Key Points

In 2024, 306 cases of Legionnaires’ disease were reported in the Île-de-France region.

• The notification rate was 2.9 cases per 100,000 inhabitants, making Île-de-France the 5th region with the highest notification rate. It has increased compared to 2023.

• Significant disparities exist between departments, with cases primarily concentrated in Paris, Essonne, and Seine-Saint-Denis; however, the departments of Essonne and Paris have the highest notification rates (number of cases relative to the resident population in the departments).

• The characteristics of the cases—mostly men, with a median age of 66—are comparable to previous years.

• The case fatality rate is 9.5% (26 deaths), comparable to previous years.

• In 92% of cases, diagnosis was made by urine antigen testing, which remained the most common diagnostic method.

• A Legionella strain was isolated in 91 cases (30%), leading to a comparison of clinical and environmental strains in 18 cases, 13 of which were found to be identical, thereby identifying the source of contamination.

• A cluster of 17 cases in southwest Paris, with a cooling tower identified as the most likely source of contamination

• No risk exposure was reported via mandatory reporting for 61% of cases.

• The study on sporadic cases of Legionnaires’ disease and exposure in the home (LEGIODOM) began on October 1, 2024. Its objective is to document the proportion attributable to contamination of household water systems, and it will last two years.