La legionellosis, or Legionnaires’ disease, is an infectious disease of bacterial origin, which remains rare but often occurs in the form ofepidemics.

On average, in Western countries, the incidence is considered to vary from 1 to 30 cases per million people per year. Thus, in France, in 2012, less than 1500 cases of legionellosis were notified (their declaration is mandatory).

The disease is transmitted by inhaling aerosols containing a bacterium of the genus Legionella (about fifty known species) and coming fromcontaminated water, especially in communities (water heaters, hot water tanks, cooling towers, spas, etc.). It is not a contagious disease.

The disease can manifest itself in two ways:

• flu-like syndrome, which usually goes unnoticed (this is called Pontiac fever)
• potentially serious pneumonia, especially if it affects people with weakened immune systems, including people in hospital.

Laboratory tests are necessary to confirm the diagnosis of legionellosis, in case of symptoms of pneumonia.

Several tests can be used, including:

• bacterial culture
• la urinary soluble antigen test
• serological analysis (late diagnosis)
• direct immunofluorescence analysis on respiratory samples
• a search for the bacteria’s genes (by PCR)

These tests each have their own specificity and advantages.

Bacterial culture (from a respiratory sample) remains the reference method, since it makes it possible to precisely identify the type of legionella involved.

However, urine soluble antigen testing is widely used because it is much faster than cultivation and easy to perform. However, this test can only diagnose one type of Legionella, L. pneumophila serogroup 1, responsible for 90% of legionellosis.

The test is done on a urine sample and consists of detecting “traces” (antigens) of the bacterium. These traces are present in the urine of the vast majority of patients 2 to 3 days after the onset of the first symptoms. The test is sensitive (80% on concentrated urine) and very specific (99%).

It is systematically carried out in the event of respiratory signs occurring in a hospitalized patient, because legionellosis is a feared nosocomial disease.

Its result can be returned in 15 minutes (thanks to commercial diagnostic kits).

If the test is positive, the diagnosis of Legionellosis will be confirmed. Culture will however remain essential for the epidemiological investigation.

The doctor is obliged to report the case to the public health authorities. Identifying the source of contamination is essential in order to limit the spread of the epidemic. Other potential cases can thus be detected early.

As for the patient, antibiotic treatment will be administered quickly, generally based on an antibiotic from the macrolide family.