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)

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.