Public Health Response for Legionnaires' Disease

Key points

  • Even a single case of Legionnaires' disease can require some level of public health response.
  • Case verification and a patient interview are important first steps to help determine if additional investigation is warranted.
  • Outbreak investigations are often complex and require careful planning and periodic reassessments.

Case verification

Public health personnel should verify that a reported Legionnaires' disease case meets the case definition for legionellosis.

Patient interview

Next, public health personnel should interview the patient about the 14 days before onset of symptoms. The incubation period is most commonly 5 to 6 days, with a range of 2 to 14 days1. However, public health personnel have reported incubation periods up to 26 days under rare circumstances2. For surveillance purposes, it's recommended that public health personnel collect exposure histories for the 14 days before date of symptom onset.

At minimum, public health personnel should capture key details related to demographics, illness, and potential exposures.

Public health personnel can use a CDC case report form or an equivalent state-specific case report form as an interview tool.

Keep Reading: Case Report Forms and Instructions

Patient interview tools‎

CDC offers forms that can be used as-is or adapted to meet specific needs. They include an interview questionnaire for cases potentially related to cruise ship travel and a medical record abstraction form.
Downloadable investigation resources

Outbreak investigation considerations

Every outbreak investigation is unique and requires careful planning and periodic reassessments to determine the most appropriate response.

In addition to general considerations, CDC provides information and guidance on the following topics:

Settings-specific considerations

CDC also provides special considerations for:

Defining an outbreak

Cluster vs. outbreak

Learn about communication considerations when deciding whether to use the term cluster or outbreak.

Keep Reading: Communication Recommendation: Consider Terminology

CDC outbreak definition

CDC defines a Legionnaires' disease outbreak as two or more cases associated with the same possible source during a 12-month period. It applies to cases associated with

Cooling towers, hot tubs, and decorative fountains have all been implicated in Legionnaires' disease outbreaks.

This definition increases sensitivity of outbreak detection, especially for outbreaks involving potable water. It also helps account for periodic changes in risk (e.g., due to seasonality). Note that under certain circumstances, the timeframe under consideration may be shorter. For example, cooling tower outbreaks tend to be more explosive and of shorter duration (e.g., 3 months).

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Content Source:
National Center for Immunization and Respiratory Diseases; National Center for Environmental Health
  1. Egan JR, Hall IM, Lemon DJ, Leach S. Modeling Legionnaires' disease outbreaks: Estimating the timing of an aerosolized release using symptom-onset dates. Epidemiology. 2011;22(2):188–98.
  2. Fraser DW, Tsai TR, Orenstein W, et al. Legionnaires' disease: Description of an epidemic of pneumonia. N Engl J Med. 1977;297(22):1189–97.
  3. Smith P, Moore M, Alexander N, et al. Surveillance for travel-associated Legionnaires disease — United States, 2005–2006. Morb Mortal Wkly Rep. 2007;56:1261–3.