Fact sheet on Legionella in drinking water

General
Legionella are bacteria of the genus Legionella. They are widespread in our environment, particularly as a natural occurrence in groundwater and surface waters. Legionella also occur in water distribution systems; multiplication of Legionella primarily takes place in deposits and coatings (biofilms). Legionella can pose a health hazard and cause potentially fatal pneumonia when they are inhaled via, for example, steam, aerosols or water droplets. The most clinically relevant species is Legionella pneumophila.

If Legionella are detected, appropriate countermeasures must be initiated immediately. Legislation requires the absence of Legionella in drinking water (0 CFU/100 ml). A positive finding of Legionella in a drinking water sample under the Drinking Water Ordinance must be reported to the competent authority.

Characteristics

• Rod-shaped bacteria
• Gram-negative
• Oxidase-positive
• Catalase-negative

Origin/Occurrence

• Natural occurrence in groundwater and surface waters, but also in artificial water systems (e.g. water pipes in buildings)

Significance

• Legionella are pathogens that can cause a range of illnesses in humans, from flu-like symptoms to severe pneumonia that can be fatal
• Legionellosis is mainly caused by inhalation of finely atomized or nebulized water (aerosols). Typical sources of infection include showers, garden hoses, humidifiers, taps, and industrial systems such as evaporative cooling systems. Requirements on this topic are contained in the Drinking Water Ordinance, VDI 6022 (for HVAC systems) and the Federal Immission Control Ordinance (42nd BImSchV) for evaporative cooling systems.
• There is generally no risk of infection from drinking contaminated material, because Legionella are killed by gastric acid. This also applies to foods that have come into contact with contaminated water

Legal regulations for drinking water

• The Drinking Water Ordinance 2023 (TrinkwV) "on the quality of water intended for human consumption" prescribes regular testing for Legionella
• This affects operators and owners of drinking water installations with large systems for water heating when the water is supplied commercially and/or publicly and when water nebulization additionally occurs
• A large system refers to a storage water heater or a central flow water heater with a volume of more than 400 litres or a volume of more than three litres in at least one pipe between the outlet of the water heater and the most remote tap
• Public supply concerns, for example, hospitals, schools, kindergartens, hotels and nursing homes. These facilities are required to test for Legionella once a year. Owners/landlords of multi-family houses, housing companies and property managers are also affected. For these the required testing interval is three years, unless other requirements are imposed by the authority
• The Drinking Water Ordinance sets a technical action value for Legionella of 100 CFU per 100 ml. If this value is exceeded, it must be reported to the competent public health authority
• If a company is not subject to a statutory testing obligation for Legionella, testing is still recommended as part of the duty of care if showers are present or if there is any other contact with atomized or nebulized water

Clinical picture

• Minimal infectious dose: varies; as few as 10² CFU/ml may be sufficient
• Incubation period: usually 24 to 48 hours, but can last up to 10 days
• Duration of illness: 2 to 5 days, and depending on severity even several weeks
• Symptoms of severe pneumonia (Legionnaires' disease): cough, chills, headache, high fever, but also abdominal pain, diarrhoea and dizziness/confusion
• Further complications can be life-threatening

Examples of affected water systems

• Taps, showers, garden hoses
• Humidifiers
• Evaporative cooling systems
• Drinking water installations with large systems for water heating

Conditions for growth

• Temperature
  • Optimum: +30 °C to +45 °C
  • Minimum: +20 °C
  • Generally no growth above +60 °C
• pH value: growth at pH 5.0 to 8.5
• a_w-value: growth at a water activity close to 1.0
• Oxygen requirement: aerobic to microaerophilic

Inactivation by heating and other technical measures

• Legionella are generally inactivated at a water temperature of +60 °C
• The water temperature at entry (circulation) into the hot water storage should be at least +55 °C and at the outlet at least +60 °C
• Cold water storage temperatures should be below <20°C
• Water exchange after 7 days, preferably after 3 days of stagnation in the pipeline network
• Remove deposits and biofilms
• Regular cleaning and disinfection and maintenance/replacement of devices/systems and aerators/hoses connected to the drinking water network

In case of Legionella in drinking water
Insufficient control temperature at the water heater and/or in the pipe network

Stagnation of water due to non-use, i.e. exchange after 7 days, preferably after 3 days (otherwise risk of biofilm formation)

Regular flushing of the pipes (check protocols, documentation)

Insufficient use of the piping system and/or "dead legs" with stagnation or other technical/structural defects in the pipe network

Deposits and biofilms

Contamination/wear of devices/systems and aerators/hoses connected to the drinking water network

1. Check correctness of processes and temperatures on site
   1. The water temperature at entry (circulation) into the hot water storage should be at least +55 °C and at the outlet at least +60 °C
   2. Cold water storage temperatures should be below <20°C
   3. Water exchange after 7 days, preferably after 3 days of stagnation in the pipeline network
2. Check for sufficient use of the piping network
   1. Remove deposits and biofilms
   2. Regular cleaning and disinfection and maintenance/replacement of devices/systems and aerators/hoses connected to the drinking water network
3. Effectiveness control by follow-up testing