Hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the principal source of infection in such cases is the drinking-water distribution system. Epidemiology
Legionnaires disease acquired its name in July 1976, when an outbreak of pneumonia occurred among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia. Of the 182 reported cases, mostly men, 29 died. On 18 January 1977, the causative agent was identified as a previously unknown strain of bacteria, subsequently named Legionella, and the species that caused the outbreak was named Legionella pneumophila. Following this discovery, unexplained outbreaks of severe respiratory disease from the 1950s were retrospectively attributed to Legionella. Legionnaires disease also became a prominent historical example of an emerging infectious disease.Outbreaks of Legionnaires disease receive significant media attention, but this disease usually occurs in single, isolated cases not associated with any recognized outbreak. When outbreaks do occur, they are usually in the summer and early autumn, though cases may occur at any time of year. Most infections occur in those who are middle-aged or older. National surveillance systems and research studies were established early, and in recent years, improved ascertainment and changes in clinical methods of diagnosis have contributed to an upsurge in reported cases in many countries. Environmental studies continue to identify novel sources of infection, leading to regular revisions of guidelines and regulations.
enter the lungs either by aspiration of contaminated water or inhalation of aerosolized contaminated water or soil. In the lung, the bacteria are consumed by macrophages, a type of white blood cell, inside of which the Legionella bacteria multiply, causing the death of the macrophage. Once the macrophage dies, the bacteria are released from the dead cell to infect other macrophages. Virulent strains of Legionella kill macrophages by blocking the fusion of phagosomes with lysosomes inside the host cell; normally, the bacteria are contained inside the phagosome, which merges with a lysosome, allowing enzymes and other chemicals to break down the invading bacteria. Diagnosis
People of any age may develop Legionnaires disease, but the illness most often affects middle-aged and older people, particularly those who smoke cigarettes or have chronic lung disease. Immunocompromised people are also at higher risk. Pontiac fever most commonly occurs in those who are otherwise healthy.The most useful diagnostic tests detect the bacteria in coughed-up mucus, find Legionella antigens in urine samples, or allow comparison of Legionella antibody levels in two blood samples taken 3–6 weeks apart. A urine antigen test is simple, quick, and very reliable, but only detects L. pneumophila serogroup 1, which accounts for 70% of disease caused by L. pneumophila, which means use of the urine antigen test alone may miss as many as 30% of cases. This test was developed by Richard Kohler in 1982.