Key Points

Legionellosis has been a notifiable infectious disease (NID) since 1987 due to its severity and the possibility of addressing exposure sources to limit its spread. With an average of nearly 320 cases per year between 2017 and 2021—accounting for 18% of all cases nationwide—the Auvergne-Rhône-Alpes region is particularly affected by this disease.

This BSP presents the surveillance system and the roles of key stakeholders, particularly the interventions by the Regional Health Agency (ARS) following the reporting of a Legionnaires’ disease case, in accordance with applicable regulations, which are summarized here. To describe the epidemiological situation and potential sources of exposure, in addition to data from the MDO surveillance system typically used, this BSP draws on data from the geographic surveillance system established in the Auvergne-Rhône-Alpes region.

After the surveillance system was scaled up through the early 2000s, the incidence rate of Legionnaires’ disease remained relatively stable until recent years, when an upward trend has been observed; in fact, 2018 and 2021 were record years both in the region and nationally. Geographic disparities in incidence are significant, with a west-to-east gradient observed both nationally and regionally, featuring particularly high rates in the Alpine departments of our region.

While climate and its changes undeniably appear to play a role in these geographic disparities and the recent increase, other hypotheses involving additional environmental factors (physicochemical parameters of distributed water, air quality, new sources of contamination, and/or changes in known sources, …), population-related factors (vulnerabilities within the population), or factors related to the surveillance system (comprehensiveness of mandatory reporting, changes in the use of diagnostic tests) may also contribute and remain to be explored.

Overall satisfactory, the completeness of mandatory reporting (MR) in the region, reassessed over the 2010–2020 period, showed that it has increased, reaching 87% in 2020. However, it is not uniform across the entire region, varying from 70% to 93% depending on the department, which makes it possible to consider targeted awareness-raising initiatives.

The epidemiological characteristics of cases have changed little over time and do not differ from those of other regions, except for the case fatality rate observed in Auvergne-Rhône-Alpes, which has been below the national average for several years, reflecting strong awareness of early diagnosis and effective care by healthcare professionals. The disease primarily affects men; the median age of reported cases is 65 years, and smoking remains the most commonly identified risk factor.

Urinary antigen testing remains the primary diagnostic method, but the use of PCR is increasing and allows for more frequent detection of non-Lp1 Legionella, although these remain very rare. A clinical strain was isolated in 26% of cases in Auvergne-Rhône-Alpes (2017–2021), with significant disparities across departments. It is important to remind healthcare professionals of the value of lower respiratory tract specimens for culture. The isolation of strains allows for the characterization of strains circulating in the region, and when environmental strains are available, for confirming the source of contamination through comparison, which was possible for 30 reported cases during the 2017–2021 period.

For the vast majority of reported cases (73%), no risk location outside the home was identified, suggesting that the home plays a predominant role in the occurrence of isolated cases, which now constitute the vast majority of situations. Travel-related settings (hotels, vacation rentals, campgrounds, temporary residences) represent the most frequent risk exposure among cases where risk locations were reported. These results are consistent with a few sources of exposure that could be identified through the comparison of human and environmental strains conducted by the CNR-L. Certain risk exposures that are becoming more common, such as hot tubs, have been subject to recent regulations that should help limit the risks.

Overall, while the data compiled in this BSP demonstrate progress in surveillance, medical care, management, and prevention of Legionella risk in the region, the trend in disease incidence remains concerning and justifies continuing to raise awareness among healthcare professionals regarding DO and the importance of deep respiratory samples. It also highlights the need to improve knowledge of unidentified environmental exposure factors and/or sources, as the majority of localized areas of excess incidence remain unexplained to date.