Legionella occurrence and risk

By the Environmental Protection Agency

Premise plumbing systems have been identified as a source of legionella infection (Stout et al., 1992; Muder et al., 1986). Within healthcare facilities such as hospitals and nursing homes, potable water supply is the most common source of exposure (Lin et al., 2011a). Exposure to legionellae has also been associated with other types of premise plumbing systems (such as hotels and other buildings with complex water distribution systems) (Silk et al., 2012; Hung et al., 1993; Tobin et al., 1981a and 1981b).

The European Centre for Disease Prevention and Control (ECDC) reports that 58% of sampling sites that tested positive for legionella in 2014 were from cooling towers and 26% were from water systems, including 66 hot water systems, 31 cold water systems, and 184 non-specified water systems (ECDC, 2016).

Legionella pneumophila

Legionella species are known to be present in finished water from water treatment plants (Lu et al., 2016; Buse et al., 2012) and can persist and grow in the biofilms of municipal water distribution systems (Lu et al., 2016; Wingender and Flemming, 2011; States et al., 1987). Lu et al. (2015) identified diverse legionella species, including L. pneumophila, L. pneumophila sg1, and L. anisa, in sediment samples from municipal drinking water storage tanks in 18 locations and across 10 states. In the Lu et al. (2015) study, quantitative polymerase chain reaction (qPCR) was used instead of a culture-based approach, and legionella species were detected with a frequency of approximately 67%. A co-occurrence of Acanthamoebae and legionella was observed. Quantitative PCR-based monitoring complements culture-based methods in the presence of disinfectants that affect cell culturability (Bédard et al., 2016).

Schwake et al. (2016) conducted two surveys of tap water: one in small buildings (for example single- storey homes and businesses) and the other in two hospitals in Flint, Michigan. They found L. pneumophila in the two hospitals but not in the small buildings. Schwake et al. (2016) looked at linkages between a legionella outbreak and changes in municipal water quality, and operational changes in the distribution system. The study mentions that water utilities may have a role to play in controlling proliferation of pathogens in premise plumbing.


“… 26% (positive tests) were from water systems, including 66% hot water systems …”


Further, L. pneumophila can form biofilm from secreted substances (that is, extracellular polymeric substances) and can multiply within such biofilms (Mampel et al., 2006). Therefore, biofilms in municipal drinking water systems can be a potential source of water contamination (Wingender and Flemming, 2011) and drinking water from municipal systems can possibly contaminate the premise plumbing systems in hospitals and other buildings with L. pneumophila (Donohue et al., 2014; States et al., 1987).

Several surveys have found legionella in premise plumbing systems, including in buildings that had not been linked to recognised outbreaks:

  • Bartley et al. (2016) traced the epidemiology of two nosocomial (hospital-acquired) cases of legionnaires’ disease at a hospital in Australia. Whole genome sequence analysis was performed on L. pneumophila isolates from the patients infected in 2013. The genome sequences were found to be closely related to those of isolates from the hospital water distribution system and to retrospective isolates from a patient infected in 2011.
  • Bédard et al. (2016) found L. pneumophila in the hot water system of a hospital from 85% of sampled taps despite copper treatment. A significant decrease in L. pneumophila count by culture was observed following heat shock disinfection. Ongoing corrective measures were implemented, which included increasing the hot water temperature from 55 to 60°C, flushing taps weekly with hot water, removing excess lengths of pipe, and maintaining a temperature of 55°C throughout the system. A low level of contamination remained in areas with hydraulic deficiencies.
  • Rhoads et al. (2016a) studied L. pneumophila trends in controlled, replicated pilot-scale hot water systems with continuous recirculating lines. They demonstrated the potential for thermal control strategies to be undermined by distal taps and corrective mixing. Rhoads et al. (2016b) surveyed a cross-section of green buildings and compared them to conventional buildings. They found increased water age, and decreased chlorine and chloramine residuals in the green buildings, as well as increased levels of total bacteria 16S rRNA genes and increased levels of gene markers for legionella. The authors concluded that the elevated water age inherent to achieving the sustainability goals of plumbing systems in green buildings raised concerns with respect to the chemicals and microbiological stability of the water quality.
  •  Donohue et al. (2014) used two qPCR assays to evaluate incidence of L. pneumophila serogroup 1 in 272 water samples collected in 2009 and 2010 from 68 public and private cold drinking water taps across the United States. L. pneumophila serogroup 1 was detected in 47% of the taps.
  • Stout et al. (2007) isolatedand L. anisafrom 14 hospital water systems. They observed high-level colonisation of the premise plumbing system (defined as 30% or more of the distal outlets being positive for)for six of the 14 hospitals with positive findings.
  • Borella et al. (2005b) studied legionella in hot water samples of 40 hotels in five Italian cities. They detected legionella in 30 hotels and 60.5% of samples. L. pneumophila was found in 87% of positive samples, and L. pneumophila serogroup 1 was in 45.8% of positive samples. Of the samples positive for L. pneumophila serogroup 1, 75.8% had concentrations of 1 000CFU/ℓ (colony-forming units per litre) or more. The authors found that L. pneumophila serogroup 1 presence correlated with soft water and higher chlorine levels (>0.1 milligrams per litre (mg/ℓ)). They also noted that P. aeruginosa was less likely to occur at these chlorine levels and more likely to occur in hard water.
  • Patterson et al. (1997) sampled hot and cold-water outlets in 69 organ transplant units in the United Kingdom for legionella and protozoa. They found legionella in 55% of units and L. pneumophila in 45%. Other legionella (the blue–white fluorescent group, which includes L. gormanii, L. bozemanii and others) were detected in 26%of organ transplant units. Protozoa of genera known to support growth of legionella were found in 58% of units. The authors found a significant association between the detection of legionella and the presence of these protozoan genera in the cold-water outlets sampled.
  • Wadowsky et al. (1985), using tap water from their laboratory, found that naturally occurring L. pneumophila multiplied at a temperature between 25 and 37°C at pH levels of 5.5–9.2, and at concentrations of dissolved oxygen of 6.0–6.7mg/ℓ. They also noted that legionella growth did not occur in tap water when the dissolved oxygen level was less than 2.2mg/ℓ. They also observed an association between the multiplication of L. pneumophila and non-legionellaceae bacteria, which were also present in the water culture.
  • Wadowsky et al. (1982) sampled showerheads, shower pipes, and water and sediment collected from the bottom of hot water tanks in 11 buildings, including five homes and three hospitals. L. pneumophila serogroups 1, 5 and 6 were isolated from the drinking water fixtures in seven buildings, including one of the five homes. Legionella species were also present in water and sediment in hot water tanks maintained at temperatures from 39 to 54°C, but not found in tanks maintained between 71 and 77°C. The authors hypothesised that hot water tanks are the major source and seed of L. pneumophila in premise plumbing systems.
  • Tobin et al. (1981b) conducted a premise plumbing system survey of 31 buildings including hospitals and hotels, six of which were associated with sporadic cases or outbreaks of legionnaires’ disease. For the six buildings (hospitals and hotels) associated with cases of legionnaires’ disease, the study found L. pneumophila in all of the premise plumbing systems and in the cooling water for each of the three buildings with cooling towers. For buildings that had not previously experienced an outbreak, the study found L. pneumophila in four out of 24 taps or showers, three out of nine cooling towers, and one out of 15 storage tanks.

The growth of legionella within a premise plumbing system may be a function of the system’s pipe or other plumbing materials, water temperature, water quality and other system-specific factors. Tai et al. (2012) found that copper inhibited biofilm growth at temperatures typically found in hot water systems (20, 37 and 44°C), whereas stainless steel and polyethylene promoted the development of biofilm and growth of L. pneumophila. Biofilm formation by L. pneumophila was found to be inhibited in iron-rich conditions (Hindré et al., 2008). Moritz et al. (2010) found that L. pneumophila and P. aeruginosa penetrated biofilms grown in cold water on different plumbing materials in the laboratory: ethylene-propylene diene-monomer (EPDM) rubber, silane cross-linked polyethylene, electron ray cross-linked polyethylene, and copper. The pathogens, added to biofilms after 14 days, became part of the biofilms in EPDM and the polyethylenes; however, only L. pneumophila grew in the copper biofilm, and only in low numbers. In a study of eight different plumbing materials, latex and synthetic rubbers (ethylene-propylene) grew the most extensive biofilm, probably because these materials leach the most nutrients (Rogers et al., 1994).

Regulatory context

EPA regulates legionella under the Surface Water Treatment Rule (SWTR). The SWTR has treatment technique requirements to control for Giardia and viruses. The SWTR’s treatment technique requirements presume that if sufficient treatment is provided to control for Giardia and viruses ((that is, 3-log (99.9%) inactivation of Giardia and 4-log (99.99%) inactivation of viruses)), then legionella risks will also be controlled. In addition, the Revised Total Coliform Rule (USEPA, 2013a) and the Ground Water Rule (USEPA, 2006a) have treatment technique requirements that address bacteria. Corrective actions related to treatment technique violations may provide some control of legionella. All of these rules apply to public water systems (PWS).

Premise plumbing systems that do not meet all the exemption criteria in the Safe Drinking Water Act (SDWA) Section 1411 and 40 CFR 141.3, are subject to federal drinking water regulations under 40 CFR Part 141. Adding certain water treatment technologies in a premise plumbing system could impact the chemical and microbial quality of the water and change the regulatory status of the premise plumbing system. The criteria for being a regulated PWS are provided at 40 CFR 141.3. Where there are questions about the application of these criteria, the primacy agency (for example, the state) typically makes the determination based on these criteria and any relevant site-specific considerations. EPA has issued guidance that primacy agencies may use as they make regulatory application decisions ((USEPA, 1976 (revised in 1998); USEPA, 1990 (revised in 1998)). States and/or local governments may have drinking water standards for such systems, even if federal regulations do not apply.

A determination of which technology is best suited for a particular premise plumbing system is case-specific in part due to the complex and diverse nature of premise plumbing systems and local water chemistry. This document does not specifically recommend the addition of treatment nor the installation of any of the technologies discussed herein; however, it does provide information regarding the operational requirements with which regulated PWSs must comply. This information is included only to provide the reader with a comprehensive understanding of the technologies.

Facility owners or operators who are considering adding treatment to their building’s premise plumbing system may wish to consult with their water supplier (that is, PWS) to better understand any potential water quality issues before making treatment-related decisions. The installation of treatment may also trigger cross-connection control measures to protect the water supplier. If a decision to add treatment in the premise plumbing system seems likely, EPA advises facility owners or operators to consult with their primacy agency for any specific requirements that may apply before they add any treatment.

In addition to the drinking water regulations under SDWA, manufacturers of pesticidal treatment technologies used to control legionella and other microbial contaminants need to comply with the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) requirements, which are independent of the SDWA requirements. Under FIFRA, pesticide devices are regulated, and unless exempt, pesticide products that contain a substance or mixture of substances and that make a pesticidal claim, must be registered by EPA prior to sale or distribution. Registration of a pesticide product under FIFRA does not mean that it meets the requirements of SDWA or vice versa.


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