Progress in the battle against nosocomial legionnaires' disease: shedding light on shades of gray.

In 1977, Fraser et al. described an outbreak of pneumonia among legionnaires attending a convention at a hotel in Philadelphia in 1976.1 The outbreak, which later became known as legionnaires’ disease, was caused by a new pleomorphic, faintly staining gram-negative bacillus, Legionella pneumophila, that was isolated at the Center for Disease Control from lung tissues of legionnaires who died.2 Legionella are a fascinating group of intracellular pathogens that often inhabit aquatic environments, where they can survive and even multiply in protozoa.3 There are 42 species of Legionella and several different serotypes, but L. pneumophila serotype 1 is responsible for most lower respiratory tract infections.3,4 Hematogenous or lymphatic spread from the lungs to the liver, spleen, lymph nodes, myocardium, and kidneys may occur.3 Each year 1,200 to 1,800 cases of Legionella infection are reported to the Centers for Disease Control and Prevention, of which 20% to 25% are considered to be nosocomial.3,4 The number of cases reported annually may underestimate the magnitude, morbidity, and cost of nosocomial legionnaires’ disease because of limited reporting, missed cases, and a lack of sensitive and specific methods of diagnosis.3,4 Diagnostic tests for Legionella are also underused, and the commonly used urinary antigen test lacks sensitivity for many Legionella species and does not identify non–serotype 1 isolates of L. pneumophila. The increased use of the new fluoroquinolones and macrolides during the past decade may have decreased the reported incidence, risk, and mortality of Legionella pneumonia. Many individuals believe that the water in hospitals is sterile. However, Legionella and other microorganisms may be isolated from institutional hot and cold water systems. The pathogenesis of nosocomial Legionella infections and the contamination of hospital water have been linked “like light and shadow” using different molecular typing methods.3-5 Recent surveys suggest that 12% to 75% of hospitals have water systems that are contaminated with Legionella.4,6,7 Legionella are transmitted to humans via contaminated aerosols from cooling towers, showers, faucet aerators, nebulizers, humidifiers, or ice machines or by aspiration of contaminated water or pharyngeal contents; person-to-person transmission is uncommon. A patient’s risk of acquiring Legionella pneumonia is related to the type and intensity of the exposure, advanced age, smoking, or a chronic underlying disease.3-5,7 Patients undergoing bone marrow or solid organ transplants and other immunosuppressed patients are at highest risk for infection. With an aging patient population in U.S. hospitals that has more chronic disease and immunosuppression, measures should be taken to prevent nosocomial Legionella infections. What can be done? Healthcare facilities should formulate a strategy based on their patient population, facility design, resources, and available methods for control.3,4 Two schools of thought have emerged for hospitals with either no cases or a few, sporadic cases. Yu et al. suggest routine culturing of the water samples from the facility.8 If the cultures for Legionella are negative, then the risk of nosocomial legionellosis is low. If the cultures are positive, physicians and other professional staff should be notified, appropriate diagnostic testing for Legionella performed, surveillance increased, and water decontamination performed.