Legionella Infections: Clinical Guidance on Testing, Diagnosis and Treatment

Legionnaires' disease cannot be told apart from pneumonia caused by other germs by symptoms or chest imaging. The right tests are needed to diagnose it and guide treatment — all the more because first-line treatment for community-acquired or health care-associated pneumonia does not always cover Legionella. CDC's guidance for clinicians also stresses asking patients about recent travel and health care exposures.

When to test

For patients with pneumonia, test for Legionnaires' disease in cases of:

• outpatient antibiotic failure for community-acquired pneumonia;
• severe pneumonia, especially illness needing intensive care;
• an immunocompromising condition;
• overnight travel away from home during the incubation period;
• risk factors together with hospitalization for health care-associated pneumonia;
• an overnight stay in a health care facility during the incubation period; or
• a link to a setting with known Legionella concerns — where the bacteria have been found, or that is associated with at least one laboratory-confirmed case.

For case investigations, public health officials collect exposure histories covering the 14 days before symptoms began.

Testing for health care-associated Legionnaires' disease is especially important if a facility has had a health care-associated case in the past year, positive environmental tests for Legionella, or recent changes in water quality that could encourage its growth.

Why exposure history matters

Most recognized outbreaks are linked to:

• health care settings, such as hospitals and long-term care facilities, where prompt reporting helps protect especially vulnerable patients; and
• travel, such as hotels, resorts and cruise ships.

About 10% to 15% of reported cases occur in people who traveled during their exposure period. Outbreaks among travelers are hard to detect because few exposed people get sick, symptoms appear some time after exposure, travelers scatter from the source, and cases go undiagnosed — so collecting and reporting overnight travel history is important.

Diagnosis

The preferred approach pairs two tests:

1. culture or molecular testing of lower respiratory secretions, such as sputum or bronchoalveolar lavage, which can detect a range of Legionella types; and
2. the urinary antigen test, which detects Legionella pneumophila serogroup 1.

Ideally both specimens are collected at the same time. The urinary antigen test usually detects only one type of Legionella, and other types also cause infection. Respiratory specimens are best taken before antibiotics begin, but treatment should not be delayed to get them; they can be collected after antibiotics have started. Specimens should be stored as the testing laboratory recommends.

Treatment

• Legionnaires' disease: follow the treatment guidelines for community-acquired or hospital-acquired pneumonia. First-line treatment does not always include antibiotics active against Legionella, such as macrolides and respiratory fluoroquinolones. Testing before antibiotics is preferred, but treatment should not wait for it.
• Pontiac fever: this self-limited illness does not benefit from antibiotics and should not be treated with them. Patients usually recover within a week.

Sources

• Centers for Disease Control and Prevention, "Clinical Guidance for Legionella Infections": https://www.cdc.gov/legionella/hcp/clinical-guidance/index.html