Epidemiology

Legionnaires' disease was isolated and named after an outbreak at an American Legion meeting in Pennsylvania in 1976, where 221 people were affected and 34 died. Cases have since been reported worldwide. Although most cases occur sporadically, clusters and outbreaks can occur, sometimes with substantial implications for public health.[4] Between 2015 and 2019, the US Centers for Disease Control and Prevention (CDC) reported 65 Legionella outbreaks associated with treated recreational water, resulting in 13 deaths.[5] There were 8890 cases of Legionnaires’ disease reported to the CDC in 2019, with an incidence of 2.71 per 100,000 population.[6] The European Surveillance network received 895 reports of travel-related Legionnaires’ disease in 2021, an increase of 38% more cases compared to 2020.[7]

In the US, among patients with community-acquired pneumonia who were tested for Legionella, 1.5% were positive.[8]​ The number of reported cases increased nearly 900% between 2000 and 2018. Notification rates have also increased in Europe. In 2021, the European Union/European Economic Area (EU/EAA) witnessed the highest annual notification rate of Legionnaires’ disease to date, with 2.4 cases per 100,000 population, up from 1.3 per 100,000 population in 2015.[7]​[9]​​​​​​ ​It is unclear if this increase is due to increased awareness or testing, increased susceptibility of the population, increased Legionella in the environment, or a combination of factors.[10]

Community-acquired Legionnaires' disease appears to be most prevalent during summer and fall months.[11]​ Studies have linked this to warmer and wetter weather conditions and higher relative humidity in these seasons.[4] Most cases of Legionnaires’ disease are community acquired. However, outbreaks can develop in healthcare facilities such as hospitals and care homes due to contamination of water supplies. National surveillance data from 2015 in the US found that among 2800 confirmed cases of Legionnaires’ disease in 21 jurisdictions (20 states plus New York City), around 20% were healthcare-associated.[12]​ In Europe, healthcare-associated infection was identified as the source in 5% of all reported cases in 2021.[7]

Approximately 70% of patients are male. In years past, the peak age group affected has been the sixth decade, but, since 2000, the CDC in the US has noted the greatest number of cases in middle-aged adults (45-64 years old).[11]​ In 2019, 82% of cases reported to the CDC were in people aged ≥50 years and incidence increased with age.[6]

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