What is the incidence of Legionella infection in the US?

Updated: Nov 13, 2018
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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An estimated 8000-18,000 cases of Legionnaires disease are reported in the United States each year. Most cases are not reported. More than 80% of cases are sporadic throughout the year, and the rest occur in outbreaks during the summer and early fall.

In adults, legionellosis causes 2-15% of all cases of community-acquired pneumonia (CAP) requiring hospitalization. Legionellosis is the second most frequent cause of severe pneumonia requiring ICU admission. Estimates for the proportion of nosocomial pneumonias caused by Legionella species widely vary, but the numbers probably represent an underestimation because most hospitals only test for serogroup 1. [15]

Serologic studies suggest that children are frequently exposed to Legionella species. However, this organism is a rare cause of acute respiratory disease in the pediatric population, with only scattered case reports available to determine its natural history in this age group. Moreover, it is not commonly seen in immunocompromised pediatric patients. [16]

The estimated frequency of Legionella pneumonia cases that require hospitalization is approximately 1-5%. [4, 17] The reported annual incidence of both CAPs and nosocomial pneumonias caused by Legionella species has increased. Most reported cases have involved neonates, children who are immunocompromised [4] (including those with prolonged courses of corticosteroids [12, 13] ), and children with underlying respiratory disease. [18]

The CDC reported 2,809 cases of Legionnaires’ disease in 2015, of which 3% were confirmed to be associated with a health-care facility and 17% were possibly associated. Among the definite health care-associated cases, 88% occurred in those 60 years of age and older. The fatality rate was 25% in definite health care- associated cases and 10% for the possibly associated cases. [19]  

A study reviewed case records to determine the epidemiology of and risk factors for the 1,449 cases reported to the New York City Department of Health and Mental Hygiene from 2002–2011. Incidence of Legionnaires’ disease in the city of New York increased 230% from 2002 to 2009 and followed a socioeconomic gradient, with highest incidence occurring in the highest poverty areas. The study also added that further studies are required to clarify whether neighborhood-level poverty and work in some occupations represent risk factors for this disease. [20, 21]

According to the CDC, passive surveillance for legionellosis in the United States showed a 249% increase in crude incidence during 2000-2011. In 2011, the Active Bacterial Core (ABC) surveillance system was instituted, Overall rates were similar to the passive system however, ABC’s data showed that during 2011-2013, 44% of patients with legionellosis required intensive care, and 9% died. [22, 23]

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