A super-spreading ewe infects hundreds with Q fever at a farmers' market in Germany

Abstract

Background: In May 2003 the Soest County Health Department was informed of an unusually large number of patients hospitalized with atypical pneumonia.

Methods: In exploratory interviews patients mentioned having visited a farmers' market where a sheep had lambed. Serologic testing confirmed the diagnosis of Q fever. We asked local health departments in Germany to identify notified Q fever patients who had visited the farmers market. To investigate risk factors for infection we conducted a case control study (cases were Q fever patients, controls were randomly selected Soest citizens) and a cohort study among vendors at the market. The sheep exhibited at the market, the herd from which it originated as well as sheep from herds held in the vicinity of Soest were tested for Coxiella burnetii (C. burnetii).

Results: A total of 299 reported Q fever cases was linked to this outbreak. The mean incubation period was 21 days, with an interquartile range of 16-24 days. The case control study identified close proximity to and stopping for at least a few seconds at the sheep's pen as significant risk factors. Vendors within approximately 6 meters of the sheep's pen were at increased risk for disease compared to those located farther away. Wind played no significant role. The clinical attack rate of adults and children was estimated as 20% and 3%, respectively, 25% of cases were hospitalized. The ewe that had lambed as well as 25% of its herd tested positive for C. burnetii antibodies.

Conclusion: Due to its size and point source nature this outbreak permitted assessment of fundamental, but seldom studied epidemiological parameters. As a consequence of this outbreak, it was recommended that pregnant sheep not be displayed in public during the 3rd trimester and to test animals in petting zoos regularly for C. burnetii.

Figure 1

Schematic display of the farmers' market on May 3 and 4, 2003 and the location of the stands of the vendors. On May 4, the predominant direction of the wind was from south-south-east. Red stars represent cases among vendors.

Figure 2

Outbreak cases of Q fever linked to the farmers' market on May 4, 2003 as notified by local health departments (N = 299). The interval between the day of onset and May 4 yields the incubation period. Superimposed is a modeled normal distribution of all outbreak cases.

Figure 3

Geographical location of Q fever outbreak cases notified to the statutory surveillance system.

Figure 4

Age and sex distribution of notified cases of the Q fever outbreak associated with the farmers' market in a spa town near Soest; 2003.

Figure 5

Attack rates among adults and children in a most likely scenario and 8 other scenarios. Most likely scenario: 3000 visitors, 83% adult visitors and 20% clinical attack rate among adults. Scenarios 1–8 varied in the assumptions made for "number of visitors", "proportion of adult visitors" and "attack rate among adults" (see Table 3). Displayed are attack rates and 95% confidence intervals.