More than 10 years of unrecognized nosocomial transmission of Legionnaires' disease among transplant patients

OBJECTIVE: To investigate a cluster of cases of legionnaires' disease among patients at a hospital. SETTING: A university hospital that is a regional transplant center. DESIGN: Retrospective review of microbiology and serology data from the hos pital laboratories and prospective surveillance via the radiology departmen t; a case-control study and environmental sampling within the hospital and from nearby cooling towers. RESULTS: Diagnosis of seven cases of legionnaires' disease in the first 9 m onths of 1996 led to recognition of a nosocomial outbreak that may have beg un as early as 1979. Review of charts from 1987 through September 1996 iden tified 25 culture confirmed cases of nosocomial or possibly nosocomial legi onnaires' disease, including 18 in bone marrow and heart transplant patient s. Twelve patients (48%) died. During the first 9 months of 1996, the attac k rate was 6% among cardiac and bone marrow transplant patients. For cases that occurred before 1996, intubation was associated with increased risk fo r disease. High-dose corticosteroid medication was strongly associated with the risk for disease, but other immunosuppressive therapy or cancer chemot herapy was not. Several species and serogroups of Legionella were isolated from numerous sites in the hospital's potable water system. Six of seven av ailable clinical isolates were identical and were indistinguishable from en vironmental isolates by pulsed-field gel electrophoresis. Initial infection control measures failed to interrupt nosocomial acquisition of infection. After extensive modifications to the water system, closely monitored repeat ed hyperchlorinations, and reduction of patient exposures to aerosols, tran smission was interrupted. No cases have been identified since September 199 6. CONCLUSIONS: Legionella can colonize hospital potable water systems for lon g periods of time, resulting in an ongoing risk for patients, especially th ose who are immunocompromised. In this hospital, nosocomial transmission po ssibly occurred for more than 17 years and was interrupted in 1996, after a sudden increase in incidence led to its recognition. Hospitals specializin g in the care of immunocompromised patients (eg, transplant centers) should prioritize surveillance for cases of legionnaires' disease. Aggressive con trol measures can interrupt transmission of this disease successfully.