Patient Has Pneumonia? Think Legionellosis

Nicholas Gross, MD, PhD


February 23, 2016

Active Bacterial Core Surveillance for Legionellosis -- United States, 2011-2013

Dooling KL, Toews KA, Hicks LA, et al
MMWR Morb Mortal Wkly Rep. 2015;64:1190-1193

Legionellosis: Underestimating the Incidence?

Legionellosis is the disease acquired by inhalation of Legionella bacteria. The most common presentation is a severe form of pneumonia, although a milder self-limited infection—Pontiac fever—can occur. A recent Morbidity and Mortality Weekly Report compared the incidence of legionellosis during 2000-2011, when case finding was passive, with data from 2011-2013, when the Active Bacterial Core Surveillance (ABCs) program was in place and confirmed cases met laboratory criteria for legionellosis.

During the earlier period, the crude incidence of legionellosis increased by 249%, from 0.39 to 1.36 cases per 100,000 persons. During the later period, ABCs data confirmed a total of 1426 cases, for an incidence rate of 1.3 cases per 100,000 persons. Therefore, disease rates were similar in both the passive and active surveillance periods. However, inconsistency in the use of the bacteriologic methods and a reliance on less sensitive urine antigen testing to diagnose legionellosis probably resulted in many missing cases, so these data probably underestimate the actual burden of legionellosis.

The ABCs data from 2011 to 2013 also showed that 44% of patients with legionellosis required intensive care, 27% required mechanical ventilation, and 9% died. Disease incidence was highest on the East Coast, especially in New York, Maryland, and Connecticut; Oregon and California had the lowest incidence. Among other demographic features, the incidence was higher among black persons than white persons and among males than females, and it increased progressively with age older 50 years. Underlying conditions that predisposed to legionella infection were current cigarette smoking, immunosuppression, chronic obstructive pulmonary disease (COPD), and diabetes.


Legionellosis has increased in frequency over the decades since its first identification. Its frequency, severity, and high mortality demand greater suspicion on the part of clinicians. Factors that might be considered warnings for the clinician are age (particularly > 70 years) and the presence of comorbid conditions (smoking, COPD, diabetes). The commonly used urine antigen test is limited to detection of the Lp1 antigen which is present in only about 80% of cases; culture is preferred if available. More sensitive tests are required.

Treatment, which will often be initiated empirically, is based on the newer macrolides, such as azithromycin (1 g initially, followed by 500 mg daily for 7-10 days[1]), or a quinolone, such as levofloxacin (750 mg once daily for 5 days[2]) for patients who are not immunocompromised. Combinations of these agents or the combination of a quinolone with rifampin have been used, but no formal trials of a combination vs monotherapy have been published. Parenteral therapy is preferred to oral therapy. For immunosuppression, a combination given for 21 days has been recommended. Isolation of patients is unnecessary because legionella is not transmitted from person to person.[3]



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