Additional epidemiologic analysis demonstrated a dose-response relationship between time spent in grocery store A and risk of disease that is, there was a direct relation between the amount of time persons spent in the store and their risk for disease. The investigators visited grocery store A and looked for potential sources of aerosolized water. An ultrasonic mist machine was operating over one section of the produce display. No one at grocery store A was familiar with the maintenance or operation of this machine. Permission was obtained to culture a specimen of water from the reservoir of the misting device. The culture from the misting device contained Legionella pneumophila serotype 1 (LP-1). Cultures from various cooling towers around town also contained LP-1, but of different subtypes. The investigators were suspicious this misting device may have been related to the outbreak.

• QUESTION 13:
  At this point, is information sufficient to make recommendations have the basic criteria of causation been satisfied? How would you proceed with this investigation?

Four additional activities were undertaken: 1) a serosurvey (that is, a survey and analysis of blood specimens obtained from a sample of persons) was conducted among all grocery store employees in Bogalusa to determine antibody status against LP-1; 2) a second case-control study was undertaken to determine if exposure to the misting device was associated with developing Legionnaires' disease; 3) ten similar misting devices from other parts of the country were cultured; and 4) the investigators asked for permission to perform autopsies on two patients who had died of pneumonia early in the epidemic.

Employees at grocery store A were more likely to have elevated antibody titers to Legionella than employees at the other grocery stores (13/48 [27%] versus 7/75 [9%] the ratio of these two percentages is defined as the prevalence ratio [the ratio of the prevalences of antibody in each group]; prevalence ratio=3, p=0.02). Analysis of the second case-control study revealed a significant association between disease and purchasing produce which was nearest the mister. Of the 10 mist machines from other parts of the country, 6 grew Legionella. The subtype of Legionella found in the grocery store misting device was also isolated in a small cooling tower which was far from public access and not in proximity to Grocery Store A. Lung tissue from the two autopsied patients revealed Legionella of the same subtype as that found in grocery store A.

Until now, the news media had not been aware of the outbreak, the investigation, or the results.

• QUESTION 14:
  Who needs to know about these findings? How would you go about reporting the findings?