Module 3 – Group Exercise: Generating Hypotheses about an Outbreak
Divide into groups by table.
1. Read the brief description of an E. coli O157:H7 outbreak that occurred in Wisconsin in.
2. Answer the questions.
3. Develop a hypothesis about the source of the outbreak.
On September 5, the Wisconsin Division of Public Health received separate reports of clusters of laboratory-confirmed E. coli O157:H7 infections in three non-contiguous counties: Manitowoc,
Ozaukee, and Dane. The Manitowoc County cluster involved five ill persons, four of whom visited an animal exhibition at a county fair.
On September 7, the Wisconsin state epidemiologist was called by the director of the Blood
Center of Southeastern Wisconsin regarding five adults who received plasma exchanges in the prior 3 days to treat illnesses consistent with hemolytic uremic syndrome. Three had a laboratory-confirmed E. coli O157:H7 infection.
A total of 30 E. coli O157:H7 infections were reported among Wisconsin residents in one week.
(On average, 15 E. coli O157:H7 infections are reported in each month.)
Question 1: Do these cases represent an outbreak? What explanations might explain the increase in cases (other than an outbreak)? What information might help you determine if the cases represent an outbreak?
Molecular subtyping of 12 of the E. coli O157:H7 isolates showed seven had PFGE patterns that were indistinguishable. Infections with this strain had only been reported sporadically in the past. The PFGE patterns from the four Manitowoc County fairgoers did not match this pattern.
Officials from the Wisconsin Division of Public Health decided that the cases of E. coli O157:H7 infection with the indistinguishable PFGE pattern represented a possible outbreak and assembled the outbreak investigation team to consider the situation.
Question 2: What are the modes of transmission for E. coli O157:H7? What vehicles have been associated with E. coli O157:H7 in past outbreaks? What is the average incubation period of E. coli O157:H7?
To search for additional cases of E. coli O157:H7 infection, the Wisconsin Division of Public
Health staff used the Wisconsin Health Alert Network and e-mail to notify local, regional, and tribal health departments; laboratories; infection control professionals; hospitals; emergency departments; and clinics of the suspected outbreak.
Group Exercise: Generating Hypotheses about an Outbreak – p. 1

For the investigation, a case was defined as a patient who was infected with the outbreak strain of E. coli O157: H7, who resided in Wisconsin and had onset of symptoms since August 1.
A total of 49 Wisconsin residents from 10 counties had illness meeting the case definition. The most frequently reported signs and symptoms among the cases included diarrhea (96%), abdominal cramps (96%), bloody diarrhea (88%), fatigue (80%), watery diarrhea (63%), and chills (57%).
Among cases, illness onset dates occurred from August 20 through September 14. (Figure 1)
Case ages ranged from 1 to 84 years; 38 (78%) cases were ≥ 20 years of age. Of the 49 cases,
35 (71%) were female, 24 (49%) were hospitalized, and 9 (18%) had hemolytic uremic syndrome.
Figure 1. Onset of illness among persons infected with outbreak strain of E. coli O157:H7, Wisconsin.
Question 3: Interpret the descriptive epidemiology of the outbreak including the epidemic curve
(Figure 1). Were symptoms among patients consistent with infection with E. coli O157:H7?
Does the epi curve suggest a mode of transmission? Was clustering of cases by selected demographic characteristics apparent?
Staff at local health departments and the Wisconsin Division of Public Health conducted telephone interviews of cases using a standard comprehensive enteric disease questionnaire.
Of the initial 9 cases interviewed, 7 (78%) ate potatoes, 5 (56%) ate romaine lettuce, and 8
(89%) ate bagged spinach.
Group Exercise: Generating Hypotheses about an Outbreak – p. 2

Question 4: Using the excerpt from the FoodNet Atlas of Exposures, are these exposures unusual?
Question 5: State your hypothesis about the source of this outbreak including causative agent, people at risk, mode of transmission, vehicle, and period of interest.
Question 6: How would you prioritize this outbreak for investigation? Why?
Exercise based on investigation by
Wendel AR, Johnson DH, Sharapov U, et al. Multistate Outbreak of Escherichia coli O157:H7
Infection Associated with Consumption of Packaged Spinach, August –September 2006: The
Wisconsin Investigation. Clinical Infectious Diseases 2009; 48:1079 –86.
Group Exercise: Generating Hypotheses about an Outbreak – p. 3