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Legionnaires' Disease on the Rise in the United States

OCT 23, 2017 | KRISTI ROSA
Dr. Cooley highlighted the surveillance systems that are used to collect data on Legionellosis. The first was the National Notifiable Disease Surveillance System (NNDSS), through which local, state, territorial, federal, and international agencies all share health information on notifiable diseases; and the Supplemental Legionnaires’ Disease Surveillance System (SLDSS). The CDC get their annual case counts from the NNDSS, as well as basic demographics on the cases and case-level data. The NNDSS collects information on clinical disease from Legionella; however, it does not differentiate between Legionnaires’ disease and Pontiac fever. Similarly, the SLDSS collects case-level data; however, it also collects epidemiologic risk factors (travel history, health care exposures, disease presentation), as well as laboratory testing information and case status. The SLDSS does differentiate between Legionnaires’ and Pontiac fever.

In 2015, 6079 cases of Legionellosis were reported through the NNDSS. Some of those cases were also reported through the SLDSS, with more information. “We got almost 3700 cases of Legionellosis reported [from the SLDSS] and 98% of those were Legionnaires’ disease. So, at least of [the cases reported to the SLDSS], Legionnaires’ disease accounts for the vast majority of Legionellosis,” Dr. Cooley said. To illustrate how these numbers have grown over time, she showed conference attendees a heat map of the United States, with the colors darkening as rates of Legionellosis increased over time. “I wanted you to notice that as I go from 2005 to 2010 to 2015, the country as a whole is getting darker,” stated Dr. Cooley, driving home the grave fact that Legionnaires’ disease incidence in the United States is increasing.

Additional key takeaways from Dr. Cooley’s presentation include the following:
  • More cases/outbreaks of Legionellosis are seen in the warmer months: summer and early fall
  • Individuals over 50 are at highest risk
  • Regardless of age, 90% of individuals who contract Legionnaires’ disease are hospitalized for treatment
The UAT and a lower respiratory culture are the preferred diagnostic methods for Legionnaires’ disease, according to Dr. Cooley. “UAT is great and it’s easy; it detects Lp1, which is a common cause of reported cases in the United States, but it has some gaps,” she explained. “[The test] isn’t completely sensitive [and] that’s why we also recommend the culture of respiratory secretions on selective media be performed at the same time. Isolation of Legionella in culture is important to pick up those gaps that are missed by the UAT.” Respiratory culture is also used in outbreak settings “to compare environmental and clinical isolates for source attribution purposes,” Dr. Cooley shared.

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