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Foodborne Illness Outbreak Investigation
Case Study: Escherichia coli O157:H7
Seven physicians from bordering counties in the states of Washington and Oregon report to
their respective State Departments of Health a total of 20 individuals with severe
gastroenteritis and stool samples positive for E. coli O157:H7. The Departments of Health
from both states notify the United States Centers for Disease Control and Prevention (CDC).
The CDC launches an investigation into the cluster of illnesses.
As an epidemiologist with the CDC, it is your job to determine how widespread the outbreak
has become, the source of infection, and means to halt the outbreak. This involves
interviewing food borne illness victims, determining common exposures, determining what
potential vehicles (food, individuals, environmental samples, other) should be tested,
interpreting laboratory results, helping to determine necessary actions to stop the outbreak,
and helping to determine actions necessary to prevent future outbreaks.
You interview the patients to determine a possible common source of infection. From these
interviews, you are able to identify one commonality: all patients had visited Rainy County
Farm during elementary school field trips within 1 to 5 days prior to the onset of symptoms.
Were their illnesses the result of consumption of one type of contaminated food or beverage,
several different contaminated foods or beverages, environmental exposure, and/or exposure
to an infected individual? Or is the farm a purely coincidental commonality and not the point
of infection?
Eleven patients are showing signs of recovery; however, 8 individuals are in reportedly in
poor medical condition and another has since died due to kidney failure. Additional cases of
severe gastroenteritis have been reported in the region. More people may become sick if you
cannot identify the source quickly and recommend actions necessary to stop the outbreak.
The public is counting on you to solve this case.
1. Aside from talking with farm patrons who contracted E. coli O157:H7 infection, who else
should you interview to help determine what could have been the source of the illness
versus what did not cause illness?
2. You have been provided with names of interviewees. Based on the information you
already have (Table 1), prepare a list of interview questions to gather more of the same
type of information from the new interviewees.
3. Using the sets of questions you developed, conduct interviews with the latest cases.
Compile interview responses in Table 1.
4. Graphs are helpful to quickly visualize patterns in large data sets. Using data from Table
1, prepare the following:
a. A bar graph of the number of patients who developed symptoms for the dates of
symptom onset.

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b. On the same graph as plotted for part a, plot the number of patients who
developed symptoms for the dates they visited the farm.
c. Table 2 completed with data on the foods consumed by all individuals and
whether the foods were positively or negatively associated with illness.
5. A review of farm employee health and absentee records and interviews with select
individuals were conducted. Could the ill farm employee listed in Table 1 have been the
original source of contamination in this outbreak? Why or why not?
6. One of the individuals whose stool sample tested positive for E. coli O157:H7 is a food
handler in a restaurant. What additional measures should be taken to protect the public?
7. News of the outbreak and the association to the farm has hit news sources including
newspapers, radio, television, and the internet. As a result, numerous individuals other
than those sought during the interviews have contacted their doctors, public health
professionals, and/or attorneys with concerns they may also be victims in this foodborne
illness outbreak. Review the symptoms and circumstances of the cases presented in Table
3, and determine which individuals may be additional victims of foodborne illness due to
this particular outbreak versus which individuals are not likely to be cases associated with
this outbreak. Provide an explanation for your assessment on each individual.
8. From the data compiled in Table 2, calculate the odds ratio to determine what
foods/events are positively associated with illness?
9. What do you want to have tested for E. coli O157:H7 contamination?
10. Give a reason why would you not necessarily want to have all foods tested.
11. In any outbreak, when there are multiple foods associated with illness, what could be the
reasons for several foods being implicated?
12. Laboratory data for various foods consumed at Rainy County Farm and stool samples
from affected individuals are presented in Figure 1. Do the laboratory results support
your hypothesis on which food(s) were associated with illness? Explain your response.
13. The United States Department of Agriculture (USDA) Economic Research Service (ERS)
has an interactive website to calculate the cost of foodborne illness outbreaks attributed to
shiga-toxin producing E. coli O157:H7. Enter the calculator at
http://www.ers.usda.gov/Data/FoodborneIllness/. In the cost component feature (left
column of the table), click on <change number of cases>. Enter the number of known
cases from this case according to how they meet the descriptions of severity of illness.
What is the estimated total cost associated with this outbreak using the ERS assumptions?
14. If it is concluded that a product grown and processed (elsewhere, by two different
organizations) but distributed at the farm is the vehicle for E. coli O157:H7 in this
outbreak, provide an opinion statement on who is responsible for contamination. Provide

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Foodborne Illness Outbreak Investigation Case Study: Escherichia coli O157:H7 Seven physicians from bordering counties in the states of Washington and Oregon report to their respective State Departments of Health a total of 20 individuals with severe gastroenteritis and stool samples positive for E. coli O157:H7. The Departments of Health from both states notify the United States Centers for Disease Control and Prevention (CDC). The CDC launches an investigation into the cluster of illnesses. As an epidemiologist with the CDC, it is your job to determine how widespread the outbreak has become, the source of infection, and means to halt the outbreak. This involves interviewing food borne illness victims, determining common exposures, determining what potential vehicles (food, individuals, environmental samples, other) should be tested, interpreting laboratory results, helping to determine necessary actions to stop the outbreak, and helping to determine actions necessary to prevent future outbreaks. You interview the patients to determine a possible common source of infection. From these interviews, you are able to identify one commonality: all patients had visited Rainy County Farm during elementary school field trips within 1 to 5 days prior to the onset of symptoms. Were their illnesses the result of consumption of one type of contaminated food or beverage, several different contaminated foods or beverages, environmental exposure, and/or exposure to an infected indiv ...
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