Legionella spp. in Thermal Facilities: A Public Health Issue in the One Health Vision

Thermal facilities comprise a wide spectrum of recreational and therapeutic activities with the number of customers increasing over time despite the break that occurred during the first phases of the COVID-19 pandemic. In order to evaluate the potential role played by this setting in the risk of contracting legionellosis, we conducted a cross-sectional study to evaluate the presence of Legionella spp. in some Italian thermal facilities. Specifically, we retrospectively analyzed the results of a 16-year surveillance carried out before the COVID-19 pandemic. Of 409 samples, 70 (17.1%) were positive with Legionella spp. but there was an overall decrease over time. L. pneumophila 2-14 were by far the most common detected serotypes, while L. pneumophila 1 accounted for only 8.8%. Of all the different kind of samples, swabs and municipal water samples were the most contaminated. Moreover, in the positive samples, bacterial load was often at intermediate values. In only a small percentage of samples was load high and, in this case, L. pneumophila 1 was often the most common strain. Our results show the importance of a continuous monitoring of Legionella risk in these settings, even more so now after the COVID-19 pandemic and the prolonged break in activities. Water is the natural environment of Legionella spp. and environment in general plays a crucial role in the transmission of these bacteria; therefore, it would be useful to frame this infection in a "One Health" key.