Large buildings with hot water tank, like hospitals, health-care units and hotels are susceptible to colonisation by Legionella. In these buildings the temperature must be kept high in order to prevent Legionella colonisation and cases of LD. The WHO guidelines and Spanish and Catalan legislation recommend maintaining temperatures of at least 50°C in circulating hot water and above 60°C in the hot water tank; however, the WHO fact sheet states that at 50°C Legionella is able to persist and colonise the pipeline system 26.
The hospital where this study was performed has a past history of Legionella colonisation. Several disinfection measures were applied, but none were able to completely eradicate the bacterium 27,28. The intervention described here to eliminate Legionella colonisation showed the importance of the pipeline material and of maintaining the water at a temperature of 60°C in the distal points. The points below 60°C were the ones that presented the highest rates of colonisation.
Before the replacement, the water temperature in the pipeline in the area nearest the hot water tank was around 60°C but the temperature was lower at points further away: most of the samples below 60°C were from the Lower-North area, the one furthest away from the tank. The larger a suboptimal pipeline system is, the greater the heat loss. Although the temperature of circulating water started at 60°C, this level was not maintained across the entire pipeline network, and at the points with lower temperatures Legionella was able to colonise the site and persist inside the circuit.
In the second year, increasing the temperature and changing the pipeline system (i.e., replacing galvanised iron with PVC) achieved a stable temperature throughout the building, with all but one sample remaining above 60°C. This measure was able to eradicate the Legionella colonisation in the building; however, since amoeba are still present in the pipeline, the risk of pipeline Legionella re-colonisation remains if the temperature falls once more.
Corrective measures and thermal regimes have been implemented in large buildings to eradicate Legionella from water systems. Gavaldà et al. concluded that a combination of corrective measures and raising the temperature at the end points to 55°C might reduce the presence of Legionella29. However, after implementing the same measures, Bédard et al. achieved a reduction of Legionella colonisation in only one of the two systems analysed 30.
Maintaining the temperature of the circulating water above 60°C entails a high energy cost. An alternative measure for preventing LD cases in hospital is the installation of point-of-use filters in the faucets. This measure is currently in use in high risk patient areas, but it is expensive and effective only at the installed points; what is more, its effects are only transitory. The installation of point-of-use filters in areas with high risk patients entails a cost of nearly 30 000€/year (for 100 points), whereas raising the temperature in the return pipe to the hot water tank from 50°C to 55°C would suppose an increase of around 32 000€/year. So, the cost is similar, but raising the temperature eradicates Legionella throughout the building – not just at the most susceptible points.
One way to reduce the cost of keeping the circulating water above 60°C would be to install local heaters in critical areas. This would mean that the temperature in the tank would not have to be 60°C in order to keep the circuit at the level required; it would only be necessary to increase the temperature at the “weak” points. Local heaters would drastically bring down the energy costs of heating the tank to high temperatures, and would also avoid overheating of the pipeline, minimise pipeline damage, and reduce the risk of scalding patients.
Local heaters are ideal for buildings with large pipelines and with numerous distal points, such as the one analysed here (with 13 floors and 28 rooms on each floor). These local heaters would also be a good solution for old buildings which use hot water tanks and recirculating water where Legionella can persist in the pipeline, and where colonisation has not been prevented by the technical changes carried out. As the WHO suggested, future buildings should be built using a pipeline material that inhibits biofilm formation, and should implement disinfection measures at the beginning of water circulation to prevent Legionella colonisation 31. Furthermore, temperature control studies should be conducted to identify critical areas where Legionella can grow, and local heaters should be installed to avoid its proliferation.
In the study by Lasheras et al., another factor shown to correlate with Legionella colonisation was water hardness. Those authors concluded that softer water was better in this situation 32. As our hospital’s water is semi-hard 33, it would be advisable to install a water softener to improve water quality.
Though Legionella was undetectable in the system after the pipeline improvement, amoebas were not affected by this change. Lasheras et al. found a correlation between the presence of amoebas and a larger number of Legionella-positive points, since amoebas are Legionella reservoirs 32. Although we did not detect Legionella in the second year, we cannot be entirely sure that itwas not present in the pipeline; after environmental stresses, Legionella can change its metabolic state to viable but non-culturable (VBNC) and settle in biofilms or parasite amoebas. In any of these states, Legionella is more difficult to detect. In the VBNC state Legionella has low metabolic activity and does not grow in culture media, but in spite of this low growth rate the bacteria retain the features of viable cells such as their integrity and virulence 34,35.
The impossibility of growing bacteria in VBNC in culture media is a limitation both of our study and of the current regulations regarding environmental Legionella control. The absence of culturable Legionella in water distribution systems does not guarantee water healthiness/safety because VBNC Legionella maintains its capacity to infect and to cause hospital-acquired LD in the most vulnerable/susceptible patients. In this situation there is a clear need to change the current regulations, and to add the detection of VBNC to the culture plate method currently in use. VBNC can be identified by the detection of specific genetic material with PCR using reagents such as propidium monoazide (PMA) or ethidium monoazide bromide (EMA), by antibody detection using flow cytometry protocols, or by reactivation of Legionella cells by adding activating agents or infecting amoeba cells 35–37. The use of these methods in environmental Legionella control would help to broaden our understanding of Legionella colonisation in water distribution systems. They would also provide information on the power of the disinfectants used in the facility, and on their modes of action.