Legionellosis (Legionnaires’ disease, Pontiac fever, and extrapulmonary legionellosis)

• Reporting criteria
• The disease and its epidemiology
• Case definition

• Laboratory testing
• Case investigation
• Disease control measures

• References
• Contact

What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency  

• All suspected, probable or confirmed cases of legionellosis (i.e., Legionnaires’ disease, Pontiac fever, or extrapulmonary legionellosis) should be reported to public health within four (4) days of diagnosis or a positive laboratory test.
• Cases should be reported using EpiTrax Reportal, fax or telephone to CDPHE or local health departments (telephone and fax numbers are listed at the end of this document).
• All potential foodborne, waterborne, and enteric disease outbreaks should be reported immediately to CDPHE or local health departments and entered into EpiTrax, even if the causative agent is not yet known.
  • In general, an “outbreak” is defined as an increase in the number of illnesses above what is normally expected among a specific population, within a given area, over a certain period of time.
  • Specifically, a Legionnaires’ disease outbreak is defined as two or more cases associated with the same possible source during a 12-month period. This definition is intended to increase the sensitivity of outbreak detection, particularly for outbreaks associated with potable water systems. It also helps account for periodic changes in risk, such as seasonality. 

Purpose of surveillance and reporting

• To identify cases for investigation and potential outbreaks
• To monitor trends in disease incidence

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Etiologic agent

Legionella pneumophila and other Legionella spp. are the causative bacterial agents of Legionellosis (Legionnaires’ disease (LD), Pontiac fever (PF), and extrapulmonary legionellosis (XPL)). L. pneumophila causes about 80 to 90% of reported human Legionella infections, with serogroup 1 accounting for about 80% of these. At least 60 species of Legionella other than L. pneumophila have been identified, most of which are considered pathogenic. Of the reported non-pneumophila Legionella infections, approximately 60% are caused by L. micdadei, 15% by L. bozemanii, 10% by L. dumoffii, and 5% by L. longbeachae.

Clinical description

Legionella infection manifests in three distinct forms: pneumonia (called Legionnaires’ disease (LD) or Legionella pneumonia), Pontiac fever (PF), and extrapulmonary legionellosis (XPL). LD includes a broad spectrum of illness, ranging from mild cough and slight fever to severe bilateral pneumonia with high fever and multisystem failure. Watery diarrhea is seen in 25-50% of cases, and change in mental status (lethargy to encephalopathy) is seen with more severe illness. LD is not easily distinguished from other causes of community-acquired pneumonia, although hyponatremia (serum sodium <130 mEq/L) is found significantly more often with LD than other pneumonias.

PF is an acute, self-limited flu-like illness without pneumonia. The predominant symptoms are malaise, myalgias, fever, chills, and headache. Non-productive cough has also been noted. The attack rate among those exposed may be higher than 90%. It is less severe than LD, rarely requiring hospitalization. PF is self-limited, meaning it resolves without antibiotic treatment.

Other manifestations of legionellosis, although extremely rare, include skin infections, joint infections, kidney infections, and heart infections via bacterial spread through the blood system. There have also been cases of infections that occur at surgical sites via contamination of the wound by water containing Legionella. All of the above are classified as cases of extrapulmonary legionellosis (XPL).

Most patients with legionellosis (i.e., pneumonia) caused by non-pneumophila Legionella spp. are immunocompromised secondary to medications, organ transplantation, or malignancy.

Reservoirs

Legionella bacteria survive and grow in freshwater environments and soil, which include natural settings such as lakes and streams, but also man-made systems including plumbing systems and recreational water features like pools, hot tubs, and decorative water fountains. The microenvironment that these bacteria thrive in includes biofilms and parasitization of other free-living organisms such as amoeba, protozoa, and slime mold. Biofilm is a group of bacterial cells surrounded by a slimy matrix that adheres to surfaces such as pipes. Ideal growing conditions include warm water, stagnant water, the presence of organic matter/sediment, and the absence of residual disinfectants. Legionella typically replicate in temperatures of 68-128^o F, and can survive up to 158^o F. In temperatures less than 68^o F, the bacteria may not be actively replicating (dormant), but can still survive and cause illness.

Places where water may be used inconsistently, leading to stagnation (such as vacation rentals or seasonally used buildings), permissive temperatures, and low residual disinfectant, including large buildings with complex water systems such as hotels and hospitals. These exposures can result in travel, hospitals, or long-term care facilities (LTCF) being risk factors for acquiring the bacteria.

Modes of transmission

Evidence exists for multiple modes of transmission of Legionella to humans including aerosolization with subsequent airborne spread, aspiration of contaminated water or soil, introduction into the lung during respiratory tract manipulation, and surgical site infection via contamination of the wound by water containing Legionella. Legionella are not known to be transmitted from person-to-person.

Legionella is primarily contracted by inhalation of aerosolized water droplets contaminated with the bacteria. This can include aerosolization directly from a water source such as a shower or hot tub, or less frequently, through aspiration via drinking water or ice containing Legionella. Aerosolized water is commonly associated with hydrotherapy tubs, sinks, hot springs, sprinklers, misters, cooling towers, evaporative towers, swamp coolers, evaporative condensers, humidifiers, respiratory therapy equipment, and ice machines. There is no evidence to suggest transmission of Legionella from air conditioners in cars or household window air conditioning units that do not use water as their coolant. Non-pneumophila Legionella spp. are commonly found in aquatic habitats and soil. Water distribution systems, including hospital water systems, may become colonized with any of a number of these species.

Incubation period

Legionnaires’ disease: The average incubation period is 5–6 days but can range from 2 to 14 days. Public health officials have reported incubation periods up to 26 days under rare circumstances.

Pontiac fever: The average incubation period is 24 to 48 hours but can range from 5 to 66 hours.

Infectious period

Legionella infections are not known to spread from one person to another. 

Epidemiology

Legionellosis is reported throughout the year in Colorado; however, cases peak between July and October. Individuals most at risk for Legionella infection (LD or XPL) include people with diabetes, chronic lung disease, kidney or liver failure, cancer, current or past history of smoking, any other immunocompromising conditions, and those aged 50 years and older. During 2023, 132 cases were reported in Colorado, with a five-year average of 101 cases per year (2019-2023).

In 2022, there were 7,512 legionellosis cases reported in the United States, with a five-year average of 8,217 cases per year (2018-2022). Between 93% and 96% of cases were hospitalized for the treatment of Legionnaires’ disease. Overall, the case fatality rate is estimated to be between 6%-7%, which, importantly, varies by age and exposure category. The case fatality rate was 10% for Legionnaires’ disease cases with a health care exposure, 3% for cases with a travel exposure, 9% in cases with assisted living or senior living exposure, and 6% for cases with none of the previously listed exposures. 

For more data, see Colorado reportable disease data (Colorado data) and CDC WONDER (national data).

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Clinical criteria

Legionellosis is associated with three clinically and epidemiologically distinct illnesses: Legionnaires’ disease, Pontiac fever, or extrapulmonary legionellosis.

Legionnaires’ disease (LD): LD presents as pneumonia, diagnosed clinically and/or radiographically. Evidence of clinically compatible disease can be determined several ways: 1) a clinical or radiographic diagnosis of pneumonia in the medical record or 2) if “pneumonia” is not recorded explicitly, a description of clinical symptoms¹ that are consistent with a diagnosis of pneumonia.

Pontiac fever (PF): PF is a milder illness. While symptoms of PF² could appear similar to those described for LD, there are distinguishing clinical features. PF does not present as pneumonia. It is less severe than LD, rarely requiring hospitalization. PF is self-limited, meaning it resolves without antibiotic treatment.

Extrapulmonary legionellosis (XPL): Legionella can cause disease at sites outside the lungs (e.g., associated with endocarditis, wound infection, joint infection, or graft infection). A diagnosis of extrapulmonary legionellosis is made when there is clinical evidence of disease at an extrapulmonary site and diagnostic testing indicates evidence of Legionella at that site.

¹Clinical symptoms of pneumonia may vary, but must include acute onset of lower respiratory illness with fever and/or cough. Additional symptoms could include myalgia, shortness of breath, headache, malaise, chest discomfort, confusion, nausea, diarrhea, or abdominal pain.

²Clinical symptoms may vary, but must include acute symptom onset of one or more of the following: fever, chills, myalgia, malaise, headaches, fatigue, nausea and/or vomiting.

Laboratory criteria for diagnosis

Confirmatory laboratory evidence:

• Isolation of any Legionella organism from lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site.
• Detection of any Legionella species from lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site by a validated nucleic acid amplification test.
• Detection of Legionella pneumophila serogroup 1 antigen in urine using validated reagents
• Fourfold or greater rise in specific serum antibody titer to Legionella pneumophila serogroup 1 using validated reagents.

Supportive laboratory evidence:

• Fourfold or greater rise in antibody titer to specific species or serogroups of Legionella other than L. pneumophila serogroup 1 (e.g., L. micdadei, L. pneumophila serogroup 6).
• Fourfold or greater rise in antibody titer to multiple species of Legionella using pooled antigens.
• Detection of specific Legionella antigen or staining of the organism in lower respiratory secretions, lung tissue, pleural fluid, or extrapulmonary site associated with clinical disease by direct fluorescent antibody (DFA) staining, immunohistochemistry (IHC), or other similar method, using validated reagents.

Epidemiologic linkage

• Epidemiologic link to a setting with a confirmed source of Legionella (e.g., positive environmental sampling result associated with a cruise ship, public accommodation, cooling tower, etc.)

OR

• Epidemiologic link to a setting with a suspected source of Legionella that is associated with at least one confirmed case.

Criteria to distinguish a new case from an existing case

An individual should be considered a new case if their previous illness was followed by a period of recovery prior to acute onset of clinically compatible symptoms and subsequent laboratory evidence of infection. The recovery period for legionellosis can vary based on patient-specific factors. CDC consultation is encouraged for case classification of individuals without clear periods of recovery or subsequent acute illness onset.

Case classification

Confirmed

• Confirmed Legionnaires’ disease (LD): A clinically compatible case of LD with confirmatory laboratory evidence for Legionella.
• Confirmed Pontiac fever (PF): A clinically compatible case of PF with confirmatory laboratory evidence for Legionella.
• Confirmed extrapulmonary legionellosis (XPL): A clinically compatible case of XPL with confirmatory laboratory evidence of Legionella at an extrapulmonary site.

Probable

• Probable Legionnaires’ disease (LD): A clinically compatible case with an epidemiologic link during the 14 days before onset of symptoms.
• Probable Pontiac fever (PF): A clinically compatible case with an epidemiologic link during the three days before onset of symptoms.

Suspect

• Suspect Legionnaires’ disease (LD): A clinically compatible case of LD with supportive laboratory evidence for Legionella.
• Suspect Pontiac fever (PF): A clinically compatible case of PF with supportive laboratory evidence for Legionella.
• Suspect extrapulmonary legionellosis (XPL): A clinically compatible case of XPL with supportive laboratory evidence of Legionella at an extrapulmonary site.

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• The Colorado State Public Health Laboratory (“State Lab”) provides testing services to support public health investigations.
• The State Lab may test clinical and/or environmental samples when determined to be necessary for public health investigations. These services vary by pathogen but may include:
  • Testing for individuals who have been restricted or excluded from work or child care by public health
  • Testing for outbreak investigations, with prior approval from CDPHE
  • Other testing services as discussed with CDPHE
• The State Lab maintains a test directory, guidance for submitting samples, and other helpful resources on their webpage.
  • Additional questions can be directed to the Lab Coordinators by email (cdphe_labcoordinators@state.co.us; preferred method) or phone 303-692-3069.
• Clinical and commercial laboratories are required to submit all respiratory specimens collected from legionellosis patients (even if diagnosed via urinary antigen test [UAT]) to CDPHE for culture; urine specimen submission is not required. While these submissions do not require prior approval from CDPHE, a complete lab requisition form is required for each isolate/clinical material submission to the State Lab.
• Isolates from health care-associated legionellosis cases should be saved for molecular typing and comparison to environmental isolates. 

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Interview all cases of legionellosis to determine:

• Potential common (i.e., for multiple cases) environmental source of infection and to implement control measures to prevent further transmission from an implicated source.
• Potential health care-associated infection and to implement control measures to prevent further transmission from an implicated source to high-risk patients.
• If probable exposures for the case have been implicated in previous investigations.

Local health departments have primary responsibility for interviews of sporadic cases in their jurisdictions. Interviews should be conducted and appropriate disease control measures implemented as soon as reasonably possible after the case is reported. If XPL is suspected, consult with CDPHE to determine an appropriate course of action for follow up with the patient and/or health care facility.

Forms 

For single cases, use the CDPHE legionellosis case investigation form for the interview, and enter the information into the case’s morbidity record (CMR) in EpiTrax. Interview all cases, regardless of specimen source (respiratory, urine, etc.).

For surveillance purposes, collect exposure histories for the 2 to 14 days before symptom onset for Legionnaires’ disease or the three days before symptom onset for Pontiac fever. Use the following criteria to determine the case’s exposure period:

1. Legionnaires’ disease:
   1. If the case did not have prior respiratory symptoms at baseline, choose the onset date of cough or shortness of breath, whichever occurs first, to determine the exposure period.
   2. If case has respiratory symptoms at baseline, use the earliest date when other symptoms compatible with Legionnaires’ Disease began to determine the exposure period. These symptoms may include diarrhea (> 3 loose stools in 24 hours), altered mental status (confusion, lethargy, etc.), fever > 100.4°F, nausea, vomiting, myalgia (body aches), headache, hemoptysis (coughing up blood), or malaise.
2. Pontiac fever:
   1. Use the earliest date when fever, myalgia (body aches), or headache began to determine the exposure period. 

If the case does not report onset of any clinically relevant symptoms (see Clinical description section above), nor are they documented in medical records or reported by a medical provider, the case does not meet the legionellosis case definition. Contact CDPHE for assistance in updating the case status and deleting the record. 

After completing the case interview, enter the information into the case’s morbidity record (CMR) in EpiTrax, including the investigation form, and conduct any necessary disease control activities. If an outbreak is suspected, create a new outbreak in EpiTrax and contact CDPHE if assistance is needed; this includes requests for additional testing at the State Lab.

Identify and evaluate contacts

Symptomatic contacts

• Individuals who have a clinically compatible illness and an epidemiologic linkage (see Case definition, Epidemiologic linkage section above) are epi-linked probable cases and are treated the same as cases with a positive lab test for disease control purposes. See Disease control measures below.
  • Complete a case interview with all epidemiologically linked probable cases.
  • Epi-linked probable cases who have been interviewed should be entered into EpiTrax as a new, probable legionellosis morbidity record (CMR). Link this newly created CMR to the laboratory-confirmed case’s CMR under the “contacts” tab in EpiTrax.
• Refer symptomatic individuals who have not previously been tested to their health care provider for testing and appropriate medical care.
• If a common source of exposure is suspected (e.g., exposure to the same facility or water feature), create an outbreak record in EpiTrax, and notify CDPHE as soon as possible.  

Reported incidence is higher than usual/outbreak suspected

If the number of reported cases of legionellosis in your jurisdiction is higher than usual, or if an outbreak is suspected, investigate the situation to identify common sources of infection. Notify CDPHE as soon as possible, and enter the outbreak into EpiTrax as appropriate. CDPHE staff can assist local public health agencies to investigate outbreaks and determine disease control recommendations to prevent further cases, as well as coordinate surveillance of cases that cross county lines.

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Legionella (Legionellosis) | CDC

Treatment

The preferred treatment for Legionnaires’ disease for hospitalized patients is azithromycin or a respiratory fluoroquinolone (e.g., levofloxacin). For patients who do not require hospitalization, acceptable antibiotics include erythromycin, doxycycline, azithromycin, clarithromycin, or a fluoroquinolone. Pontiac fever requires no specific treatment. For the most recent treatment guidelines, please refer to the IDSA-ATS guidelines for treatment of community-acquired pneumonia and the IDSA-ATS guidelines for treatment of hospital-acquired pneumonia.

Prophylaxis

No prophylactic treatment of close contacts is recommended.

Education

If appropriate, sporadic cases may be educated about the prevalence of Legionella spp. in the environment, especially water and other probable environmental exposures; and about increasing age, smoking, and underlying disease as risk factors for acquiring legionellosis. It should also be explained that it is extremely difficult to determine the specific environmental source of sporadic cases and, therefore, environmental testing is not typically indicated. Cases may be referred to CDC’s Preventing Waterborne Germs at Home and Toolkit for Controlling Legionella in Common Sources of Exposure webpages for more information about preventing growth and spread of Legionella. Cases who use respiratory therapy equipment should be counseled to never use tap water in such devices, only distilled or sterile water. See Environmental Measures, below, for additional information about the prevention of Legionella.

Managing special situations

Single case of community-acquired legionellosis

One case of legionellosis does not require any further investigation other than interviewing the case and completing the CDPHE investigation form in EpiTrax. Since Legionella can be found in a wide variety of water sources at low levels, it is difficult to prove a particular source was the cause of illness unless another case occurs that also implicates the suspected source. Alleged sources should not be tested or decontaminated based on one community-acquired case. However, recommending the suspected source be cleaned or serviced by a professional company is advised. For cooling towers and water features, there are many different models, so a professional would be able to clean them based on that unit’s manufacturer guidelines. For pools and hot tubs, you can provide them with the Model Aquatic Health Code (MAHC) cleaning standards to pass along to a pool company. Use the guidelines found in Chapter 6.5, “Fecal/Vomit/Blood Contamination Response,” of the MAHC and the CDC hot tub disinfection guidelines. Short-term vacation rentals can also be a source of Legionella, particularly those that have hot tubs on the property, or that have prolonged vacancy periods (e.g., property used seasonally). Owners or managers of vacation rentals where a case report staying should be provided with notification of the case and CDC’s guide to preventing the growth and spread of Legionella at their property. 

Health care-associated legionellosis

A laboratory-confirmed case of legionellosis that occurs in a patient who has been hospitalized ≥ 10 days of continuous stay at a health care facility during the 14 days before onset of symptoms is considered a presumptive health care-associated legionellosis case. A case that occurs in a patient who has spent a portion of the 14 days before date of symptom onset in one or more health care facilities, but does not meet the criteria for presumptive HA-LD is considered a possible healthcare-associated legionellosis case. 

When a presumptive health care-associated legionellosis case is identified, CDPHE should be contacted, and certain activities should be initiated, including enhanced surveillance within the facility and an environmental investigation to determine the source of Legionella spp. If the facility is a hospital, skilled nursing facility, or a facility that shares a water system with a skilled nursing section, then a full investigation is necessary. A full investigation typically includes an environmental assessment, sampling, and active case finding. Further details about environmental investigations can be found in the Public Health Handbook for Management and Investigation of Legionellosis (Legionnaires’ disease, Pontiac fever, and extrapulmonary legionellosis).

Environmental measures

• After outbreaks or health care-associated cases, vigilant monitoring of proven sources should be maintained.
• Buildings or facilities that serve vulnerable populations (primarily housing people > 65 years, people with chronic/acute medical conditions, or weakened immune systems) or have high-risk devices or plumbing systems (cooling towers, hot tubs, or large, complex water systems) should consider implementing a water management program (WMP) to mitigate the risk of Legionella growth and spread.
  • Identify buildings with increased Legionella risk
  • Toolkit: Developing a Legionella Water Management Program
• Cooling towers should be drained when not in use and mechanically cleaned and maintained according to the manufacturer’s recommendations.
• Hotels and other owners/operators of whirlpool spas or hot tubs, pools, decorative fountains, and ice machines should maintain them according to the manufacturer’s recommendations and keep current on protocols for public health safety, including but not limited to Colorado’s Swimming Pools and Mineral Baths Regulations and the MAHC.

You may also consult the Public Health Handbook for Management and Investigation of Legionellosis (Legionnaires’ disease, Pontiac fever, and extrapulmonary legionellosis) for more information on environmental investigations.

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American Academy of Pediatrics. Red Book: 2024-2027 Report of the Committee on Infectious Diseases, 33rd Edition. Illinois, Academy of Pediatrics, 2024.  

CDC. “Legionella: Clinical Features.” Page last reviewed March 25, 2021.

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CDC. (2022, December). Legionnaires’ Disease Surveillance Summary Report, United States 2018-2019. https://www.cdc.gov/legionella/php/surveillance/surveillance-report-2018-2019. 

CDC. Legionella Ecology & Intro to Env Health & Engineering for Outbreaks. YouTube. (2015). https://www.youtube.com/watch?v=RV0bmdliQjQ

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Lau and Ashbolt. (2009). “The role of biofilms and protozoa in Legionella pathogenesis: implications for drinking water.” Journal of Applied Microbiology.

Legionellosis: Legionnaires’ disease, pontiac fever or extrapulmonary legionellosis 2020 case Definition | CDC. (n.d.). https://ndc.services.cdc.gov/case-definitions/legionellosis-2020/

Metlay, J.P., Waterer, G.W., Long, A.C., et al. (2019). Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. American Journal of Respiratory and Critical Care Medicine.

National Notifiable Diseases Surveillance System. Annual Summary Data 2018-2022 Request. (n.d.). https://wonder.cdc.gov/nndss-annual-summary.html

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CDPHE Communicable Disease Branch

• Phone: 303-692-2700 or 800-866-2759
• Fax: 303-782-0338
• After hours: 303-370-9395

CDPHE Lab Coordinators: 

• Email (preferred): cdphe_labcoordinators@state.co.us  
• Phone: 303-692-3480

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