Legionella are rod-shaped, sporeless, gram-negative bacteria grouped in the family Legionellaceae. The single genus Legionella subsumes over 40 named species with more than 60 different serogroups, only a small number of which are pathogenic to humans. The epidemiologically most important species is Legionella pneumophila with at least 15 serogroups, whereby serogroup 1 has been detected in about 90% of human legionelloses in Austria.
Legionellosis (Legionnaires' disease) is a severe, often fatal pneumonia, mainly caused by inhalation of aerosol contaminated with Legionella pneumophila or, more rarely, with other Legionella species. The bacterium can occur in hot water supply systems of large buildings such as hospitals, nursing homes, retirement homes, lodging houses, spas, less commonly residential facilities, and in other man-made water-bearing systems where it finds conditions conducive to reproduction (such as in water-bearing systems of cooling towers, whirlpools, humidifiers, indoor decorative fountains, pool baths, or in natural thermal springs and their distribution systems) in numbers of concern to humans.Legionnaires' disease primarily affects adults. Despite the availability of effective antibiotic treatment and the lack of evidence of antibiotic-resistant pathogens to date, between 10-15% of people affected by Legionnaires' disease die each year. The most important measure in the fight against this infectious disease is therefore prevention.
Inhalation of legionella-contaminated aerosols (finest water droplets) or fine dusts can lead to severe pneumonia, the so-called Legionnaires' disease. Drinking water contaminated with legionella does not pose a health risk. People with weakened immune systems, chronic lung disease, and smokers are at increased risk. Transmission from person to person is not possible.
In Europe, Legionnaires' disease is mainly caused by Legionella pneumophila. However, there are also other Legionella species that are widespread in the environment and can also lead to infection under certain circumstances. Increasingly, cases of Legionnaires' disease caused by the species Legionella longbeachae are being recorded worldwide, particularly in Australia, but also in Europe. The risk of contracting Legionnaires' disease by inhaling fine dust particles containing Legionella from plant soil or compost is very low: In 2009, two cases were documented by AGES. In Australia, where infections with Legionella longbeachae are more widespread, plant soil is labelled with a risk notice and it is recommended to moisten the soil before use to avoid dust formation, to wear gardening gloves and to wash hands thoroughly after work. Wearing fine dust respirators is also sometimes advised for handling plant soil.
After the discovery of the infectious disease in 1976, national surveillance systems for Legionella infection were established in many countries. In Austria, every suspected case, case of illness and case of death must be reported. Epidemiological surveillance based on reports from physicians and laboratories together with the surveillance system of the National Reference Centre for Legionella Infection enables rapid identification of clusters. An adequate outbreak clarification with prompt identification and elimination of the source of infection can be carried out by the competent district administrative authorities with the support of infection epidemiologists of AGES.