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Legionnaires' Disease Associated with Cooling Towers -- Massachusetts, Michigan, and Rhode Island, 1993

From July through October 1993, outbreaks of Legionnaires' disease (LD) were reported from communities in Massachusetts and Rhode Island and from a state prison in Michigan. Cooling towers (CTs) were identified as the source of all three outbreaks. This report summarizes investigations by state and local health officials and CDC and efforts to control these outbreaks. Massachusetts

During July-August 1993, LD was diagnosed in 11 persons living in Fall River, Massachusetts. The mean age of patients was 59 years (range: 40-72 years); six were men. Three persons died. Three persons had Legionella pneumophila serogroup 1 (Lp-1) isolated from respiratory secretions, four had Lp-1 antigens detected in respiratory secretions by direct fluorescent antibody testing, three had fourfold rises in serum antibody titer to Lp-1, and one had both a fourfold rise in serum antibody titer and Lp-1 antigens detected in urine by radio-immunoassay.

A case-control study, matching the 11 patients and 22 controls by primary physician, age, sex, and underlying medical condition, indicated that patients were more likely than controls to have visited sites within a 0.04-square-mile (0.1-square-km) neighborhood of Fall River in the 2 weeks before onset of illness (matched odds ratio {OR}=14.0; 95% confidence interval {CI}=1.6- 120.8); no other activities were significantly associated with acquiring LD.

Water samples from seven CTs within the neighborhood and from the homes of culture-positive patients were taken approximately 1 month after onset of the last identified case of LD in the community and cultured for legionellae. All samples from potable water taps in patients' homes were culture-negative. Five isolates were cultured from four CTs. Lp-1 was cultured from two conjoined CTs on a building within the neighborhood and had the same monoclonal antibody subtype (MAS) and pulsed-field gel electrophoresis (PFGE) patterns as all three clinical isolates.

The conjoined CTs were decontaminated on an emergency basis according to guidelines previously developed by a technical work group (1). The onset of the last identified case was August 10, and the CT was decontaminated on September 24. No additional cases were identified after decontamination. Michigan

During August-September 1993, LD was diagnosed in 17 persons with pneumonia at a state prison in Michigan; 16 patients were inmates, and one was an employee. One patient died. The mean age of the patients was 47 years (range: 29-81 years); all were men. One person had Lp-1 cultured from respiratory secretions and, for 11, LD was diagnosed by a fourfold rise in titer of antibodies to Lp-1; five patients with pneumonia had evidence of LD by single convalescent-phase antibody titers of 512 or more.

Water samples from wells and potable water taps in the prison and the prison hospital, from the prison hospital CT, and from a CT near the prison were cultured for legionellae. All of the potable water samples were culture-negative. Lp-1 was isolated from both CTs. The isolate from the CT located on the roof of the prison hospital had the same PFGE pattern as the single clinical isolate.

Fourteen (0.6%) of 2253 prisoners who used exercise yards each day adjacent (within 100 yards) to the prison hospital had LD, compared with two (0.1%) of the 2270 inmates who used yards at least 400 yards from the prison hospital (relative risk=7.1; 95% CI=1.6-31.0).

The CT on the prison hospital was shut down on September 17 and decontaminated according to published guidelines (1). No new cases of LD were identified with onset after September 1. Rhode Island

During August 30-October 20, 1993, LD was diagnosed in 17 patients who lived or worked in eastern Rhode Island. The patients' mean age was 54 years (range: 28-86 years); 11 were men. Two patients died. Seven patients had Lp-1 cultured from respiratory secretions and 10 had Lp-1 antigen detected in urine.

A case-control study, matching the 17 patients with 33 controls by physician practice, age, sex, and underlying medical conditions, indicated that patients were more likely than controls to visit a 0.04-square-mile (0.1-square-km) section of downtown Providence (matched OR=6.5; 95% CI=1.4-30.9) in the 2 weeks before onset of illness.

Water samples from the homes of six culture-positive patients were negative for legionellae by culture, but samples from 10 of 24 CTs and one of three decorative fountains in downtown Providence were positive for Lp-1. The environmental isolates were tested by MAS and PFGE; one isolate from a CT on a building located within the area had the same MAS and PFGE patterns as isolates cultured from four case-patients who reported visiting the LD-associated section of downtown Providence. No other sources of transmission were identified in the community. These Lp-1 isolates had MAS and PFGE patterns that were different than those from the Fall River outbreak (approximately 19 miles away); however, the PFGE patterns suggested that the isolates were genetically related.

The CT was shut down and decontaminated on an emergency basis on October 26. No additional cases of LD associated with the area were identified after decontamination of the CT.

Reported by: TE Gecewicz, L Saravo, Fall River Dept of Public Health; SM Lett, MD, PE Kludt, MPH, A DeMaria, Jr, MD, State Epidemiologist, Massachusetts Dept of Public Health. MG Stobierski, DVM, D Johnson, MD, W Hall, MD, S Dietrich, H Stiefel, S Robinson-Dunn, PhD, S Shah, Michigan Dept of Public Health; C Hutchinson, MD, Michigan Dept of Corrections. LA Mermel, DO, CH Giorgio, Rhode Island Hospital, Providence; L D'Agostino, M Rittman, U Bandy, MD, M Stoeckel, BT Matyas, MD, State Epidemiologist, Rhode Island Dept of Health. Div of Field Epidemiology, Epidemiology Program Office; Childhood and Respiratory Diseases Br and Emerging Pathogens Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Approximately 1000-1300 cases of LD are reported to CDC annually. However, because previous studies indicate that most cases are not diagnosed, the incidence of disease may be substantially higher (2). Legionella causes 1%-5% of community-acquired pneumonia in adults (3); most cases occur sporadically. The case-fatality rate of LD is 5%-30% (2).

Diagnosis of LD requires heightened clinical suspicion. Culturing respiratory secretions for legionellae and testing urine for presence of antigen are not routinely performed for patients with community-acquired pneumonia. Although not widely used, urinary antigen detection is a sensitive (60%-80%), highly specific (more than 99%), and rapid method for diagnosing infection caused by Lp-1 (the cause of 90% of cases of LD) (4). In comparison, serial serum antibody titers require several weeks for definitive results. Single serum antibody titer results have low predictive value (positive and negative) and are not useful for diagnosing LD in nonoutbreak situations. However, they may be useful in identifying cases during outbreaks of LD when serial serum specimens are unavailable -- as for some patients in the Michigan investigation -- and when Legionella is suspected to be the cause of a substantial proportion of pneumonia under investigation.

Although most cases of LD are not associated with outbreaks, investigations of outbreaks have provided most of the knowledge about transmission of the disease. LD can be transmitted by aerosol-producing devices (e.g., CTs {5,6}, evaporative condensers {7,8}, whirlpool spas {2}, humidifiers {9}, and decorative fountains {2}), and by potable water aerosolized by shower heads and tap-water faucets (2,10).

CTs and evaporative condensers have been identified as sources of transmission of LD since the late 1970s. Although legionellae can be cultured in up to 40% of CTs, these devices are rarely associated with outbreaks of LD (1). To reduce CT-related LD, CDC recommends maintenance of all CTs in accordance with published guidelines.

Although the attributable risk of CTs in sporadically occurring LD is unknown, the findings in this report indicate that CTs remain an important cause of outbreaks of LD. In each investigation, molecular typing of isolates confirmed the epidemiologic findings. CDC, in collaboration with other agencies, is establishing guidelines for prevention of LD, targeting CTs as well as other known sources of LD.


  1. Wise M, Addiss D, LaVenture M, et al. Control of Legionella in cooling towers: summary guidelines. Madison, Wisconsin: Wisconsin Department of Health and Social Services, 1987.

  2. Breiman RF. Modes of transmission of epidemic and nonepidemic Legionella infection: directions for further study. In: Barbaree JM, Breiman RF, Dufour AP, eds. Legionella: current status and emerging perspectives. Washington, DC: American Society for Microbiology, 1993:30-5.

  3. Hoge CW, Breiman RF. Advances in the epidemiology and control of Legionella infections. Epidemiol Rev 1991;13:329-40.

  4. Edelstein PH. Laboratory diagnosis of Legionnaires' disease: an update from 1984. In: Barbaree JM, Breiman RF, Dufour AP, eds. Legionella: current status and emerging perspectives. Washington, DC: American Society for Microbiology, 1993:7-11.

  5. Dondero TJ Jr, Rendtorff RC, Mallison GF, et al. An outbreak of Legionnaires' disease associated with a contaminated air-conditioning cooling tower. N Engl J Med 1980;302:365-70.

  6. Garbe PL, Davis BJ, Weisfeld JS, et al. Nosocomial Legionnaires' disease: epidemiologic demonstration of cooling towers as a source. JAMA 1985;254:521-4.

  7. Cordes LG, Fraser DW, Skaliy P, et al. Legionnaires' disease outbreak at an Atlanta, Georgia, country club: evidence for spread from an evaporative condenser. Am J Epidemiol 1980;111:425-31.

  8. Breiman RF, Cozen W, Fields BS, et al. Role of air sampling in an investigation of an outbreak of Legionnaires' disease associated with exposure to aerosols from an evaporative condenser. J Infect Dis 1990;161:1257-61.

  9. Mahoney FJ, Hoge CW, Farley TA, et al. Communitywide outbreak of Legionnaires' disease associated with a grocery store mist machine. J Infect Dis 1992;165:736-9.

  10. Hanrahan JP, Morse, DL, Scharf VB, et al. A community hospital outbreak of legionellosis: transmission by potable hot water. Am J Epidemiol 1987;125:639-49.

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