KWR, has since 1980 been providing advice, analyses and scientific research regarding this bacterium, which causes legionnaires’ disease among others.

Legionella is a bacterium that can cause legionnaires’ disease, which is a life-threatening pneumonia that was first recorded in 1976 among a group of war veterans (legionnaires) in the United States. Infection occurs when water droplets with the bacterium are spread through the air (aerosols) and are breathed in, for example during showering. The disease – also known as legionellosis – is not transmitted from one person to another, and one can drink water containing Legionella bacteria with no danger. A mild form of legionellosis, so-called Pontiac fever, is associated with flu symptoms. Locations where Legionella bacteria can multiply include installations for warm-water distribution, cooling towers, whirlpool baths and in certain industrial water installations at water temperatures above 25ºC. Legionella bacteria belong to the Legionella genus, of which more that 50 strains have in the meanwhile been identified. The most dangerous strain is Legionella pneumophila. Legionella anisa, a strain that is far more frequent in water distribution installations, is practically not pathogenic.

In the Netherlands cases of legionnaires’ disease were recorded as early as 1980. Following a 1986 recommendation of the Health Council of the Netherlands, compulsory reporting was declared for the disease in 1987, and the number of legionellosis cases began to be registered. In the Spring of 1999, more than 30 visitors to the Westfriese Flora flower festival in Bovenkarspel died as a result of legionellosis. An exhibited whirlpool was the source of the infection. As a result of this outbreak, legal measures were instituted with the aim of preventing legionellosis. Every year, a few hundred cases of legionellosis are reported in the Netherlands.  

There are a variety of sources of exposure to Legionella bacteria and a number of laws have been passed to limit exposure. In 2004, the Decree for the amendment of the Water Supply Decree and the Decree for Swimming Facilities Hygiene and Safety (prevention of Legionella in mains water) were issued. With these regulatory measures, the Ministry of Housing, Spatial Planning and the Environment aims to minimize the risk of infection from collective water distribution installations. The owners of the affected installations are obligated to conduct – or have conducted – a risk analysis, to develop a management plan, to have the mains water examined periodically for Legionella bacteria and, if required, to take corrective measures. The Environmental Protection Inspectorate is the supervisory agency – as an extension of this supervision, the water distribution companies conduct checks of their connected collective water distribution installations.

The responsibility for the timely detection and treatment of people infected with Legionella, as well as the research into the possible source of the infection, lies with the Ministry of Health, Welfare and Sport. The Area Health Authority carries out these activities.

Risk factors for the growth of Legionella
Legionella bacteria can reproduce in a variety of water installations. Exposure to the bacteria occurs when the water is atomised so that droplets can be breathed in. Reproduction of L. pneumophila is dependent on the following risk factors: water temperature, the presence of biofilm and sediment, and the stagnation of the water in the distribution network.

Legionella reproduces at temperatures between 25 and 45ºC. At temperatures above 60ºC, they die off quickly, and at temperatures below 20ºC, while the bacteria do survive, they hardly reproduce at all.

Biofilm and sediment
Legionella reproduces primarily in biofilms and in sediment. A biofilm is a thin slime coating on a surface that is in contact with water, for example, the walls of water mains. The slime coating is made up of bacteria that grow on degradable substances that are present in the water. Certain protozoa, single-cell animal organisms (including amoeba), that feed off bacteria in this slime coating and in sediment can play the role of hosts to Legionella. Limiting the formation of biofilm and sediment is therefore important for containing the growth of protozoa and Legionella.

If the (mains) water in an installation stagnates or does not move for several days, Legionella has more of a chance to grow in the biofilm and in the sediment.

Preventing and combating Legionella
Growth of Legionella in water distribution installations can be limited through thermal management. This means that the temperature of the cold water is kept below 25ºC, while that of the warm water in the mains, up until the point-of-use (thermostatic mixer tap), has a minimum temperature of 60ºC. In situations where the heating of the cold water to temperatures above 25ºC is unavoidable and/or where other risk factors cannot be eliminated, the application of an alternative management measure is permitted under certain conditions. These alternative management measures are: copper-silver ionisation, anodic oxidation, ultrafiltration and ultraviolet (UV) light. The application of such a measure can be decided upon on the basis of the results of a risk analysis. A precondition for an effective application of alternative measures is the reduction of other risk factors – for example, the removal of dead-end mains and the cleaning of the installation (elimination of biofilm and sediment).

How are water companies combating Legionella?
Legionella is naturally present in very low amounts in surface water, for example. During the purification of surface water into drinking water, the number of Legionella bacteria, particularly L. pneumophila, is reduced to the point that it can no longer be measured. The water distribution companies in the Netherlands ensure that drinking water contains no nutrients, that the distribution network remains clean, and that safe materials are used in the mains. In this way, any Legionella bacteria that remain in the water have no chance of reproducing. Moreover, the temperature of the drinking water in the distribution network does not exceed 25ºC.

Drinking water, showering
Legionella bacteria can only infect us when we breath them in. Drinking tap-water is therefore safe. Shower water is of course heated and does atomise, so it can indeed be breathed in. But even in this case, there is no danger since boilers, and similar equipment, are for safety reasons installed by certified professionals at standard temperature settings of a minimum of 60ºC. The Legionella bacteria cannot survive at temperatures above 55ºC. Neither can the use of gas water heaters lead to the growth of Legionella, since the water is heated and mixed with cold water just before the point-of-use. 

KWR’s activities
In the framework of the research programme that KWR conducts for the water companies, we pay a great deal of attention to the Legionella problem. This research also receives financial support from the government (Ministry of Housing, Spatial Planning and the Environment Inspectorate). The subjects of our research are:

  • development of rapid detection methods for Legionella bacteria in water;
  • identification of strains of Legionella bacteria present in mains water;
  • determination of the influence of the water temperature on the growth of various Legionella strains in (mains) water;
  • determination of the influence of mains materials on the formation of biofilms and the growth of Legionella bacteria;
  • determination of the influence of the water composition on the formation of biofilm and sediment in water distribution installations;
  • determination of the effectiveness of management measures.

Apart from research activities, we also provide consultancy services. In many cases this involves specifically: the detection of Legionella and the organisation of collaborative studies for the analysis of Legionella bacteria with standardised culture methods (NEN6265). KWR is also engaged in policy-supporting and -promoting studies on commission from the government and school programmes.

Laboratory analyses
In addition to employing culture methods according to NEN 6265, KWR is also able to conduct rapid analyses – within one day – using PCR methods, and has methods for genotyping Legionella strains. These latter methods are particularly important in determining whether an encountered bacterial strain is actually the pathogen.

Collaborative studies
If you have a laboratory at your disposal that itself conducts Legionella analyses, you would be well advised to participate in one of the collaborative studies that KWR organises to give laboratories the opportunity to have their performance anonymously assessed in comparison to that of others.  Laboratories recognised by the Dutch Accreditation Council participate in these studies and are accepted by the supervisory agency.

Harm Veenendaal, sample collection and Legionella and other microorganism research
tel. (030) 606 95 67
e-mail: Harm.Veenendaal@kiwa.nl.

The results of KWR research conducted into Legionella are set out in reports and publications, which are available via the KWR website – search for Legionella. Place link here. 

Other agencies
Ministry of Housing, Spatial Planning and the Environment
Kiwa Certification and Inspection
Kiwa Industry & Water
Foundation for Water Education



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