Key Points

• Legionellosis has been on the rise since 2017 in the Hauts-de-France region, as well as nationally and across Europe.

• 206 cases reported in 2023 in the Hauts-de-France region (+33% compared to 2022) and a regional rate (3.6 cases per 1,000,000 inhabitants) never before seen since surveillance began.

• Favorable meteorological factors (temperatures and precipitation) contributed to three significant peaks in reported cases in 2018, 2021, and 2023.

• Spatial disparities in incidence, with:

  • a west-to-east national gradient

  • a significant and sustained excess incidence in certain areas of Hauts-de-France

• An increase in the median age of cases (61 years over the 2010–2023 period versus 56 years from 1995 to 2009), driven by men (59 years versus 54 years before 2010).

• Risk exposures within 14 days prior to symptom onset were identified in only one-third of cases

• Improved microbiological surveillance over the past 3 years with increased use of culture (1 in 3 cases in 2023) and PCR (1 in 5 cases)

• In 2023, two outbreaks were investigated in the Hauts-de-France region:

  • a large-scale Legionnaires’ disease outbreak (about 30 cases) that affected the Creil metropolitan area in the Oise department for several months.

  • 6 clustered cases of Legionnaires’ disease (2 cases) and Pontiac fever (4 cases) linked to visits to a balneotherapy center in the Nord department.