Legionellosis Incidence Increases; Better Diagnostics Needed

Laurie Barclay, MD

October 30, 2015

Passive surveillance for legionellosis showed a 249% increase in crude incidence in the United States between 2000 and 2011, according to a report from the Centers for Disease Control and Prevention (CDC) published in the October 30 issue of the Morbidity and Mortality Weekly Report.

Between 2011 and 2013, nearly half of patients with legionellosis required intensive care, and nearly 1 in 10 died, with higher disease incidence seen among blacks than whites, write Kathleen L. Dooling, MD, from the Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, and Epidemic Intelligence Service, CDC, and colleagues. These later data are from the Active Bacterial Core (ABC) surveillance program initiated in 2011. This program provides more complete demographic and clinical data than does passive surveillance, although overall disease rates were similar in both systems.

"Findings from the first 3 years of [ABCs] for legionellosis in the United States highlight the severity of legionellosis, the need to better understand racial and regional differences, and the need for better diagnostics to detect non–Legionella pneumophila serogroup 1 infections,” they say

The rising burden of legionellosis, which includes Legionnaires' disease and the milder Pontiac fever, mandates appropriate use of available diagnostic tests and future development of better tests. In 2011, the CDC began ABCs for legionellosis to clarify the clinical course, use of diagnostic tests, and incidence disparities across population groups.

From 2011 to 2013, ABCs identified 1426 legionellosis cases (1.3 cases per 100,000 population during the 3 years). Legionellosis incidence increased with age, was higher in blacks than in whites (1.5 per 100,000 vs 1.0 per 100,000), and varied among the states, with New York having the highest incidence (4.0 per 100,000) and California the lowest (0.4 per 100,000).

"The racial/ethnic differences in legionellosis incidence might reflect disparities in the prevalence of underlying medical conditions, socioeconomic determinants, and environmental exposures," the authors write. "Geographic differences in incidence might be influenced by regional differences in environmental exposures, testing practices, or the prevalence of underlying medical conditions."

Among patients with legionellosis, approximately 40% required admission to the intensive care unit, and 9% died. Despite guideline recommendations, most clinicians did not routinely collect respiratory cultures and depended only on urinary antigen testing for diagnosis, even though this detects only one serogroup of Legionella (Lp1) infections and is approximately 70% to 90% sensitive.

"Until better diagnostics are developed, obtaining respiratory cultures from persons suspected to have legionellosis continues to be important for diagnosing disease and detecting the source of infection in outbreaks," the report authors conclude. "The underlying reasons for geographic and racial differences in legionellosis incidence need further exploration, which will be done through additional analyses in ABCs."

The report authors call for development of molecular-based tests to detect more species and serogroups from respiratory specimens. More sensitive urine diagnostics are also needed, because nearly half of patients with legionellosis do not produce sputum.

For now, the investigators recommend obtaining respiratory specimens for culture from persons suspected to have legionellosis infection. Cultures facilitate diagnosis, guide initiation of specific therapy, and point to links between individual patients and environmental sources in outbreaks.

Limitations of this report include surveillance of only a portion of the US population, limiting generalizability to the entire population, and missed cases because of reliance on urine antigen testing in most patients, they note.

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2015;64:1190-1193. Full text

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