Impact of meteorological factors on Legionella colonisation of water systems and the incidence of Legionnaires' disease.

Climate change will inevitably have consequences for the distribution and activity of infectious diseases. A clear example of this is Legionnaires' disease (LD), a pneumonia caused by the waterborne pathogen Legionella when inhaled from the environment in aerosol form. LD presents a seasonal pattern, with peaks in infection occurring during warmer periods of the year. Coincidentally, some man-made water systems easily reach the warm temperatures necessary for multiplication. In the present study, we analysed relationships between the temperature of the water in water samples, atmospheric temperature, accumulated precipitation and relative humidity on the one hand, and the presence of Legionella in water samples as well as LD incidence for the region of Catalonia during the years 2018-2023. To our knowledge, the present study is the first to assess the correlation between climatic factors and Legionella positivity in water systems using samples form routine testing. All LD cases reported to the Catalonia Public Health Agency during the study period were included. To analyse relationships between the variables, a maximum lag of 5 months was considered, and the estimated values of the cross-correlation function were plotted. The main findings from this study include evidence of an association between both atmospheric temperature and the temperature of the water in sanitary cold-water systems and cooling towers on the one hand, and sample positivity for Legionella on the other. We found that atmospheric temperature was also correlated with LD incidence. No correlation was found between accumulated precipitation and sample positivity or LD incidence, while the average relative humidity showed an inverse correlation with LD incidence. These results suggest that in Catalonia, meteorological factors are associated with the colonisation of water systems by Legionella and LD incidence. This study highlights that as climate change advances, an increase in LD incidence is likely to be observed.