons who become ill, when exposed to the source of an outbreak Less than 5% Greater than 90%
Treatment Antibiotics Supportive care (self-resolving)
Isolation of organism Possible Never possible
Outcome Hospitalization common
Case-fatality rate: 10%, (25% for healthcare-associated)
Hospitalization uncommon
Case-fatality rate: 0%

* 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 has been reported to be up to 19 days in rare cases. For routine surveillance purposes, exposure histories are collected for the 10 days prior to onset. However, in outbreak settings where it is important to consider a wide range of possible sources, use of a 14-day incubation period is often desirable.

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