Legionella pneumophila

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.