Legionella species are consistently recognized as one of the most common causes of pneumonia. Legionella pneumophila serogroup 1 is responsible for most human infections and is the causative agent of Legionnaires' disease, also known as legionellosis. The bacteria occur naturally in freshwater however, artificial water systems, such as large building air-conditioning systems, provide environments conducive to growth and can cause a disease outbreak.
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The genus Legionella has over 50 species comprising 70 distinct serogroups. One species of Legionella, L. pneumophila, is the main cause (90%) of Legionellosis, and 70-80% of those cases are attributed to infection with serogroup type I. Legionellosis can be acquired by the inhalation of aerosols containing Legionella bacteria or by micro-aspiration of ingested water contaminated with Legionella. Person-to-person transmission is not thought to be a risk. The likelihood of contracting Legionellosis depends on the level of contamination in the water source, the susceptibility of the person exposed, and the intensity of exposure. Legionellosis can appear in two distinct clinical presentations: Legionella pneumonia (Legionnaire’s disease) and Pontiac fever. Legionnaire’s disease is a serious form of pneumonia that can cause death in 10-15% of cases. Pontiac fever is a milder form of the disease that presents as an influenza-like illness. It is usually self-limiting and typically does not require treatment. The attack rate is much higher for Pontiac fever (up to 95% of those exposed) than for Legionnaire’s disease (5%). Individuals not only attain varied titers but also give heterological responses to different serotypes and species. The heterologic response is variable and is probably characteristic of the individual and does not seem to be mediated by the antigenic nature of the infecting strain.
Diagnosis of Legionellosis is based on the detection of L. pneumophila by PCR, culture, or antigen detection in urine. The most useful diagnostic test has proven to be antigen detection in urine since it is simple, quick, and very reliable. However, in most situations, the use of both the urinary antigen plus sputum culture are the best diagnostic combination due to the heterological response of infected individuals to different serotypes and species. Symptoms and chest x-rays alone are not enough to distinguish Legionnaires’ disease from other types of pneumonia.
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