Legionnaires’ Disease Shows Steady Increase in US Over 15+ Years

Legionnaires’ disease (LD) in the United States appears to be on an upswing that started in 2003, according to a study from the Centers for Disease Control and Prevention (CDC).

The reasons for this increased incidence are unclear, the researchers write in Emerging Infectious Diseases.

“The findings revealed a rising national trend in cases, widening racial disparities between Black or African American persons and White persons, and an increasing geographic focus in the Middle Atlantic, the East North Central, and New England,” lead author Albert E. Barskey, MPH, an epidemiologist in CDC’s Division of Bacterial Diseases in Atlanta, Georgia, said in an email to Medscape Medical News.

“Legionnaires’ disease cannot be diagnosed based on clinical features alone, and studies estimate that it is underdiagnosed, perhaps by 50%,” he added. “Our findings may serve to heighten clinicians’ awareness of this severe pneumonia’s etiology, so with an earlier correct diagnosis, appropriate treatment can be rendered sooner.”

Barskey and his co-authors at CDC — mathematical statistician Gordana Derado, PhD, and epidemiologist Chris Edens, PhD — used surveillance data to investigate the incidence of LD in the US over time. They compared LD incidence in 2018 with average incidence between 1992 and 2002. The incidence data, from over 80,000 LD cases, were age-standardized using the 2005 US standard population as the reference.

The researchers analyzed LD data reported to CDC by the 50 states, New York City, and Washington, DC, through the National Notifiable Diseases Surveillance System. They performed regression analysis to identify the optimal year when population parameters changed, and for most analyses, they compared 1992–2002 data with 2003–2018 data.

Legionnaires’ Disease Up in Various Groups

  • The overall age-standardized average incidence grew from 0.48 per 100,000 people during 1992–2002 to 2.71 per 100,000 in 2018 (incidence risk ratio [RR], 5.67; 95% CI, 5.52 – 5.83).

  • LD incidence more than quintupled for people over 34 years of age, with the largest relative increase in those over 85 (RR, 6.50; 95% CI, 5.82 – 7.27).

  • Incidence in men increased slightly more (RR, 5.86; 95% CI 5.67 – 6.05) than in women (RR, 5.29; 95% CI, 5.06 – 5.53).

  • Over the years, the racial disparity in incidence grew markedly. Incidence in Black persons increased from 0.47 to 5.21 per 100,000 (RR, 11.04 95% CI, 10.39 – 11.73) compared with an increase from 0.37 to 1.99 per 100,000 in White persons (RR, 5.30; 95% CI, 5.12 – 5.49).

  • The relative increase in incidence was highest in the Northeast (RR, 7.04; 95% CI, 6.70 – 7.40), followed by the Midwest (RR, 6.13; 95% CI, 5.85 – 6.42), the South (RR, 5.97; 95% CI, 5.67 – 6.29), and the West (RR, 3.39; 95% CI, 3.11 – 3.68).

Most LD cases occurred in summer or fall, and the seasonal pattern became more pronounced over time. The average of 57.8% of cases between June and November during 1992–2002 grew to 68.9% in 2003–2018.

Although the study “was hindered by incomplete race and ethnicity data,” Barskey said, “its breadth was a strength.”

Consider Legionella in Your Diagnosis

In an interview with Medscape, Paul G. Auwaerter, MD, a professor of medicine and the clinical director of the Division of Infectious Diseases at Johns Hopkins University School of Medicine in Baltimore, Maryland, said he was not surprised by the results. “CDC has been reporting increased incidence of Legionnaires’ disease from water source outbreaks over the years. As a clinician, I very much depend on epidemiologic trends to help me understand the patient in front of me.

“The key point is that there’s more of it around, so consider it in your diagnosis,” he advised.

“Physicians are increasingly beginning to consider Legionella. Because LD is difficult to diagnose by traditional methods such as culture, they may use a PCR test,” said Auwaerter, who was not involved in the study.Legionella needs antibiotics that differ a bit from traditional antibiotics used to treat bacterial pneumonia, so a correct diagnosis can inform a more directed therapy.

“Why the incidence is increasing is the big question, and the authors nicely outline a litany of things,” he said.

The authors and Auwaerter proposed a number of possible contributing factors to the increased incidence:

  • an aging population

  • aging municipal and residential water sources that may harbor more organisms

  • racial disparities and poverty

  • underlying conditions, including diabetes, end-stage renal disease, and some cancers

  • occupations in transportation, repair, cleaning services, and construction

  • weather patterns

  • improved surveillance and reporting

“Why Legionella appears in some locations more than others has not been explained,” Auwaerter added. “For example, Pittsburgh always seemed to have much more Legionella than Baltimore.”

Barskey and his team are planning further research into racial disparities and links between weather and climate and Legionnaires’ disease.

The authors are employees of CDC. Auwaerter has disclosed no relevant financial realtionships.

Emerg Infect Dis. Published online February 17, 2022. Full text

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