Hospital characteristics associated with colonization of water systems by Legionella and risk of nosocomial Legionnaires' disease: A cohort study of 15 hospitals

OBJECTIVE: To investigate an increase in reports of legionnaires' disease b y multiple hospitals in San Antonio, Texas, and to study risk factors for n osocomial transmission of legionnaires' disease and determinants for Legion ella colonization of hospital hot-water systems. SETTING: The 16 largest hospitals in the cities of San Antonio, Temple, and Austin, Texas. DESIGN: Review of laboratory databases to identify patients with legionnair es' disease in the 3 years prior to the investigation and to determine the number of diagnostic tests for Legionella performed; measurement of hot-wat er temperature and chlorine concentration and culture of potable water for Legionella Exact univariate calculations, Poisson regression, and linear re gression were used to determine factors associated with water-system coloni zation and transmission of Legionella RESULTS: Twelve cases of nosocomial legionnaires' disease were identified; eight of these occurred in 1996. The rise in cases occurred shortly after p hysicians started requesting Legionella urinary antigen tests. Hospitals th at frequently used Legionella urinary antigen tests tended to detect more c ases of legionnaires' disease. Legionella was isolated from the water syste ms of 11 of 12 hospitals in San Antonio; the 12th had just experienced an o utbreak of legionnaires' disease and had implemented control measures. Noso comial legionellosis cases probably occurred in 5 hospitals. The number of nosocomial legionnaires' disease cases in each hospital correlated better w ith the proportion of water-system sites that tested positive for Legionell a (P=.07) than with the concentration of Legionella bacteria in water sampl es (P=.23). Hospitals in municipalities where the water treatment plant use d monochloramine as a residual disinfectant (n=4) and the hospital that had implemented control measures were Legionella-free. The hot-water systems o f all other hospitals (n=11) were colonized with Legionella. These were all supplied with municipal drinking water that contained free chlorine as a r esidual disinfectant. In these contaminated hospitals, the proportion of si tes testing positive was inversely correlated with free residual chlorine c oncentration (P=.01). In all hospitals, hot-water temperatures were too low to inhibit Legionella growth. CONCLUSIONS: The increase in reporting of nosocomial legionnaires' disease was attributable to increased use of urinary antigen tests; prior cases may have gone unrecognized. Risk of legionnaires' disease in hospital patients was better predicted by the proportion of water-system sites testing posit ive for Legionella than by the measured concentration of Legionella bacteri a. Use of monochloramine by municipalities for residual drinking water disi nfection may help prevent legionnaires' disease (Infect Control Hosp Epidem iol 1999;20:798-805).