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


In our study, Legionella spp. were isolated in 22.6% of domestic hot water samples, with a mean number of legionellae in positive samples of 1.17 x 103 CFU/L (geometric mean); the highest concentration was 8.7 x 104. In previous studies in Finland and Germany, the occurrence of legionellae was similar (30% and 26%, respectively) as well as the contaminating concentration.[20,21] In an Italian study of hot water samples taken from swimming pool showers, 27% were positive for Legionella spp. and 46% for P. aeruginosa,[18] findings in line with results of our study on domestic water plants. According to a survey in Germany,[22] L. pneumophila is by far the most abundant species in potable and environmental water samples, as >75% of positive samples were contaminated by L. pneumophila.

We could not verify seasonal variability in the contamination, because all samples were taken in the spring. Recent studies, however, found that contamination was consistent throughout the year, both in terms of the species of legionellae isolated and in the concentration of organisms,[18] suggesting that the occurrence of Legionnaires' disease most frequently in the summer is not necessarily linked to a higher water contamination.

By comparing the environmental factors associated with Legionella and Pseudomonas occurrence, substantial differences in the microbes' sensitivity to these factors were observed. Pseudomonas was not influenced by system characteristics but strongly affected by water parameters. Thus, free chlorine and operating temperature appeared to inhibit these microbes, whereas groundwater origin, which influences higher degree of hardness, was found to favor Pseudomonas occurrence. The negative effect of chlorine and the positive influence of hardness, particularly higher calcium level, have been already observed in other studies on Pseudomonas water contamination.[17,23]

Conversely, system and building characteristics were the main predictors for Legionella in domestic hot water. Thus, residing at higher floors of large buildings with many apartments and with older, centralized water heating systems increased the risk for Legionella contamination compared to living in apartments with independent water heater systems and a short distance from the sampling point to the hot water distribution site. Among independent heaters, electric ones appeared to be most protective against contamination, whereas the opposite was observed in previous studies in Quebec City, where temperature of electric heaters was significantly lower than that of fossil-fuel heaters.[24] In a representative sample of Wellington, New Zealand, domestic residences with electrically heated hot water systems, no Legionella spp. were isolated by culture, but PCR tests were positive for Legionella in 12 homes, some with hot water temperatures >60°C, suggesting that the bacteria are killed during passage through the hot water tank.[11]

In our study, Legionella presence was not affected by the origin of water (groundwater vs. mixture), pipe materials, water temperature, or concentration of chlorine, and the negative association of Legionella with hardness and oxidizability disappeared in the multilogistic regression analysis. When the potable system was adopted, Legionella was found in both chlorinated and untreated water, confirming the low efficacy of this disinfecting system on microbe eradication.[25] In addition, bacteria from a chlorinated water system may be more resistant to combined and free chlorine than bacteria from unchlorinated systems.[26]

The examined domestic water samples were not colonized by multiple serotypes or strains, a common finding in hospitals, hotels, and spas.[27,28,29] This result could depend on different distribution systems and frequency of water use between private and public buildings.

Because the contaminating organism (L. pneumophila serogroup 1, L. pneumophila serogroups 2-14, or non-pneumophila Legionella spp.) was specific to a system, we could examine differences in distribution of species according to the system and water characteristics. These differences have been insufficiently evaluated in previous studies, but recent studies demonstrated that intracellular replication, cytopathogenicity, and infectivity to mammalian and protozoan cells also vary with Legionella species.[30,31]

Our hypothesis is that Legionella strains substantially differ in their sensitivity to environmental risk factors and, as a consequence, may have different ecologic niches. L. pneumophila serogroup 1, responsible for approximately 80%-90% of Legionnaires' disease cases,[32] was predominantly isolated from independent water heating systems, despite the fact that they were less frequently contaminated. Furthermore, compared with the other legionellae, serogroup 1 was found in water with a lower temperature, less Pseudomonas contamination, and a relatively higher residual chlorine concentration. Taking results together, L. pneumophila serogroup 1 appears to survive and grow in systems with a short distance between the hot water distribution site and the distal outlets. In agreement with our findings, a recent study on contaminated dental units recovered L. pneumophila serogroup 1 in nearly all sites positive for Legionella species.[33] In these conditions, the possibility of contaminated aerosol inhalation might be more frequent for L. pneumophila serogroup 1, despite the fact that this serogroup is not the most frequently isolated in hot water systems. If our hypothesis is correct, most probably simple hygienic procedures, like good cleaning practice and periodically replacing shower heads, would be effective in reducing the number of infections. From our experience with epidemic clusters of nosocomial legionellosis in a hospital mainly contaminated by L. pneumophila serogroups 2-14 with rare isolates of L. pneumophila serogroup 1, we observed that introducing adequate cleaning procedures in the bathroom and surveillance by health personnel was sufficient to avoid further cases, even when the central hot water distribution systems were not decontaminated.[34]

Our findings show the possible effect of trace elements on Legionella in hot water samples. Experimental studies have shown that Legionella spp. are affected by osmolarity[35] and metal concentration[36] and that iron limitation in vitro reduces bacteria growth and expression of the zinc-metalloprotease that is an important pathogenicity factor.[37] We show that hot water samples low in iron, zinc, and manganese, but rich in copper, predicted the absence of Legionella colonization, confirming their roles as growth promoters or inhibitors.

Of particular interest is the inverse relationship between copper levels and Legionella presence. In the examined water, the risk of Legionella contamination was approximately six times lower when copper levels exceeded 50 µg/L, without influencing Pseudomonas contamination. In other studies, copper concentrations low enough to be commonly found in drinking water reduced numbers of coliform bacteria.[13] Thus, we emphasize that this trace element influences some, but not all, bacterial growth.[33] To control Legionella in hot water systems, methods that release copper and silver ions electrolytically in water may represent a promising solution.[38,39,40,41] Although both metals play a role in limiting bacterial colonization, copper seems to better penetrate biofilm. Amoebae, the natural hosts of legionellae, have not been controlled successfully in vitro by adding metal,[42] suggesting that legionellae survive inside protozoa and are destroyed by metal ions when released into free water.

The risk of getting pneumonia was 1.95 higher among residents in the legionellae-positive homes than in residents of the legionellae-negative buildings, but the difference was not significant and was similar to that found in previous studies.[19] Legionellae concentrations of 3-7,000 CFU/L could be sufficient to produce one case per year in a susceptible population,[43] and these contamination levels correspond to those found in our study at the domestic level. In a recent epidemiologic survey on seropositivity in residents of homes with and without Legionella in the water systems, the prevalence of anti-Legionella antibodies was twice as high in persons in homes with legionellae as in those persons whose homes did not have legionellae.[44] The antibodies were most likely the result of asymptomatic infections caused by exposure in their home water supply, as no cases of pneumonia in the exposed population were reported. Most cases of sporadic legionellosis are not reported to health authorities in Italy as well as in other countries, and finding an association with a specific source of infection such as domestic contamination is rare.[45]

Our observations suggest that Legionella species should be considered when examining environmental contamination, which is essential to better evaluate environmental risk factors and select the most appropriate prevention and control measures.[46] To limit Legionella colonization at the domestic level, we suggest simple and general measures: 1) use independent domestic water heaters, 2) maintain high cleaning standards, 3) periodically replace components of the system which could favor presence or dissemination of bacteria, and 4) have a water copper content >50 µg/L. We do not believe disinfecting measures at the domestic level are needed, considering that our retrospective study on pneumonia in residents did not show a relevant evidence of risk in colonized buildings.