Infectious diseases

Legionella pneumophila

Legionella pneumophila is a Gram-negative bacillary bacterium. It lives in stagnant water at high temperatures and its growth is favoured by the presence of organic matter. Requires oxygen to breathe and has a flagellum to move. 16 serogroups of L. pneumophila have been identified.

From the 30 known Legionella species, Legionella pneumophila serogroup 1 is responsible for most human infections. Accounts for 10% of cases of pneumonia, in both community acquired and in the hospital.

In some countries, prevention controls for legionellosis are compulsory in facilities using water for its operation, producing aerosol and being located inside and outside buildings of collective use, industrial facilities and means of transport.

Clinical features: Legionellosis can have two different clinical presentations, Legionnaires' disease and Pontiac fever. In the first, the disease usually manifests as pneumonia, although the clinical spectrum can vary from mild to moderate to severe disease with multiple organ failure. Pontiac Fever is a self-limited disease which results in a clinically similar to the flu.

Diagnosis: Direct methods of diagnosis include culturing, direct fluorescent staining, and antigen detection in urine. The first two methods display low and variable sensitivities.

Culture on BCYE plates is considered the gold standard for the laboratory diagnosis of Legionella infections. However, Legionella is slow-growing and fastidious bacteria, and successful culture requires selective medium and prolonged incubation periods.

On the other hand, urine antigen detection test has become a reference technique in most laboratories, enabling easy and early diagnosis of legionellosis. However, those test detect a limited number of serogroups (especially L. pneumophila serogroup 1), and antigenuria is not always present in all patients. For those reasons, urine antigen tests have sensitivities between 60 and 85%.

Serological tests provide good specificity results, with sensitivities close to 80%, and have proven to be an ideal complement to direct detection methods. It is widely used in clinical diagnostic laboratories. It is necessary to prove seroconversion in order to confirm a serological diagnosis since high titers may be found in healthy population. A high titer in a single serum sample together with clinical symptoms suggests illness.

Nucleic acid amplification techniques are attractive tools for detection of L. pneumophilagenes in sputum, urine and blood.

Treatment: Antibiotics are used to fight the infection. Treatment is started as soon as Legionnaire's disease is suspected, without waiting for confirmation by lab test.

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