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5.2.1 Concerned employers, building owners and operators, facility managers, and others seek to prevent real and potential health hazards, if possible. Water system operators may identify undesirable situations by monitoring routinely for legionella and may be able to implement control measures before the bacterium reaches an amount sufficient to cause human illness (see 6.2.4.2). The CDC 2000 Guidelines for Preventing Opportunistic Infections Among Hematopoietic Stem Cell Transplant Recipients advises that because transplant recipients are at much higher risk for disease and death from legionellosis compared with other hospitalized persons, periodic culturing for legionella in water samples from a center's potable water supply could be regarded as part of an overall strategy for the prevention of Legionnaires' disease in transplant centers and other facilities housing persons at high risk of infection if exposed (see 6.4.2). There is some evidence that environmental legionella surveillance should be considered a proactive strategy for the prevention of hospital-acquired Legionnaires’ disease (1). However, the optimal methodology (that is, frequency or number of sites) for environmental surveillance cultures in transplant centers has not been determined, and the cost-effectiveness of such a strategy has not been evaluated for either transplant centers or other health-care settings nor for institutional, commercial, or residential buildings.

5.2.2 Some experts advise against testing water systems for legionella in the absence of illness, particularly in buildings other than hospitals or health-care facilities, given that absolute exclusion of this bacterium from water systems may not be necessary to prevent legionellosis nor may it be achievable without considerable expense. Microbiological water monitoring increases operational costs, and interpretation of test results may be difficult (see 6.2.4). Identification of legionella in environmental samples also may cause unwarranted alarm and unnecessary remediation. The WHO publication states that legionella testing cannot be considered a control measure, but does provide some evidence that the water safety plan is effective and that control measures are operating properly. Sampling for legionella cannot provide results sufficiently quickly to be useful in operational monitoring, which instead should be by measures that provide real-time results, for example, monitoring of the biocide concentration, temperature, and pH of the water.

5.3.1 Testing potential legionella sources as soon as possible after confirmation of legionellosis may increase the likelihood of identifying the responsible source. Environmental conditions and equipment operation may change frequently, which may affect the likelihood of legionella detection. Inspectors may fail to identify the responsible source if they postpone sampling until an illness is confirmed as legionellosis (see 6.6 and 6.7) or until a search for other cases identifies common exposures (see Section 9).

5.3.2 Persons with legionellosis often have been exposed to more than one possible source during the disease's incubation period (see 6.4.3, 6.5.3) and may not recognize or recall all possible exposures. Isolation of apparently identical legionellae from clinical and environmental samples (see 6.2.1, 6.6.2.4, and Section 8) is suggestive, but does not identify a source absolutely as the site of a patient's exposure because the distribution of legionella species, serogroups, and subtypes (see 6.1.1 and 6.1.2) in the environment is not known, that is, the same legionella could colonize more than one water system. Identification of the environmental source responsible for legionella transmission may be difficult if no clinical isolate is available for comparison with environmental isolates (see 6.2.1, 6.6.2.4). Legionella has been found in a substantial proportion of water systems tested in prevalence surveys and outbreak investigations. Without a clinical isolate, identification of the probable source of legionella transmission must be based on environmental and epidemiological information (see Sections 7 – 9).