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Information for Local Health Departments and Public Health Laboratories​

Legionellosis refers to three distinct clinical and epidemiological presentations of infection with Legionella bacteria:

      • Legionnaires’ disease – a serious illness characterized by pneumonia

      • Pontiac fever – a milder illness characterized by fever and muscle aches

      • Extrapulmonary legionellosis – an illness that occurs when Legionella infect tissues outside of the lungs and cause disease

All three conditions are reportable in California, though Legionnaires’ disease accounts for approximately 98% of reported legionellosis cases.

There are more than 60 species of Legionella, most of which can cause disease in humans. Legionella pneumophila serogroup 1 is the most commonly identified strain among laboratory-confirmed legionellosis patients and causes the majority of reported cases.​

Legionella spp. are found naturally in freshwater sources but can grow and multiply within biofilms in the built environment, especially in large buildings with complex water systems or industrial and household devices that use water. Exposure to Legionella occurs primarily through inhalation of contaminated aerosols or water droplets.

Legionellosis in California

Approximately 500 laboratory-confirmed cases of Legionnaires’ disease are reported each year in California. Outbreaks of legionellosis in California have been associated with multiple settings and devices including:

      • Hospitals

      • Congregate living facilities

      • Hotels

      • Cooling towers

      • Hot tubs

While most legionellosis cases in California appear to be sporadic rather than outbreak-associated, it is important for local health departments (LHDs) to follow up with legionellosis patients to identify any potential actionable sources of exposure.

Reporting Cases and Interviewing Patients

  • Obtain detailed information about water, healthcare, residential/congregate living, travel, and occupational exposures in the 14 days prior to symptom onset.

  • Please use the CDPH Legionellosis case report form (CalREDIE tabs or CDPH 8588 PDF in the CalREDIE Document Repository).

Legionellosis has been a nationally notifiable disease since 1976. Clinical laboratories and healthcare providers are required to report Legionella infections by electronic transmission (including FAX), telephone, or mail within seven days of identification.

Laboratory Testing

Clinical isolates are of critical importance during outbreak investigations because they can be compared to environmental isolates to confirm outbreak causes. However, molecular analysis of clinical isolates from sporadic cases is also important to continuously expand repositories of Legionella molecular data, allowing for improved understanding of virulence factors and other epidemiologic patterns.

The CDPH Microbial Diseases Laboratory (MDL) is able to confirm and identify species for both environmental and clinical isolates of Legionella spp. and conduct whole genome sequencing (WGS) on isolates, but does not perform primary isolation. MDL may assist LHDs in sending clinical specimens and isolates to CDC or the Legionella Reference Center for primary isolation and/or additional testing. Please follow up with the clinical laboratory to make sure that any isolates have been appropriately submitted to a public health laboratory, and document the final laboratory reports in the legionellosis case report form (either in CalREDIE or CDPH 8588 PDF in the CalREDIE Document Repository).

The CDPH Drinking Water and Radiation Laboratory Branch (DWRLB) Microbiology Unit is certified by CDC’s Environmental Legionella Isolation Techniques Evaluation Program (ELITE), and can isolate Legionella from environmental samples (i.e., water and swabs). DWRL may be able to assist with testing of environmental samples to support outbreak investigations. Additional laboratory testing support for environmental samples may also be available through the CDC laboratory or the Legionella Reference Center.

​​To coordinate submission of isolates for molecular analysis, or to inquire about laboratory testing capacity for primary clinical or environmental specimens, please contact the Infectious Diseases Branch (IDB) at 510-620-3434.​

Outbreak Response

Situations that the meet CDC's definitions for travel-associatedcommunity-associated, or health-care associated outbreaks of legionellosis warrant a full investigation. Suspected outbreaks in any setting should be reported to CDPH within 24 hours of identification. 

Response needs for legionellosis clusters and outbreaks can vary depending on the scope, setting, and population at risk. However, investigation steps typically include:

  1. Case finding and hypothesis generation

  2. Environmental assessment and sampling

  3. Laboratory testing

  4. Remediation of contaminated water systems and devices

  5. Implementation of long-term prevention measures

For assistance with travel- or community-associated Legionnaires’ disease (including outbreak investigation support), call the CDPH Infectious Diseases Branch (IDB) at 510-620-3434.

For assistance with healthcare-associated Legionnaires’ disease (including outbreak investigation support), call the CDPH Healthcare-Associated Infections (HAI) Program at 510-412-6060.​​​

For detailed information about reporting and legionellosis case investigation guidelines, please see:

CDPH Infectious Diseases Branch (IDB) Guidance for Managing Legionellosis (PDF)

March 2023


Data and Statistics

 California Data


 National Data

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