Clinical Pearls & Morning Reports

Posted by Carla Rothaus

Published December 1, 2021


What is the best diagnostic test for legionella?

The clinical and radiographic features of legionnaires’ disease are often indistinguishable from pneumonia caused by other bacteria. Legionnaires’ disease should always be included in the differential diagnosis of severe pneumonia. Read the NEJM Clinical Problem-Solving Article here.

Clinical Pearls

Q: Has the incidence of legionellosis changed in recent decades?

A: Legionella are gram-negative, facultative, intracellular bacteria with water and soil environmental reservoirs. Humans acquire legionella through inhalation of contaminated aerosols, and infection results in a spectrum of illness, including legionnaires’ disease, a form of pneumonia (named after a famed outbreak at the 1976 Philadelphia Legionnaires’ convention), as well as Pontiac fever, a nonspecific febrile illness observed predominantly in outbreak settings. The incidence of legionellosis is 6 times as high as it was two decades ago. This rise in incidence has occurred because of a confluence of factors including improved diagnostic capabilities, an aging infrastructure and population, and rising global temperatures resulting in a warmer, wetter climate.

Q: Is legionella a common cause of hospital-acquired pneumonia?

A: Health care facilities are notable potential legionella reservoirs because of their complex plumbing formations and vulnerable populations with increased susceptibility to legionellosis; an estimated 20% of reported cases in the United States are nosocomially acquired. It is therefore critical for clinicians to consider legionella as a potential cause of hospital-acquired pneumonia, particularly in patients who do not have a response to beta-lactam monotherapy.

Morning Report Questions

Q: What is the best diagnostic test for legionella?

A: Because legionella grows poorly on routine culture media, diagnosis has historically relied on urinary antigen testing. This method detects Legionella pneumophila serogroup 1, the predominant cause of legionellosis, but not other L. pneumophila serogroups or the 30 other species of legionella that have been isolated from humans. Urinary antigen testing consequently has a limited diagnostic sensitivity of only 75%, and the worldwide burden of legionnaires’ disease is estimated to be up to 10 times as high as the number of cases reported. PCR testing for legionella can detect all serogroups of L. pneumophila, as well as species other than pneumophila. PCR-based tests are now the method of choice for the diagnosis of legionella owing to their high sensitivity and rapid availability of results.

Q: Describe first-line treatment for legionella pneumonia.

A: The first-line treatment of legionella pneumonia is either azithromycin or levofloxacin, as determined primarily on the basis of observational data. Combination antimicrobial therapy is frequently used for severe disease, but data supporting efficacy are lacking.

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