Abstract

Incidence of Legionnaires’ disease is increasing, particularly in the Mid-Atlantic states in the United States; since 2015, New Jersey has documented ≈250–350 legionellosis cases per year. We used SaTScan software to develop a semiautomated surveillance tool for prospectively detecting legionellosis clusters in New Jersey. We varied temporal window size and baseline period to evaluate optimal parameter selections. The surveillance system detected 3 community clusters of Legionnaires’ disease that were subsequently investigated. Other, smaller clusters were detected, but standard epidemiologic data did not identify common sources or new cases. The semiautomated processing is straightforward and replicable in other jurisdictions, likely by persons with even basic programming skills.

Legionellosis, a bacterial disease caused by Legionella, can manifest as either Legionnaires’ disease or Pontiac fever. Legionnaires’ disease causes severe pneumonia, often requiring hospitalization, and has a fatality rate of 10%–25%, whereas Pontiac fever is generally milder and resolves on its own. In extremely rare cases, Legionella can cause extrapulmonary infections, such as endocarditis or wound infections.

Legionella bacteria are found naturally in freshwater environments, such as lakes and streams, but if the bacteria enter human-made water systems with conditions favorable to growth, such as hot tubs, cooling towers, and plumbing systems, Legionella can become a health concern. People develop Legionnaires’ disease or Pontiac fever primarily by inhaling aerosolized water droplets containing the bacteria. Any source of aerosolized water is a potential mode of transmission: shower heads, faucets, hot tub jets, decorative fountains, medical devices. Less commonly, aspiration of contaminated drinking water can transmit the bacteria. Extrapulmonary infections result from direct inoculation or secondary hematogenous spread from the lung. Since 2000, the incidence of Legionnaires’ disease has been on the rise, particularly in the Mid-Atlantic states in the United States (1).

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