Legionella Infection Risk From Domestic Hot Water

Paola Borella; M. Teresa Montagna; Vincenzo Romano-Spica; Serena Stampi; Giovanna Stancanelli; Maria Triassi; Rachele Neglia; Isabella Marchesi; Guglielmina Fantuzzi; Daniela Tatò; Christian Napoli; Gianluigi Quaranta; Patrizia Laurenti; Erica Leoni; Giovanna De Luca; Cristina Ossi; Matteo Moro; Gabriella Ribera D'Alcalà


Emerging Infectious Diseases. 2004;10(3) 

In This Article


Table 1 shows general characteristics of the examined water in terms of supply and distribution systems. Five heating systems were recognized, corresponding to those more frequently used at the domestic level, although with geographic differences, as the centralized systems were mainly adopted in northern Italy.

Table 2 shows chemical and microbiologic qualities of hot water samples. When samples were grouped according to their origin (mixture or groundwater), groundwater had significantly higher levels of calcium (105.7 ± 32.1 mg/L vs. 68.8 ± 31.3 mg/L, p < 0.001) and magnesium (21.5 ± 16.0 mg/L vs. 14.9 ± 6.0 mg/L, p < 0.01) and was harder (35.4 ± 13.0 vs. 22.1 ± 8.8°F, p < 0.001). Pseudomonas spp. were isolated from 56 of 146 (38.4%) samples, with levels ranging from 1 to 6.4 x 104 CFU/100 mL; 85.7% of positive samples contained fewer than 103 CFU/100 mL.

A total of 33 (22.6%) samples of 146 were contaminated by Legionella spp., and L. pneumophila ( Table 3 ) was the most frequently isolated species (75.8% of isolates). In the positive samples, the mean number of legionellae was 1.17 x 103 CFU/L (range 25 to 8.7 x 104 CFU/L); three samples (9.2%) contained ≥104 CFU/L, none of which were L. pneumophila serogroup 1. Although we examined colonies with different morphologic traits, the agglutination test did not reveal multiple species or serotypes in a single water sample.

The risk for microbial contamination according to the system characteristics was evaluated by applying a univariate logistic regression ( Table 4 ). A central warm water system and distance of the water from the heating point >10 m were strongly associated with the risk for Legionella contamination (OR 9.24 and 8.10, respectively, p < 0.001). Other significant and positive associations were observed with tank volume, plant age, flooring in the home, and total number of flats in the building. Pseudomonas contamination was positively associated with heating system age and negatively with tank distance and water operating temperature. Pseudomonas was also associated with the particular water source, with 65.3% of groundwater colonized versus 12.3% of mixture water (OR 13.44; 95% CI 5.02 to 36.03, p < 0.001).

The univariate regression was then applied to study the association between microbiologic data and water chemical parameters ( Table 5 ). Seven factors were independently protective against Legionella colonization: high levels of copper, hardness, oxidizability, and free chlorine and low concentrations of zinc, iron, and manganese. Lower levels of zinc and manganese were also associated with lower total count at 36°C and 22°C. Pseudomonas was positively associated with total hardness and iron <20 µg/L, whereas residual free chlorine significantly inhibited Pseudomonas. When water samples were grouped according to their trace element levels, water samples (n = 12) characterized by concentrations of zinc <100 µg/L, iron <20 µg/L, and copper >50µg/L were all negative for Legionella. No other system or water parameters were associated with bacterial contamination of the examined samples.

The data were reanalyzed by means of multivariate conditional logistic regression models. A central heating system, distance from heating point >10 m, and a system >10 years old were each independently associated with higher risk of Legionella colonization, whereas water with levels of copper >50 mg/L and zinc <100 mg/L were predictive of no contamination ( Table 6 ). For Pseudomonas, only groundwater remained highly predictive of colonization (OR 12.69; 95% CI 2.66 to 44.00, p < 0.001), whereas an operating temperature >50°C was predictive of noncontaminated samples (OR 0.25; 95% CI 0.11 to 0.98, p < 0.05).


The percentage distribution of Legionella species differed significantly according to the heater system (chi-square = 14.00, p < 0.05). Electric heaters were legionellae-free; the gas-heated independent systems had little contamination (10.0% of those with tank and 16.4% of those without) and were mainly colonized by either L. pneumophila serogroup 1 or non-pneumophila Legionella species. In the centralized heating systems of both single buildings and neighborhoods, Legionella colonization was higher (52.8% and 66.7%, respectively), and L. pneumophila serogroups 2-14 were the most frequently isolated serotypes. Germ concentration did not differ according to the heater type.

Table 7 shows that water temperature, level of free chlorine, and Pseudomonas contamination differed according to Legionella species ( Table 7 ). Water samples contaminated by L. pneumophila serogroup 1 were characterized by significantly lower operating temperature compared to that of the other groups, whereas water samples positive for L. pneumophila serogroups 2-14 had lower residual chlorine and higher Pseudomonas count.

The reported frequency of pneumonia symptoms was double among persons living in the legionellae-positive homes compared to those living in legionellae-free buildings (8 cases in 95 residents vs. 15 cases of 333 residents), but the difference was not significant (OR 1.95; 95% CI 0.80 to 4.75). Results did not change by correcting for the duration of residence of each person in the examined house.