Investigation of the prevalence of Legionella species in domestic hot water systems

Aims. To investigate the prevalence of Legionella spp. in the hot water sys tems of a representative sample of Wellington domestic residences with elec trically heated hot water systems, and to investigate risk factors (eg wate r temperature, plumbing materials) for such contamination. Method. 100 households with electrically heated hot water systems in the We llington area were investigated. Samples of hot water from several hot wate r outlets were collected, and characteristics of the plumbing system were r ecorded. Water samples and swabs were cultured and further examined by poly merase chain reaction (PCR) and direct fluorescence antibody (DFA) testing to identify Legionella spp. and serogroups. Results. No Legionella spp. were isolated by culture. PCR tested positive f or Legionella in specimens from twelve residences. Six of these were also p ositive by DFA testing. The only environmental factor found to be associate d with the presence of Legionella was recent plumbing work on the hot water system. Five of the twelve PCR-positive residences, and four of the six DF A-confirmed residences had hot water delivery temperatures in excess of 60 degrees C. Conclusions. The results suggest that either Legionellae colonise domestic hot water reticulation systems and/or that the organisms are killed during passage through the hot water tank. Both possibilities may be correct. Furt her work to characterise the microbial ecology of Legionella-positive hot w ater distribution systems would be useful, as would the development of impr oved methods for culturing the organisms from potable water.