Prevalence and distribution of Legionella in municipal drinking water supply systems in Madrid (Spain) and risk factors associated

The presence and concentration of Legionella in drinking water supply systems, in hot water (DHW) for human consumption in public buildings in Madrid with potential health risk was studied. Sampling covered a total of 1695 DHW samples and 30 cold water (DCW) as a control taken in the 21 districts of the city over a period of 14 years (2007-2020). The detection and quantification of Legionella was carried out by plate culture and quantitative qPCR. The study evaluated a series of variables including sampling year, districts, type of building, seasonality, sampling points (taps, tanks and showers), water temperature and type of disinfection used. The degree of compliance of Legionella in the water supply network of Madrid was very high (96.1 %). The degree of colonization of the positive samples ranged from 0.3 × 10³ and 1.5 × 10⁵ GU/L for a 97 % of the samples. A higher presence of this bacterium was detected in older facilities in the peripheral districts and end points able to produce aerosols such as showers. The highest number of samples with Legionella growth occurred in the 35-40 °C range. The strategies implemented have contributed to a remarkable decrease in the presence of Legionella in the last years of sampling.