Legionella

(endorsed 1996)

Guideline

No guideline value has been set for Legionella in drinking water.

General description

The family Legionellaceae contains a single genus, Legionella, with 26 currently reported species, of which L. pneumophila serogroup 1 is most frequently associated with human disease. Other serogroups of L. pneumophila and occasionally other Legionellae have also been reported to cause disease.

Legionella organisms are widespread in natural sources of freshwater and may also be found in soils. They occur commonly in man-made water systems, particularly in hot water and cooling water systems.

Legionella spp. appear to infect humans by inhalation, and their presence in drinking water per se seems irrelevant until they are amplified by growing in specific sites under specific conditions (usually thermal enrichment), from which infective aerosols, and droplet nuclei, may be created.

Conditions in cooling towers, spas, warm water systems in buildings, hot water systems operated below 60°C, or ‘dead legs’ of hot water systems operated at higher temperatures, may favour the growth of Legionella organisms. Spraying water in cooling towers or water agitated in spas may then produce aerosols; water from hot water systems can also form aerosols in showers, through nozzle heads, or by splashing in sinks, baths etc.

Legionella organisms can be ingested by the trophozoites of certain amoebae (Acanthamoeba, Hartmanella, Valkampfia and Naegleria) and then grow intracellularly and become incorporated in their cysts. This may explain the difficulty in eradicating Legionella organisms from water systems, and it could be a factor in the aetiology of Pontiac fever.

Australian significance

Legionella spp. have been found in cooling tower waters in many parts of Australia. However, very few Legionella organisms have been isolated from drinking waters. No published reports are available on the presence of L. pneumophila in drinking waters.

Treatment of drinking water

Treatment of water with chlorine or chloramines will eliminate these organisms.

Method of identification and detection

Legionella spp. are Gram-negative, rod-shaped, nonsporing bacteria that require L-cysteine for growth and primary isolation. Cellular fatty acids in Legionella organisms are unique for Gram-negative bacilli in that they contain primarily branched chains.

An Australian Standard has been developed for the detection of Legionella organisms in water (AS3896 1991).

Isolation of Legionella spp. from environmental samples may require pre-concentration if numbers are low. Immunofluorescence techniques may also be used to detect Legionella spp. in the environment.

Health considerations

Legionella spp. are not known to cause disease by the ingestion of drinking water.

Legionella infections can lead to two types of disease: legionellosis and Pontiac fever. The epidemic form of legionellosis associated with a common infection source is also known as Legionnaires’ disease. This is a form of pneumonia with an incubation period usually of 3 to 6 days. Males are more frequently affected than females, and most cases occur in the 40 to 70 year age group. Risk factors include smoking, alcoholism, cancer, diabetes, chronic respiratory or kidney disease, and severe immunosuppression, as in transplant recipients. Ten per cent or more of cases are fatal, even though Legionnaires’ disease can be treated effectively by antibiotics such as erythromycin and rifampicin.

Pontiac fever is a milder disease with a high attack rate. The incubation period is 5 hours to 3 days, and symptoms are similar to those of influenza: fever, headache, nausea, vomiting, aching muscles and coughing. No fatal cases have been reported and few outbreaks have been recognised, possibly because the nonspecific nature of the symptoms of the disease hinders its detection.

Infection through human-made water systems such as cooling towers and hot water supplies proceeds through inhalation of aerosols which are small enough to penetrate lungs and be retained by the alveoli – the degree of risk depending on four factors: the density of the bacteria in their source, the extent of aerosol generation, the number of inhaled bacteria, and the susceptibility of the exposed individual.

The number of inhaled bacteria depends on the size of the aerosol generated (<5 μm being most dangerous), the dispersal of the aerosol in the air, and the duration of the exposure. Host defence is important in determining whether exposure to Legionella organisms will lead to clinical disease, and differences in susceptibility largely explain the fact that in some cases, high counts of L. pneumophila in water systems have been reported in the absence of disease, whereas in other cases similar or lower counts have been associated with epidemics. It is also likely, although not yet adequately proven, that differences in virulence between strains account partly for these observations.

Advice on disinfection

It is not necessary to monitor water systems for Legionella spp. routinely or to disinfect all environmental sites where Legionellae are detected. The following are generally accepted indications for disinfection:

• sites which are implicated in an outbreak of Legionnaires’ disease or Pontiac fever

• hospital wards housing high-risk patients, such as organ transplant units

• buildings in which the water system has not been used for some time and where high numbers are likely to be found.

Vulnerable systems should be designed and maintained in such a way that colonisation by Legionella spp. is prevented or minimised. The main points to consider are:

• preventing the accumulation of sludge, scale, rust, algae and slime and removing such deposits regularly

• maintaining hot water temperatures permanently above 60°C or at intervals above 70°C, and keeping cold water supplies below 20°C

• selecting materials in contact with water which do not release nutrients that support the growth of Legionella spp.

These measures are preferable to, and more effective than, the use of biocides to control Legionella organisms in water supplies within buildings; however, biocides are essential to prevent the build-up of microbial slimes in airconditioning systems that use wet evaporative cooling towers. Such systems should be kept clean and well maintained. They should be inspected weekly for fouling and accumulated slime, scale and corrosion, and thoroughly cleaned and disinfected twice yearly. Biocides are best used intermittently in clean systems.

Derivation of guideline

No specific guideline value can be established for Legionella spp. The absence of test mechanisms does not guarantee the total absence of the organism. Warm-water handling systems should always be regarded as being at risk of contamination by Legionella spp.

NOTE: Important general information is contained in PART II, Chapter 5

---

Reference

AS 3896, (1991). Waters – Examination for legionellae. Australian Standard, Standards Association of Australia, Sydney, NSW.