Legionella bacteria are aerobic, pleomorphic, gram negative bacilli found in fresh water environments and are usually transmitted through inhalation aerosols from contaminated water or soil. Legionnaire’s disease is a severe form of pneumonia caused by legionella species and can be community acquired or hospital acquired. The reported incidence of Legionnaires’ disease is approximately 1.4–1.8 cases per 100,000 persons and immunocompromised state is a very important risk factor. Some of the other important risk factors include old age, impaired cellular immunity, hematologic malignancies, solid organ transplantation, splenectomy, tumor necrosis factor-alpha inhibitors, chronic respiratory disease, diabetes and end stage renal disease. Legionella pneumophila serotype 1 is the most commonly reported cause of human Legionella infections. The pathogenesis of legionnaire’s disease involves invasion of alveolar macrophages and cell mediated immunity is the primary means of immune control. The prevalence of Legionnaires disease has risen possibly from increased awareness and reporting. The symptoms of the disease are nonspecific requiring a high index of suspicion in vulnerable hosts, as effective treatment could be life-saving. Sensitivity of urinary antigen testing is lower in immunocompromised patients because of higher likelihood of infections caused non L. pneumophila species. Extrapulmonary manifestations and higher mortality are particularly more common in immunocompromised patients than in immunocompetent hosts.
Part of the book: Hospital Acquired Infection and Legionnaires' Disease