Legionellosis (Legionnaires' disease)

Legionellosis is first and foremost a disease of the respiratory tract. It is caused by bacteria of the Legionella genus which are found naturally in water. Infection occurs primarily through inhaling aerosols – fine water droplets – that contain Legionella bacteria.

Legionellose


Pathogen and transmission

Legionellosis is caused by certain types of the Legionella genus of bacteria. A number of pathologies of the disease have been identified. The two most important ones are Legionnaires’ disease, a severe form with pneumonia, and Pontiac fever, a mild form with no pneumonia.

Legionella bacteria occur naturally in almost all water and damp conditions. But they propagate particularly well in water systems in which the water is not constantly renewed and the temperatures remain between 25 and 45°C – conditions that are found in such places as water pipes, taps, shower heads, hot tubs, air-conditioning equipment and cooling towers.

Infection occurs mainly through inhaling atomised water droplets (aerosols) that contain the Legionella bacteria. In rarer cases, infection can occur via Legionella-contaminated water which accidentally gets into the lungs instead of the oesophagus, a process known as aspiration. Drinking or swallowing Legionella-contaminated water poses no danger of infection, as the stomach’s gastric acids will kill the bacteria. Human-to-human transmission is basically possible, but is extremely rare.

Pathology

The first symptoms of Legionnaires’ disease generally appear after an incubation period of between two and ten days, and consist of fever, coughing, muscle pains and headaches and a loss of appetite. Disorientation and gastrointestinal problems may also occur. Legionnaires’ disease will typically also produce mild to severe pneumonia, which can be fatal. On average, despite treatment with antibiotics, Legionnaires’ disease leads to death in 5 to 10 per cent of cases.

The milder Pontiac fever has a shorter incubation period of between a few hours and three days, after which fever and flu-like symptoms develop. Patients with Pontiac fever generally recover within a few days with no antibiotic treatment required.

Distribution and frequency of occurrence

Cases of legionellosis are diagnosed worldwide. In Switzerland, several hundred cases with a positive laboratory confirmation of Legionella infection are reported to the Federal Office of Public Health each year. Of these, the cases which develop pneumonia – i.e. those of Legionnaires’ disease – are subjected to further evaluation. These cases show some seasonality, with most cases recorded in summer. The risk of developing legionellosis increases with age, and the disease is also seen far more in men. Smokers and persons with a weakened immune system are at particular risk.

Prevention

The best way of preventing Legionella bacteria from propagating in water and plumbing systems is by ensuring that hot water leaves the boiler at 60°C and is kept at 55°C in the plumbing system. Cold water should be kept at less than 25°C. For all other facilities and equipment (air-conditioning, hot tubs, air humidifiers etc.), the recommendations of the manufacturers or their trade associations should be observed. Located on the website of the FSVO (Federal Food Safety and Veterinary Office) are the issued jointly by the FOPH and FSVO (Empfehlungen zu Legionellen und Legionellose in German, also available in French and Italian). The various modules are aimed at enforcement authorities, the sanitation sector and homeowners, among others. The recommendations are intended to help reduce the risks posed by legionella and lower the number of cases of illness.

Facts and figures on legionellosis

Detailed data on legionellosis

(Page available only in German, French and Italian)

Trends in weekly case numbers, based on the mandatory reporting system developed for physicians’ reports.

Weekly case numbers

(Page available only in German, French and Italian)

Basis: Swiss mandatory case reporting system 

Last modification 11.09.2024

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Federal Office of Public Health FOPH
Division Communicable diseases
Schwarzenburgstrasse 157
3003 Bern
Switzerland
Tel. +41 58 463 87 06
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