Legionella bacteria are Gram-negative, non-spore-forming, mobile rod-shaped bacteria. The most medically significant type is Legionella pneumophila, which exists in 14 serogroups, of which groups 1, 4 and 6 are the most important. In addition to this type, which causes about 90 % of all cases of legionella, there are more than 40 other species with more than 60 serogroups.

The primary reservoir for legionella bacteria is fresh water. There, they multiply intracellularly into protozoa (e.g. amoebas) at temperatures ranging from 25 °C to 55 °C. The multiplication of these bacteria is best fostered on damp surfaces (hot water tanks, air conditioning units, etc.).

Route of infection
Infection is primarily caused by inhaling bacteria-contaminated water in the form of aerosols. A crucial condiction for this is the presence of infected amoebae, since legionella activate their virulence genes intracellularly. There is no risk to immunocompetent people from drinking contaminated water. Human-to-human transmission has not been established. In particular, transmission has been associated with the following types of technical systems:

  • Hot water supply systems
  • Jacuzzis
  • Air conditioning systems
  • Dental equipment

Incubation period:
Legionella pneumonia: 2-10 days
Pontiac fever: 1-2 days

Clinical Picture

Legionnaire’s disease:
Starts with uncharacteristic prodromal symptoms, such as malaise, headache, pain in the limbs and an unproductive cough. Within a few hours, it leads to chest pain, chills, a rise in temperature to 39 °C – 40.5 °C, sometimes even abdominal pain with diarrhoea and vomiting. Involvement of the CNS can cause drowsiness and states of confusion. Overall, the disease is characterised by the occurrence of strikingly severe forms of pneumonia. The mortality rate is 15 % and, in untreated immunodeficient patients, can reach up to 80 %.

Pontiac fever:
The disease begins with head, limb and chest pain, cough, fever, and sometimes states of confusion. It is characterised by a more mild course of the illness, similar to a viral infection.


  • Culture
    Sample material: Respiratory secretions, particularly BAL, pleural aspirate, bronchial secretion, lung tissue and sputum only to a certain extent.
    Result: proof of infection after 5-14 days
  • Antigen detection using EIA
    Sample material: Urine from the acute illness phase
    Result: proof of infection in 4 hours with L. pneumophila serogroup 1
  • Antibody detection using indirect immunofluorescence
    Test material: Serum pair (acute phase / 10th to 15th day of illness)
    Result: proof of infection in 4 hours only when there is a fourfold increase in titres; it therefore only has a retrospective value

Since there is no single diagnostic procedure which can demonstrate Legionella infection with a high degree of certainty, it is necessary to apply several methods at the same time. All of the named diagnostic procedures are performed by our laboratory.

Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin)
Alternatives: macrolide antibiotics (azithromycin, clarithromycin, erythromycin)


  1. Measures against contamination of water-conducting systems, such as strict requirements for the new construction of drinking water, heating and pipework systems.
  2. Measures for patients and contact persons
    When legionellosis and serious illness is suspected, in-patient treatment is indicated. No special measures are required for contact persons. The route of infection should be clarified as much as possible.
  3. Measures in the event of outbreaks
    It is important to identify the source spreading the pathogens quickly so as to be able to initiate appropriate measures (heating, disinfection). The competent office of health should be informed immediately.

To clarify contamination of water-conducting systems and prevent illness, the water can be tested for legionella by the microbiological laboratory. This requires filling approximately 200 ml of water into sterile glass bottles, which are available from the laboratory.

Reporting obligation
According to sec. 7 (1) of the German Infection Protection Act (IfSG) (2001), it is obligatory to report the detection of the pathogen. Accordingly, the obligation to report to the competent office of health is assumed by the diagnosing laboratory.

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