Epidemic curve of pneumonia outbreak, Bogalusa, Louisiana, 1989
PART II (time to completion: 20 minutes)
Discussions were held among staff of the Louisiana health department and the CDC. The health department felt capable of leading the epidemiologic investigation, but requested assistance with laboratory support. A second EIS Officer was sent from Atlanta to assist in the investigation, and CDC provided laboratory support. The field investigation team arrived in Bogalusa on November 8.
The EIS Officers were given the opportunity to address the hospital staff about this outbreak. In addition to being certain that hospital staff could recognize (diagnose) and appropriately treat patients with Legionnaires' disease, the investigators needed to enlist from the hospital staff their support, cooperation, and assistance in the investigation. Some of the points they covered during the meeting were:
A possible case of Legionnaires' disease was defined as illness in a resident or visitor of Washington Parish (where Bogalusa is located), >=20 years of age, admitted to one of the 5 local hospitals after October 1, 1989, with X-ray evidence of pneumonia or a chest radiograph consistent with pneumonia. A confirmed case had to meet the criteria for a possible case, plus have lab evidence of Legionnaires' disease (four-fold rise in antibody titer, a single elevated convalescent antibody titer, positive urine antigen test, positive sputum culture, or positive biopsy).
By November 19, investigators had identified 83 patients who met the definition of possible Legionnaires' disease (see the "epidemic curve" Figure 1; the epidemic curve is a simple graph of the number of new cases by date of onset or date of diagnosis). Fourteen of these patients had died without Legionella testing. Of the 83, 65% were female. About 3/4 of the case-patients were residents of Bogalusa; about half (41) resided on the east side of town. Most case-patients had been admitted to the hospital in mid-October; few if any new cases were occurring in mid-November (see Figure 1 on the following page). To date, no sputum culture had shown growth for Legionnaires' disease or other pathogens.
Epidemic curve of pneumonia outbreak, Bogalusa, Louisiana, 1989
Before designing the analytic portion of the investigation, the investigators considered their leading hypotheses.
At this point in the investigation, the leading hypothesis was outdoor exposure to cooling towers primarily because previous studies had demonstrated the role of cooling towers as sources of the Legionella pneumophila in other outbreaks, and there were several such towers in the town. However, rather than jumping to conclusions based on this information alone, investigators began to compile a list of retail stores and other commercial establishments which were frequently mentioned by some of the case-patients who had been interviewed. Investigators also noted the unusual preponderance of female cases.