Legionnaires' disease in Philadelphia

The American Legion held its convention in Philadelphia from 21 to 24 July 1976. Within days, attendees began falling ill with cough, fever and pneumonia, and onsets peaked between 24 and 28 July. Investigators counted a case as Legionnaires' disease if the patient had attended the convention or entered an implicated hotel. People who fell ill after only walking within a block of the hotel were classed separately, as Broad Street pneumonia.

The outbreak was explosive and came from a common source. Of 182 cases, 29 were fatal. In their December 1977 report, D. W. Fraser and 11 coauthors found that the agent seemed to travel through the air, not from person to person, and that exposure may have happened in the lobby of the headquarters hotel or just outside it. Many hotel employees appeared to be immune, which suggested the agent had been in the area, perhaps on and off, for two years or more. The source was never found.

Identifying the cause was hard. Joseph McDade, working with Charles Shepard, chief of the CDC's Leprosy and Rickettsia Branch, tested patients' blood and tissue for toxins, bacteria, fungi, chlamydiae, rickettsiae and viruses. From the lungs of four of six patients they grew a gram-negative bacillus in guinea pigs and then in the yolk sacs of fertilized eggs; it did not grow on ordinary laboratory media. With yolk-sac cultures as the antigen in a fluorescent antibody test, they found diagnostic antibody rises in 101 of 111 patients, in 54 sporadic cases of severe pneumonia, and in stored serum from two earlier unsolved outbreaks.

In 1979 the organism was named Legionella pneumophila, in a new genus and a new family, the Legionellaceae. Members of the family, it turned out, had been isolated from clinical specimens as early as 1943. A 1988 review by W. C. Winn noted that cooling towers and evaporative condensers had caused some epidemics but judged potable water systems perhaps the more important source. CDC tells building owners that a water management program helps control the bacterium; among people who catch Legionnaires' disease during a stay in a health care facility, about 1 in 4 die.