Legionella in water: prevention and control

Legionella is a widespread bacterium in the environment which, in low concentrations, also finds its way into our groundwater or drinking water. However, they must be eliminated if they are present in drinking water or groundwater in excessive concentrations. This article explains what legionella is, how to prevent it, what to do in case of contamination and what the drinking water directive stipulates.

What is legionella?

Legionella (Legionella spp.) are widespread environmental germs or water bacteria that live in water. As legionella spread naturally, they are often found in water bodies such as rivers and lakes, in groundwater and also in drinking water. They thrive in pipe systems at a temperature of 25°C to 45°C. Legionella are not only found in hot water, but also in cold water. However, they do not multiply significantly at temperatures below 20°C. They have monopolar flagella and can therefore move around in liquid habitats. Over 60 species of Legionella are currently known. Of these, Legionella pneumophila is the most common pathogen, responsible for around 90% of all illnesses, including Legionnaires' disease (legionellosis).

How do legionella bacteria develop?

Legionella can form in various habitats. Decisive factors are the temperature of the water, which can contribute to reproduction, as well as the length of time spent in the pipe system (stagnation).

The most common habitats are:

• Drinking water installations with water temperatures above 25°C.
• Re-cooling plants and ventilation systems.
• Swimming pools or other type of pools. Especially when it comes to hot whirl pools.
• Technical devices such as inhalers, oral irrigators or humidifiers.

Stagnation lines (pipe sections without a functional tap) promote the multiplication of legionella in water and worsen the drinking water quality. Other contributing factors are a dysfunctional hot water system, water contaminated with germs flowing back from the wastewater to the fresh water, sources of germs in the water tank or drinking water heater or an incorrect water temperature.

💡 An incorrect water temperature can be caused by water that is too cold in the hot water pipes, water that is too hot in the cold water pipes or stagnant water due to shutdowns, such as temporary closures, construction phases or if water is not regularly flushed from the drinking water system.

Recent Trends in Legionellosis in England and Wales (2017–2023)

The UK Health Security Agency's report on Legionellosis in England and Wales from 2017 to 2023 highlights several key findings:

• Increase in Cases: In 2023, there were 604 confirmed cases of Legionnaires' disease, marking a significant rise from previous years – the increase in reported cases may partly result from enhanced testing and diagnostic practices, rather than an actual increase in disease incidence.
• Demographic Distribution: Individuals aged 60 and over accounted for 64.2% of cases
• Fatality Rate: The crude fatality rate was approximately 3.1%, consistent with the past decade.
• Exposure Sources: Community settings were linked to 62.7% of cases, travel abroad to 35.1%, and healthcare settings to 2.2%.

Despite healthcare settings accounting for a smaller proportion of cases, patients' vulnerability in these environments necessitates stringent preventive measures. That is crucial to mitigate the risk of Legionella exposure, especially among immunocompromised individuals.

Legionella in water: testing for legionella according to the EU Drinking Water Directive

Testing for legionella in the drinking water of large-scale public or commercial water heating systems is mandatory according to the EU Drinking Water Directive. This sets the maximum permitted level of legionella bacteria at 1000 CFU/l. CFU is the abbreviation for "colony-forming unit" and is used to quantify the presence of microorganisms such as legionella.

The European Centre for Disease Prevention and Control also establishes the measures for the treatment and prevention of legionella outbreaks. These include disinfecting hot water systems with high levels of chlorine for 2–4 hours after work on the system and water heaters, and before the beginning of every warm season and regularly cleaning and disinfecting water filters, cooling towers and associated pipes used in air conditioning systems.

Getting rid of legionella: measures against legionella in water

Prevention is the most important measure against a legionella infestation. According to the EU Drinking Water Directive, an infestation is unlikely If the limit values are not exceeded. It is also important to prevent stagnant water and to ensure that a minimum temperature of 60°C is maintained in the entire hot water circuit of a domestic installation. This also significantly limits the proliferation of pathogens in general.

However, a temperature of ≥ 70°C is required to eliminate legionella. Many drinking water installations cannot technically support this temperature. 

Immediate measure: Legionella filters

The use of legionella filters for taps and shower heads can be a sensible immediate measure, and not just in cases of heavy contamination. The use of such filters is also often a recommended measure to protect health in the case of lower levels of contamination.