Things to Consider: Healthcare-associated Cases and Outbreaks

How health department investigators respond to healthcare-associated cases and outbreaks of Legionnaires’ disease depends on several factors. These factors include (but are not limited to):

  • Type and size of the healthcare facility
  • Existing capacity of the facility and health department
  • Number of cases
  • Water management program performance
  • Routine environmental sampling results

Public health officials should work closely with healthcare facility staff at each step in the process. The appropriate healthcare facility point of contact (e.g., administrator, infection preventionist, clinician, quality assurance representative, facility manager or engineer) may vary, depending upon the step.

Deciding Whom to Test for Legionnaires’ Disease

See general indications for Legionnaires’ disease testing. In healthcare settings, public health officials and healthcare providers should consider Legionnaires’ disease as a possible diagnosis in any patient at risk for Legionnaires’ disease with healthcare-associated pneumonia (pneumonia with onset ≥48 hours after admission).1 This is especially important among patients with severe pneumonia (e.g., those requiring intensive care) or if you identify any of the following in a healthcare facility:

  • Other patients with healthcare-associated Legionnaires’ disease diagnosed in the past 12 months
  • Positive environmental tests for Legionella
  • Current changes in water quality that may lead to Legionella growth (e.g., low residual disinfectant levels, temperatures permissive to Legionella growth, nearby construction, areas of stagnation)

CDC receives reports of cases of healthcare-associated Legionnaires’ disease from across the country. According to an analysis published in 2017, 76% of the jurisdictions analyzed reported a definite case of healthcare-associated Legionnaires’ disease. When it occurs, healthcare-associated Legionnaires’ disease is often severe. The same analysis found that the case fatality rate associated with definite healthcare-associated cases was 25%,2 compared with 10% for cases that are not healthcare-associated. Because you cannot clinically or radiographically distinguish Legionnaires’ disease from pneumonia due to other pathogens, performing appropriate testing to make the correct diagnosis is important, particularly because first line treatment for healthcare-associated pneumonia does not always include Legionella-directed antibiotics.

Specimen Collection

The preferred diagnostic tests for Legionnaires’ disease are culture of lower respiratory secretions (e.g., sputum, bronchoalveolar lavage) on media that supports growth of Legionella paired with the Legionella urinary antigen test.

  • Isolation of Legionella by culture is important for detection of all species and serogroups of Legionella. In contrast, the urinary antigen test only detects Legionella pneumophila serogroup 1.
  • Culture also allows for comparison of clinical and environmental isolates during an outbreak investigation.
  • Best practice is to obtain both lower respiratory culture and the urinary antigen test concurrently.
  • You should ideally obtain lower respiratory culture prior to antibiotic administration. However, antibiotic treatment should not be delayed to facilitate this process (and culture can be attempted even after antibiotic therapy has been initiated).
  • The urinary antigen test can detect Legionnaires’ disease in some cases for days to weeks (or longer, under rare circumstances) after treatment.
  • Laboratories sometimes reject lower respiratory specimens during a work-up for pneumonia based on specimen quality (e.g., due to lack of white blood cells in the sample, contamination with other bacteria). However, laboratories should not reject lower respiratory specimens for these reasons when working-up Legionnaires’ disease because Legionella can often be recovered.
    • Sputum produced by patients with Legionnaires’ disease may not have many white blood cells.3
    • Contaminating bacteria do not negatively impact isolation of Legionella on selective media (e.g., Buffered Charcoal Yeast Extract [BCYE] agar plus antibiotics).4

Do You Need to Conduct a Full Investigation?

CDC recommends that public health officials perform a full investigation for the source of Legionella in a facility upon identification of:

  • ≥1 case of presumptive healthcare-associated Legionnaires’ disease at any time
  • ≥2 cases of possible healthcare-associated Legionnaires’ disease within 12 months of each other

Having one presumptive case or two possible cases within a defined period of time raises concern regarding the potential for ongoing transmission of Legionella to others. Healthcare Infection Control Practices Advisory Committee (HICPAC) guidance provides a 6-month timeframe for linking possible healthcare-associated cases with a common site of exposure. Although it is not a specific HICPAC recommendation, public health officials may prefer to use a 12-month timeframe. It increases sensitivity of outbreak detection, especially for outbreaks involving potable water (i.e., water used for drinking and bathing). The longer timeframe 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, such as during cooling tower outbreaks. These outbreaks tend to be more explosive and of shorter duration.

Available epidemiologic evidence may not be strong enough to warrant a full investigation when only a single possible healthcare-associated case has been identified. If public health officials determine that a full investigation is not warranted, they should consider at least conducting—or recommending that healthcare facility staff conduct—an environmental assessment. An environmental assessment can help determine if conditions for Legionella growth exist in the building water system(s) where the case(s) may have been exposed. This also provides an opportunity to stress the importance of surveillance and prevention measures to healthcare facility staff. These additional steps might be particularly useful if the facility has one or more of the following characteristics:

  • Has been associated with Legionnaires’ disease in the past
  • Legionella has been recently identified in the facility’s water system(s)
  • There has been a recent disruption of the facility’s water system(s)
  • Other buildings in the area have been associated with Legionnaires’ disease

In general, state, territorial, and local health departments are best positioned to provide oversight for each step of the investigation process to ensure adherence to public health recommendations. However, if the health department does not have the necessary resources to conduct all of the steps, they may defer certain responsibilities to the owner/manager of the building. For example, the health department may not have personnel with environmental expertise specific to Legionella or essential equipment.

The assistance of a consultant with Legionella-specific healthcare environmental expertise may sometimes be helpful. The CDC Legionella team is available to assist, either remotely or in person (see Request CDC Assistance for more information).

Steps Involved in a Full Investigation

Once public health officials decide to conduct a full investigation, they should:

  • Perform a retrospective review of cases in the health department surveillance database to identify earlier cases with possible exposures to the healthcare facility
  • Develop a line list of possible and definite cases ever associated with the healthcare facility
  • Work with healthcare facility staff to identify all new and recent patients with healthcare-associated pneumonia and test them for Legionella using both culture of lower respiratory secretions on media that supports growth of Legionella and the Legionella urinary antigen test
  • Obtain post-mortem specimens, when applicable
  • Consider recommendations for restricting water exposures in the facility or other immediate control measures
  • Facilitate an environmental assessment to evaluate possible environmental exposures
  • Facilitate environmental sampling, as indicated by the environmental assessment
  • Make recommendations for remediation of possible environmental source(s), if indicated
  • Communicate with stakeholders as appropriate and develop a risk communications plan if necessary
  • Determine how long heightened disease surveillance and environmental sampling should continue to ensure the outbreak is over
  • Work with healthcare facility staff to develop or review and possibly revise the water management program
  • Subtype and compare clinical and environmental isolates, if available
  • Follow up to assess the effectiveness of implemented measures for Legionella control

Conducting Additional Case Finding

Looking for patients with Legionnaires’ disease with healthcare exposures is an important part of a healthcare-associated Legionnaires’ disease outbreak investigation. To find cases, investigators should:

  • Determine if the facility routinely conducts surveillance for healthcare-associated pneumonia and if the facility tests any identified patients for Legionella. If so, what tests does the facility use?
  • Perform a retrospective chart review of patients for the past 12 months to identify pneumonia cases that could have been healthcare-associated. If additional cases are identified, determine if patients were tested for Legionella.
  • Review facility laboratory records for all Legionella testing and any positive results. Consider also reviewing laboratory records for other healthcare facilities in the same catchment area, especially for patients recently discharged from the healthcare facility of concern.
  • Consider evaluating trends in infections due to other water-related pathogens, such as gram-negative bacteria (e.g., Pseudomonas, Burkholderia, Stenotrophomonas) and nontuberculous mycobacteria.5 This step may help investigators evaluate the facility’s water management program.

CDC’s Line List Template is a helpful tool to summarize case demographic, clinical, and exposure information specific to a healthcare-associated outbreak. The CDC Legionnaires’ Disease Medical Record Abstraction Form Template may also assist with record review for healthcare facility investigations.

Conducting Active Clinical Surveillance

Timeframe for active clinical surveillance

Once initiated, active clinical surveillance should continue for at least 2 months, as described in the Healthcare Infection Control Practices Advisory Committee (HICPAC) guidance. Many public health jurisdictions will recommend active clinical surveillance for up to 6 months or longer, depending upon factors such as identification of additional cases or concerns regarding performance of the water management program.

Active clinical surveillance is a period of enhanced surveillance during which healthcare facility staff proactively and systematically identify patients with healthcare-associated pneumonia (pneumonia with onset ≥48 hours after admission)1. During this time, they also ensure that clinicians perform Legionella-specific testing for each of those patients. It is crucial for detecting additional cases of healthcare-associated Legionnaires’ disease in a healthcare facility.

Options for identifying patients with healthcare-associated pneumonia diagnoses could include:

  • Daily review of chest radiographs and CT scans ordered to diagnose pneumonia
  • Daily review of new pneumonia diagnoses occurring in patients in intensive care units
  • Daily review of laboratory testing ordered to diagnose pneumonia (e.g., sputum Gram stain and culture)

Once cases of healthcare-associated pneumonia are identified, healthcare facility staff should perform Legionella testing. Legionella testing includes using both culture of lower respiratory secretions on media that supports growth of Legionella and the Legionella urinary antigen test. Healthcare facilities should retain clinical specimens for the duration of the investigation.

During a full investigation, public health officials should:

  • Review the healthcare facility’s protocol for active clinical surveillance (or work with healthcare facility staff to create one)
  • Coordinate submission of clinical specimens for additional testing, as indicated
  • Coordinate submission of post-mortem tissue specimens to pathology and/or microbiology for Legionella testing, as indicated (procedures will vary by facility; feel free to contact CDC if you have questions)
  • Specify that laboratories performing Legionella urinary antigen tests should have a rapid turnaround time (within ~48 hours)

Immediate Control Measures for Healthcare Facilities

If a healthcare facility’s potable water (i.e., water used for drinking and bathing) system is a likely source of Legionella transmission, consider implementing immediate control measures. Examples of immediate control measures to consider include water restrictions or installing point-of-use filters (either globally or in areas of greatest risk). These steps can reduce the possibility of ongoing transmission to susceptible patients. You should tailor options to the structural characteristics of the facility and circumstances of the outbreak. If implemented, you should continue water restrictions and/or point-of-use filters until the investigation has been completed. This means that you have identified the possible source(s), performed remediation, and that control measures are believed to have controlled the risk. Timing will vary by outbreak.

Water restrictions may lead to areas of stagnation in the facility’s water system(s); safe strategies for periodic flushing may be necessary to prevent Legionella growth.

Examples of immediate control measures include:

  • Restricting showers (using sponge baths instead)
  • Avoiding exposure to hydrotherapy tubs
  • Avoiding use of water from sink/tub faucets in patient rooms to avoid creating aerosols
  • For hematopoietic stem cell or solid-organ transplant patients, using sterile water for tooth brushing, drinking, and flushing feeding tubes pdf icon[179 pages]; for other susceptible patients, using bottled water
  • Installing point-of-use (POU) microbial filters with an effective pore size of 0.2 microns or less that comply with the requirements of ASTM F838 on any showerheads or sink/tub faucets intended for use
    • POU filters protect only the connected fixture. Correct location selection is critical to Legionella exposure prevention across the water system.
    • Follow the manufacturer recommendations regarding frequency of replacement and appropriate operating conditions.
    • POU filters may need to be removed before performing an acute remediation procedure.
  • Avoiding consumption of non-sterile ice from facility ice machines for anyone at risk for aspiration or who has swallowing difficulties
  • Halting new admissions or temporarily closing the building, affected area, or device
  • Ensuring that contingency responses and corrective actions are implemented if the building already has a water management program
  • Distributing notification letters to the appropriate audience(s); see Communications Resources for more information

Note: In healthcare settings, you should only use sterile (not distilled, non-sterile) water to fill and clean reservoirs of respiratory equipment intended for nebulization under all circumstances (not just during an outbreak).

Environmental Assessment

Learn about performing an environmental assessment in all types of facilities, including healthcare facilities.

Environmental Sampling

Learn about performing environmental sampling  in all types of facilities, including healthcare facilities.


Learn about remediation in all types of facilities, including healthcare facilities.

Consider removing aerosolizing devices that are not necessary for the function of the healthcare facility (e.g., decorative fountains, waterfalls). ASHRAE Guideline 12-2020 specifies that decorative fountains should not be placed in areas or buildings where people at increased risk for Legionnaires’ disease are likely to be, such as healthcare facilities.6

Environmental Sampling Following an Outbreak

Legionella can regrow in a building water system following remediation events if the conditions are right. This is particularly true during the period before a water management program is fully implemented. Thus, follow-up sampling for Legionella can provide important information regarding the effectiveness of implemented measures in working to control the hazard. Note that post-remediation samples should be collected at least 48 hours after the water system or device has been restored to normal operating conditions.

The specific approach to follow-up sampling can vary according to the circumstances of the outbreak. One common approach, described in HICPAC guidance, consists of collecting environmental samples for culture at 2-week intervals for 3 months. If you do not detect Legionella in cultures during 3 months of monitoring at 2-week intervals, you should collect cultures monthly for another 3 months.

You can adjust the sampling plan over time based on trend data. If you detect Legionella in one or more cultures, you should:

  • Review and modify the water management program
  • Perform additional remediation, if indicated
  • Implement a new 6-month period for post-remediation follow-up sampling

Once you have completed the period of heightened sampling, you can continue routine environmental sampling for Legionella as a component of a comprehensive water management program.

When Is the Outbreak Over?

Learn about how to determine when an outbreak is over.


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  2. Soda E, Barskey A, Shah P, et al. Vital Signs: Health care-associated Legionnaires’ disease surveillance data from 20 states and a large metropolitan area — United States, 2015. MMWR Morb Mortal Wkly Rep. 2017;66(22):584–9.
  3. Murdoch DR. Diagnosis of Legionella infection [6 pages]. Clin Infect Dis. 2003;36:64–9.
  4. Mercante JW, Winchell JM. Current and emerging Legionella diagnostics for laboratory and outbreak investigations. Clin Microbiol Rev. 2015;28:80–118.
  5. Kanamori H, Weber DJ, Rutala WA. Healthcare outbreaks associated with a water reservoir and infection prevention strategies [13 pages]. Clin Infect Dis. 2016;62:1423–35.
  6. Managing the risk of legionellosis associated with building water systems. ASHRAE Guideline 12-2020. Atlanta, GA: ASHRAE; 2020.