The Epidemiology of Legionnaires' Disease 



William B. Baiiie 



The temi "Legionnaires' disease" (LD) is currently used to refer to illness caused by the 

 bacterium that caused the 1976 outbreak of respiratory illness in Philadelphia or by another 

 strain of the same organism. Legionnaires' disease can occur in large common-source outbreaks, 

 as a series of cases in areas of apparently high endemicity, or as sporadic cases without apparent 

 temporal or geographic clustering. 



The earliest documented outbreak of LD occurred in 1965 at St. Elizabeth's Hospital, a 

 large psychiatric facility in Washington, D.C. Eighty -one patients became ill, and 14 died. Epi- 

 demiologic evidence suggested a link between infection and wind-blown dust from excavations on 

 the hospital grounds. In 1968 at least 144 cases of self-limited Olness occurred in employees or 

 visitors who had entered a health department building in Pontiac, Michigan. Investigation at that 

 time demonstrated that the etiologic agent was present in water in the evaporative condenser of a 

 malfunctioning air-conditioning system. The Pontiac agent is now known to have been the 

 Legionnaires' disease bacterium (LDB). In July 1973, an estimated eight Scottish vacationers 

 contracted LD while vacationing in Benidorm, Spain; three died. In 1974 at least 20 persons 

 attending an Oddfellows Convention at the Bellevue Stratford Hotel in Philadelphia developed 

 pneumonia, and two died. This outbreak has since been shown to have been a cluster of cases of 

 LD. 



The existence of the LDB was first recognized in laboratory investigations of a large out- 

 break of pneumonia among persons associated with Philadelphia's Bellevue Stratford Hotel in 

 1976. Of those who were known to have been inside the hotel at some point during an American 

 Legion Convention, 182 became ill, and 29 died. An additional 39 cases, of which 5 were fatal, 

 were reported among persons who had been near the hotel in that period. Subsequently, in 1977 

 and 1978, there were clusters of documented LD cases from Dallas, Texas; and in Columbus, 

 Ohio; Burlington, Vermont; Kingsport, Tennessee; Nottingham, England; Los Angeles, California; 

 Bloomington, Indiana; Atlanta, Georgia; New York, New York: Memphis, Tennessee; and Nor- 

 walk, Connecticut. 



As methodology for the diagnosis of Legionnaires" disease has developed, it has become 

 possible to detect clusters of cases that might otherwise have gone unrecognized. In some in- 

 stances, LD has continued to affect people in localized areas without evidence of the temporal 

 clustering that would suggest a point-source epidemic. A special instance of such areas of high 

 endemicity is seen in several clusters of nosocomial LD. Although the LDB can infect persons 

 who have no serious underlying illness, LD has frequently been associated with patients who have 

 impaired immunologic defenses. Thus, cases may be associated with a given hospital only because 

 the institution is a focal point for patients with renal heterografts or advanced malignancy. In 

 some instances, though, hospitals, like other public buildings appear to have been the site of 

 acquisition of infection with the LDB. 



A characteristic of the epidemiology of LD is an association between infection and reser- 

 voirs of the LDB in the inanimate environment. The bacterium has been isolated from water from 

 cooling towers or evaporative condensers in air-conditioning systems in Atlanta, Bloomington, 



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