Legionellosis is essentially a lung infection caused by Legionellaceae bacteria, commonly by Legionella pneumophila. The disease may take one of two clinical pictures a severe pneumonia (legionellosis) that can be fatal, or a milder form, although associated with fever, displaying symptoms similar to influenza.
The first outbreak of the disease occurred in July 1976 at the Bellevue Stratford Hotel (Philadelphia) where the 58th annual meeting of the American Legion (a US Army veterans association) took place, therefore; the disease named legionellosis.
Epidemiology of legionellosis
Epidemiological studies suggest that Legionella infection represent 2-15% of pneumonia cases admitted to hospitals. Case fatality rate in USA is 5-30% depending on associated risk factors as immunosuppressant drugs, immunodeficiency diseases, age, smoking…etc. In Europe, case fatality rate is nearly 8.5%.
Microbiology of Legionella organism
Legionellaceae is a single bacteria family, Legionella pneumophila was the first species isolated; now there are more than 50 species of Legionella isolated. Microbiologists isolated three strains of Legionella like ameba pathogens causing pneumonia in humans. Severity of disease caused by Legionella depends on a specific cell wall protein (called epitope).
Legionella is an aerobic gram negative bacillus, which can grow on Mueller- Hinton medium or KCYE or CMH agar media.
The way Legionella organisms infect human phagocyte defense cell (macrophages) is comparable to the way it infects ameba protozoon. In both instances infection is facilitated by avoiding blending (fusion) with phagocyte-lysosome enzymes. Once it gets into the target cell, it starts multiplication. Virulence of the Legionella bacteria is the ability to infect and multiply within the cell, this occurs in steps.
The organism binds itself to specific receptors on the surface of the macrophage cell, and then it penetrates the cell.
The organism then avoids the blending with the phagocyte-lysosome enzyme, and then it forms a vacuole within the cell where it multiplies.
The difference between the life cycle with ameba and human macrophage lies principally in the progression to penetrate the cell. In human cells it is acting dependent, however; Legionella multiplied in protozoa found in water reservoirs becomes more virulent and can produce severe forms of disease when infect humans.
When the organism enters the lung, macrophages present in the alveoli phagocytozed it and the disease process starts.
Ecology of Legionella
Water is the natural reservoir of the bacteria whether natural or artificial, whether cooling towers (as was the case in 1976) or other water systems. However, water temperature is an essential factor for colony formation; Legionella can stand 50 degrees temperature for few hours but cannot multiply in temperatures below 20 degrees. Transmission of Legionella infection occurs through sprays or droplets of water containing the organism not from one person to another.
Diagnosis of Legionellosis
Legionella can produce two different clinical entities of disease. Legionnaires' disease is a full picture of pneumonia with radiological signs. Second form is Pontiac disease, which is a milder, flu like with clinical or radiological signs of pneumonia. Diagnosis is confirmed by laboratory investigation. Isolation of the organism from the sputum or lung tissue or body fluids of a patient confirms the diagnosis. Detection of Legionella pneumophila serogroup 1 patient's urine, or increase in the serum specific Legionella antibody titer four times or more are diagnostic. Increase in the antibody titer less than four times, detection of the organism antigen or detection of the organism by immunochemical methods raise doubts but not enough to diagnose. Documentation of where infection took place, travel associated or nor is an important information to carry out surveillance needed for epidemiological studies.
Treatment and Prevention
Treatment of a diagnosed case is chiefly antibiotics. In legionellosis, quinolones, and macrolides are the first choice treatment, new macrolides (azithromycin, and clarithromycin) are more effective since they can work on intracellular organism and have better tissue penetration compared to the parent macrolide (erythromycin). Beta-lactamase antibiotics are not a choice in legionellosis, although they are the first choice in pneumococcal pneumonia.
Specific antibiotics are advised according to possible patients' risk factor. Examples are fluoroquinolone in transplant patients (receiving immunosuppressant treatment), combined treatment of rifampicin and macrolides in severely ill patients, broad spectrum antibiotics for mild (Pontiac cases), and intravenous azithromycin in immunocompromised patients (HIV/AIDS).
Legionellosis is a human infection by legionella bacteria. It does not transmit from one person to another but the organism is present in water systems to infect man by droplets of contaminated water. Virulence of the organism and patients' risk factor decide the severity of the disease. Antibiotics remain the main treatment of diagnosed cases, however, as the disease may occur in epidemics (community environment or hospital acquired infection) prevention becomes of chief importance. Planning and carrying out water safety planes should include drinking, and recreational environments as well as hotels and cruise ships. Such a plan should have three main pillars; identification of water systems and possible public health risks, monitoring and surveillance, and managing difficulties as documentation, developing supporting programs, and preparation of management measures.
An epidemic of legionellosis can be a public health risk and a cause of much mortality. To reduce such a community risk, it is better prevented than dealt with, knowing that elimination of sources of infection is unachievable. Since water systems are the primary reservoirs of infection, a water systems safety planes should be made and carried out. The planes should provide a comprehensive and methodical evaluation of possible risks, which should be ranked by importance. Identification and monitoring of difficulties and control measures, and continuous surveillance of water distribution systems, cooling towers, and evaporative condenser in hotels, cruise ships and health care facilities should be a continuous process.
