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Legionellosis Information for Health Professionals & Clinical Labs

Legionellosis

Information for Health Professionals and Clinical Laboratories

Legionella are important respiratory bacterial pathogens in the United States.

Legionella spp. are gram-negative bacteria found naturally in freshwater sources that can grow and multiply within the built environment, especially in large buildings with complex water systems (e.g., hospitals, hotels, etc.) or devices that use water (e.g., cooling towers, hot tubs, respiratory therapy equipment, etc.). Within these environments, Legionella can live and multiply within biofilms, or as intracellular parasites within protozoa.

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

Legionnaires’ disease (LD) – a serious illness characterized by pneumonia with substantial morbidity and mortality

Pontiac fever (PF) – a milder, acute, and self-limited, illness without pneumonia, characterized by fever and muscle aches

Extrapulmonary legionellosis (XPL) – occurs when Legionella infect and cause disease at sites outside of the lungs (e.g., associated with endocarditis, or wound, joint, or graft infections)

Infections caused by Legionella have risen steadily since 2000, both in California and nationwide. Nationally, annual case counts averaged over 9,000 between 2018 and 2019. However, these totals are likely to underestimate the true burden of Legionella infections in the United States; CDC estimates that the true number of cases may be 1.8–2.7 times higher than what is reported. Improved provider awareness of legionellosis, including testing for Legionella in patients presenting with pneumonia, and increasing use of Legionella culture and nucleic acid amplification methods, could improve legionellosis identification, reporting, and initiation of appropriate treatment.

All three conditions are reportable in California, though the vast majority of reported legionellosis cases are Legionnaires’ disease (LD). Most people with LD are hospitalized, and about 10% of patients die. Mortality is higher (approximately 25%) among patients with healthcare-associated LD.

Transmission

Infection occurs via inhalation of aerosolized water or aspiration of water contaminated with Legionella bacteria. Exposure to hot tubs or a recent overnight stay outside of the home (e.g., in a healthcare facility or hotel) are also considered risk factors for disease; hotels, resorts, hospitals, and long-term care facilities often have complex water systems in which Legionella can grow and spread; then, exposure can occur when susceptible people breathe in aerosols emitted by devices attached to the water system. Notably, about 2/3 of LD patients in California do not report high-risk exposures, underscoring the potential role of exposures in homes and broader communities.

Most people exposed to Legionella do not become ill. However, personal risk factors for LD include being ≥ 50 years of age, current or historical smoking, chronic lung disease, malignancy, immune system deficiencies due to disease or medication, or other underlying illnesses such as diabetes, renal failure, or hepatic failure.

Diagnosis

L. pneumophila serogroup 1 is the most frequently identified strain among reported cases in California (the causative agent in about 95% of California patients). Most cases are currently diagnosed by a urine antigen test (UAT), which is highly specific for only L. pneumophila serogroup 1; thus, disease caused by other serogroups or species can be missed if only urine is tested. Culture and nucleic acid amplification (e.g., polymerase chain reaction or PCR) methods are critically important for LD surveillance and outbreak investigation. Both culture and PCR methods can detect all species and serogroups, and culture yields isolates that can be compared to other clinical or environmental samples during outbreak investigations. Therefore, CDC recommends that both a UAT and culture of a lower respiratory specimen be performed simultaneously. In addition, if a patient is being tested via PCR, consider retaining some of the lower respiratory specimen for a reflex culture if PCR is positive. Ideally, respiratory specimens should be collected prior to antibiotic treatment or as soon as possible after treatment has started, as antibiotic treatment may reduce the likelihood of successfully isolating Legionella.

Key Points

Risk factors for LD include advanced age, current or historical smoking, chronic lung disease, and immune system disorders due to disease or medication.
At least 95% of reported cases of LD result in hospitalization, and about 10% result in death.
Only one-third (1/3) of LD patients have known high-risk exposure (e.g., use of hot tub or overnight stay away from home).
CDC recommends testing with both UAT and culture or PCR of lower respiratory specimens, as cultured isolates are critically important for LD surveillance and outbreak investigation.
Respiratory specimens should be collected prior to antibiotic treatment or as soon as possible after treatment has started.
See CDC’s Indications for LD Testing.

Indications for Testing

Because infection with Legionella can present similarly to illness caused by other, more common respiratory pathogens, providers might not consider Legionella for their differential nor order testing. However, CDC suggests testing for LD in the following scenarios:

Patients who have failed outpatient antibiotic treatment for community-acquired pneumonia
Patients with severe pneumonia, in particular those requiring intensive care
Immunocompromised patients with pneumonia
Patients with a travel history (patients who have traveled away from their home overnight within 14 days before symptom onset)
Hospitalized patients with healthcare-associated pneumonia (pneumonia with onset ≥ 48 hours after admission) at risk for Legionnaires’ disease
Patients with an overnight stay in a healthcare facility within 14 days before symptom onset
Patients with an epidemiologic link to a setting with a confirmed source of Legionella or that has been associated with at least one laboratory-confirmed case of Legionnaires’ disease

Treatment and Management

To aid in proper management of legionellosis:

LD and XPL are treated with antibiotics.
PF is a self-limited illness that is managed with supportive care and does not benefit from antibiotics.

Reporting

Healthcare providers are required to report cases of legionellosis to the local health department (LHD) within one week (seven working days) of identification or immediately by telephone if an outbreak is suspected.

More Information

CDC Clinical Overview of Legionnaires’ Disease

Clinical Features of Legionnaires’ Disease and Pontiac Fever

CDC Clinical Guidance for Legionella Infections

Resources

CDC What Clinicians Need to Know About Legionnaires’ Disease (PDF)

Information for Clinical Laboratories

Note: If you need assistance with identifying a laboratory with capacity to culture clinical specimens for Legionella, please contact your LHD. If your local public health laboratory is unable to culture for Legionella, your LHD and CDPH can help connect you with laboratory testing resources available through CDC or the Legionella Reference Center.

Resources