1. Epidemiologic Investigation
Epi-Ready
Epidemiologic Investigation
Module 6

2. Module Objectives
By the end of this module, participants will be able to evaluate the epidemiologic approaches used in an effective foodborne outbreak response.
Recognize the various methods used to gather information during an investigation.
Describe the measures of association calculated during an epidemiologic investigation.
Explain the concept of statistical significance as it relates to measures of association.

3. The Epidemiologic Investigator
Conducts Surveillance Activities
Identifies and Interviews Cases
Generates Hypothesis and Case Definitions
Identifies Comparison Groups (Controls)
Conducts Analytic Studies

4. Cross-Disciplinary Activities
Environmental
Investigator
Focus Environmental Assessment Activity
Product Tracing
Laboratory
Identify Appropriate Clinical Samples for Collection and Analysis

5. Case Definition Uniform criteria employed to define a case
Based on the best available information at the time
Subject to change
Will include:
Person
Place
Time
Clinical Criteria

6. Building a Case Definition
Person: Attended a bar-b-que on July 4th
Place: Great Bear Wilderness Club
Time: Illness onset July 6th or later
Clinical Criteria: Anyone with diarrheal illness (3 or more loose stools in the past 24 hours)
A case associated with this outbreak is a person attending a Bar-B-Que on July 4th at the Great Bear Wilderness Club with diarrheal illness with onset July 6th or later.

7. Case Finding Guest Lists Coordination Between Response Team
Credit Card Receipts
Loyalty Card Information
Media Releases
On-line Complaints

8. Hypothesis Generation
A suggested explanation of what is making people sick
Ongoing process subject to change
Testing hypothesis may require a multidisciplinary approach
Food and environmental surface laboratory analysis
Product tracing activities by environmental health
Additional epidemiologic investigation activities
Hypothesis-generating questionnaires

9. Hypothesis-Generating Questionnaires
Consumption of foods from a standard list
Meals consumed during the time of likely exposure
Food shopping habits
Travel during the time of likely exposure
Consumption of food away from the home
Restaurant
Events
Likely exposure period

10. Challenges of Hypothesis Generation
Poor recall due to time lag from onset of illness to interview
Ingredient-level contaminated food (stealth food items)
Foods with high consumption rates
Foods not disclosed due to their sensitive nature (illegal substances)
Interview fatigue of the interviewer and interviewee

11. Supportive Tools for Hypothesis Generation and Testing

12. Interpretation of Gathered Information
Line Lists
Epi Curves
Spot Maps

13. Activity
Working in pairs or in your table group, answer the questions in the participant manual.
This activity should take no more than 5 minutes.

14. Epi Curves
Visually depicts the time element associated with illness onset
May provide a pattern of onsets that can lead to better understanding of how the illness was contracted
Point Source
Continuous Common Source
Propagated

15. Point Source Outbreak Exposure to same source over brief time
Cases rise rapidly to a peak and fall off gradually
Majority of cases within one incubation period
Exposure

16. Continuous Common Source Outbreak
Exposure to same source over a prolonged time
Epidemic curve usually rises gradually
Curve may flatten
Recall

17. Propagated Outbreak Spread from person to person
Often the series will have progressively taller peaks
Peaks one incubation period apart

18. Spot Maps
Distribution of Cases Reveals Clues Regarding the Source of the Outbreak
Over broad area: commercial product with wide distribution
Clustering: locally sold product, point source, or person-to-person spread
Concentrated areas with outliers: travel to affected area or importation of product
** Bars represent cases

19. Epidemiologic Studies-Understanding Association
Case Series
Cohort Study
Case-Control Study
Case-Case Comparison

20. Case Series Detailed Examination of Cases No Comparison Group
Supported by Other Disciplines

21. Multistate Outbreak of Salmonella Enteritidis
Using shopper card information, it was determined that 7 of 9 cases bought Turkish pine nuts from chain store the week before onset of illness
Background rate: <1% of all shoppers bought Turkish pine nuts at store in previous six months
Lab testing identified outbreak strain of S. Enteritidis in pine nuts/pesto from store. Strain was indistinguishable to the clinical isolates
Store and producer voluntarily recalled pine nuts

22. Cohort Study Analytic Study in a Well-defined Group (Cohort)
Compares Attack Rates of People Exposed Against People That Were Not Exposed to a Certain Food
Cohort
Ate Food
Illness
No Illness
Did Not Eat Food

23. Attack Rates
Is the Incidence of Disease in a Group of People (Cohort)
Food-specific Attack Rates are calculated in the following manner:
Attack Rate
Number of ill that consumed the food item
Total number that consumed the food Item
100 ?
10 ill persons consumed the salad
20 persons consumed the salad
100

24. Attack Rate Table Ate the Food Did Not Eat the Food Foods Served
Ate the Food
Did Not Eat the Food
Foods Served
ill (a)
Not ill (b)
Attack Rate
ill (c)
Not ill (d)
Chicken Salad 12 18
40% 10
50%
Hamburger 20 7
74% 3
23%
Baked Beans
55% 8
44%
Veggie Burger 2
20% 22 5
81%
Iced Tea 31
61% 6
54%

25. Relative Risk Measure of association for a cohort study
Answers: “How much more likely is it for people who ate the food to become ill than people who did not eat the food?”
Relative Risk
attack rate among exposed
attack rate among unexposed

26. Interpreting Relative Risk
Close to 1.0: risk of disease is similar among people eating and not eating the food  food not associated with illness
Greater than 1.0: risk of disease is higher among people eating the food than people not eating the food  food could be risk factor
Less than 1.0: risk of disease is lower among people eating the food than people not eating the food  food could be “inversely associated with”
Magnitude of Relative Risk reflects strength of association between eating food and illness.

27. Attack Rate Table with Relative Risks
Ate the Food
Did Not Eat the Food
Foods Served
ill (a)
Not ill (b)
Attack Rate
ill (c)
Not ill (d)
Relative Risk
Chicken Salad 12 18
40% 10
50%
0.80
Hamburger 20 7
74% 3
23%
3.21
Baked Beans
55% 8
44%
1.23
Veggie Burger 2
20% 22 5
81%
0.25
Iced Tea 31
61% 6
54%
1.13

28. Case-Control Study
Cases (people with illness) and controls (people with no illness)
Compare foods eaten by cases and controls
Foods more commonly eaten by cases than controls might be associated with illness
Ate Food
Did Not Eat Food
Controls
Population at Risk
Cases

29. Odds Ratio Measure of association for a case-control study
Answers: “How much higher is the odds of eating the food among cases than controls?”
odds of eating food among cases
odds of eating food among controls
odds ratio =

30. Botulism Outbreak In Vancouver B.C.
36 cases of botulism among patrons of Restaurant X
Case-control study undertaken
20 (91%) of 22 cases ate beef dip sandwich
3 (14%) of 22 controls ate beef dip sandwich

31. Determining the Odds Ratio
20 of 22 cases ate beef dip sandwich (2 didn’t)
3 of 22 controls ate beef dip sandwich (19 didn’t)
An odds ratio of 63 means the odds that cases ate the beef dip sandwich was 63 times higher than the odds among controls. Eating the beef dip sandwich might be a risk factor for botulism in this outbreak.
odds of eating food (cases) /2
odds of eating food (controls) /19
odds ratio =
odds ratio = 63

32. Interpreting Odds Ratios
Close to 1.0: odds of eating food is similar among cases and controls  no association between food and illness
Greater than 1.0: odds of eating food among cases is higher than among controls  food could be risk factor
Less than 1.0: odds of eating food among cases is lower than among controls  food could be “inversely associated ”
Magnitude reflects strength of association between illness and eating the food.

33. Case-Case Study Design
Yields odds ratio similar to case-control studies depending on risk factors under investigation
May eliminate the need to match controls
Reduces selection and recall bias
Saves valuable resources as case-patients are interviewed as part of an effective surveillance program

34. Choosing a Study Design
Case series – when the number of cases is small (generally less than five) and no controls are available
Cohort study – when investigators can easily identify the population at risk (i.e., outbreak has occurred in a well-defined group) and the population at risk can be enumerated
Case-control study – when the population at risk (people potentially exposed to source of outbreak) is unknown or cannot be enumerated or the illness is rare

35. Statistical Significance
Odds ratios and relative risks are estimates
Observed results could be due to chance alone
Role of chance explored through
Confidence interval (CI)
p-value

36. Confidence Intervals
Range of values for the measure of association that are consistent with study findings
Has specified probability of including “true value” for the measure of association. Generally a probability of 95% is used.
Positive association when range of values are greater than 1.0
Inverse association when range of values between 0 and 1.0
Ranges that include 1.0 are not significant!

37. Confidence Intervals of the Cohort Study
Ate the Food
Did Not Eat the Food
Confidence Interval
Foods Served
ill (a)
Not ill (b)
Attack Rate
ill (c)
Not ill (d)
Relative Risk
Lower Limit
Upper Limit
Chicken Salad 12 18
40% 10
50%
0.80
0.43
1.48
Hamburger 20 7
74% 3
23%
3.21
1.16
8.88
Baked Beans
55% 8
44%
1.23
0.64
2.33
Veggie Burger 2
20% 22 5
81%
0.25
0.07
0.86
Iced Tea 31
61% 6
54%
1.13
0.65
1.96

38. P-Value Probability that findings due to chance alone
Ranges from 0 to 1.0 (0% to 100%)
Closer to 1.0 (100%)  high probability the findings are due to chance
Closer to 0.0 (0%)  low probability the findings are due to chance
p-value = 0.02
Interpretation: the probability of the finding occurred by chance is 2 in 100 times

39. P-Values Calculated for Attack Rate Table
Ate the Food
Did Not Eat the Food
Foods Served
ill (a)
Not ill (b)
Attack Rate
ill (c)
Not ill (d)
Relative Risk
p-value
Chicken Salad 12 18
40% 10
50%
0.80
0.48
Hamburger 20 7
74% 3
23%
3.21
0.002
Baked Beans
55% 8
44%
1.23
0.53
Veggie Burger 2
20% 22 5
81%
0.25
0.0005
Iced Tea 31
61% 6
54%
1.13
0.65

40. Summary
Recognize the various methods use to gather information during an investigation
Describe the measures of association calculated during an investigation
Explain the concept of statistical significance as it relates to measures of association

41. Environmental Investigation
Coming Up Next
Environmental Investigation