Management of Legionella in Water Systems

Legionella bacteria reside in many natural environments including rivers, lakes, and soils. These bacteria are also found in a variety of engineered systems that support biofilm growth, including drinking water supplies, cooling towers, hot tubs, fountains, and building plumbing systems and their outlets like faucets and showerheads. These water systems are often characterized by warm temperatures, stagnant water, and a lack of chemical disinfectants—conditions that promote the growth of biofilms and their associated protozoa, which are the hosts for Legionella bacteria. People are exposed to Legionella primarily through inhalation of contaminated aerosols into the respiratory system from these contaminated water systems. Those at higher risk for developing Legionnaires’ disease include the elderly, men, smokers, and the immunocomprised.

Incidence of Legionnaires' Disease Has Increased

Although the Safe Drinking Water Act has been effective in reducing U.S. disease rates of enteric waterborne organisms, it has had little impact on managing Legionella in water systems and buildings. In fact, the incidence of Legionnaires’ disease has increased more than five-fold from 2000 to 2017 (see Figure 1). Facilities managers can play a major role in preventing the growth of Legionella in engineered environments like building water systems.

FIGURE 1
The reported incidence of Legionnaires’ disease in the United States has increased more than five-fold from 2000 to 2017. Many factors contribute to this increase: more people are at risk because of the aging population, increased use of immunosuppressant drugs, and higher prevalence of comorbid conditions such as diabetes and pulmonary disease. There is also a growing dependence on heating, ventilation, and cooling systems, as well as increasingly complex indoor plumbing systems in large buildings. In addition, efforts to conserve water within buildings are likely to result in an increased risk of Legionella growth due to water stagnation, temperature changes, and loss of disinfectant residual. Higher water temperatures as a result of climate change may also play a role.

Source: Adapted from Shaw et al. (2018) with 2016 and 2017 data from the National Notifiable Diseases Surveillance System.

Design and commissioning of a large building is a key opportunity to ensure that Legionella control is prioritized, including the appropriate design and implementation of hot and cold water systems and heating, ventilation, and air conditioning (HVAC) features. Furthermore, they should be configured to facilitate the collection of water for Legionella monitoring and for the implementation of maintenance and remediation (such as sampling and injection ports on hot water lines). Hospitals or other buildings where sensitive populations are housed should be designed with the ability to conduct remediation in the case of contamination by Legionella or other pathogens. Unfortunately, the majority of existing large buildings were not designed in this manner and present numerous complex challenges with Legionella control.

It is clear from research and practice that, in most situations, “zero” is not an achievable target for evaluating whether Legionella has effectively been controlled, for several reasons. Some level of Legionella is common in drinking water systems in the absence of an outbreak. The following are the fundamental factors that building managers must consider in controlling Legionella.

KEY TAKEAWAYS FOR CONTROLLING LEGIONELLA

• Hot water heater temperatures should be maintained above 140°F (60°C) and the hot water temperature to distal points (the point of connection to a fixture such as a faucet, showerhead, or thermal mixing valve that blends hot and cold water right before they reach the tap) should exceed 131°F (55°C) for all building types.
• There is growing evidence that, compared to free chlorine, a monochloramine residual better controls Legionella risk from building water systems, although the reasons for the improved performance are not yet clear.
• Water utilities, plumbers, and building contractors should consider Legionella control when making decisions about pipe and fixture materials, hot water heaters, and plumbing system design.
• Low-flow fixtures should not be allowed in hospitals and long-term care facilities due to their high-risk occupant populations.

The report recommends that all public buildings have a Water Management Plan for Legionella. Today, only hospitals and healthcare facilities in New York State, those in the Veterans Health Administration network, and those that receive Medicaid and Medicare funding are required to have such plans.

Once the locations that will provide a good indication of system performance are identified, the interval for sampling can be determined. In cases where initial testing indicated there was no presence of Legionella anywhere in a facility, and the building use composition indicated no risk of exposure to building occupants, sampling may be done once every 6 months or even once per year. The sampling interval is also driven by the building’s risk tolerance. A hospital with a large immunocompromised patient population and zero tolerance for Legionella may opt for more frequent sampling. In either case, the sampling strategy is dictated by risk and the Water Management Plan parameters.

Determining a Threshold for Environmental Legionella

In order to quantify the relationship between levels of Legionella bacteria and outbreaks of Legionnaires’ disease, the report’s authoring committee reviewed dozens of studies on the occurrence of Legionella in various building types during outbreaks and routine monitoring. The committee found that a Legionella concentration of 5 × 10⁴ colony forming units per liter should be considered an “action level” (i.e., a concentration high enough to warrant serious concern and trigger remediation in a building water system). A lower action level may be necessary to protect those at higher risk for Legionnaires’ disease such as hospital patients, particularly those in intensive care, cancer, and solid-organ transplant units.