Legionella are rod-shaped, non-spore-forming bacteria living in the water. They were first detected in July 1976, after 180 people were diagnosed with life-threatening pneumonia at an American Legion congress in a hotel in Philadelphia, USA, of whom 29 died. The largest outbreak in Germany with 64 infected and 5 dead occurred in January 2010 in Ulm. The cause of the epidemic was the cooling tower of a combined heat and power plant.


Legionella are particularly prevalent at a constant water temperature between 25 ° C and 50 ° C. In the reach of humans, therefore, especially hot water circulation is affected by excessive reproduction. If there is a high retention time of the water in the piping system as well as existing biofilm, optimal conditions for rapid propagation and distribution are given. Meanwhile, more than 48 species and about 70 so-called serogroups are known. The most widespread and especially dangerous for humans is Legionella pneumophila.


Basically, transmission of Legionella by mere contact with contaminated water is possible, but unlikely. Even the drinking of legionella-containing tap water provides for healthy people i.d.R. There is no danger. However, if it comes to the inhalation of legionella-containing water in the form of an aerosol, as it arises, for example, when showering or the operation of air conditioning, there is a not insignificant health risk of getting a legionellosis especially for immunocompromised persons. In particular, hot water systems, air conditioners, humidifiers and other aerosol-causing systems are associated with the transmission of Legionella.


As already mentioned, Legionella can cause Legionellosis Disease. The most common forms are the so-called Legionnaire’s disease as well as the Pontiac-Fieder.


Legionnaire’s disease is a pneumonia caused by droplet infection. Especially in people with weakened immune systems or existing heart and lung diseases, this disease can take a life-threatening course. Estimates assume a mortality rate of up to 70%. The Pontiac fever is a rarer, but milder form of legionellosis without Lugenentzündung.



Due to the high lethality as well as other factors, according to § 7 of the Infection Protection Act for infections by Legionella in Germany there is a duty to report. the health authorities. Nationwide, approximately 400 cases are reported each year, with the number of unreported cases being significantly higher.

In addition to §37 Infection Protection Act also §6 Drinking Water Ordinance (TrinkwV 2011) “the freedom of drinking water from pathogens”. Further regulations and recommendations for action can be found, inter alia. in DVGW worksheet W551. According to this, drinking water is considered as contaminated already at a value of 100KbE / 100ml (colony-forming units). For values greater than 10000KbE / 100ml, the worksheet calls for immediate action, such as disinfecting the piping network, imposition of a shower ban (e.g., in sports halls), and other countermeasures. In addition, the health authorities have u.a. the authority to temporarily close buildings.


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