The Setting The day is the Monday after Thanksgiving. You have just returned from a weekend with your family, when your friend Joe calls you. He is upset, and hopes that your public health experience can help him. He tells you that he had a Thanksgiving dinner on Thursday for many of the students still in town. During the weekend, over half of the people who attended the party got ill, experiencing fever, diarrhea, and vomiting. While it may have just been the flu, Joe suspects their illness may be a result of something he served at the dinner. He asks you to find out what happened. First, you ask him about the dinner itself. Joe tells you that preparation for the dinner started Wednesday night, when he made several pumpkin pies. The pies included both milk and eggs in the filling. (This filling would be considered a type of custard.) After baking them, he left them on the counter to cool. They stayed on the counter until dinner on Thursday, although the recipe said to put the pies in the refrigerator after they had cooled. On Wednesday night, Joe also took the turkey out of the freezer and left it on his counter to defrost. Because of the late night of baking, Joe overslept Thursday morning. He expected guests to start coming at 1:00, and realized that if he baked the turkey at the temperature suggested, it could not get done on time. He decided to turn up the temperature on the oven assuming that the additional temperature would make up for less baking time. While the turkey was baking he went out to buy milk. When he returned he set down the bag with the milk in it to speak to a friend calling on the telephone to ask what he could bring to dinner. Joe forgot to put the milk in the refrigerator until he noticed it several hours later as he was passing by the table. He pulled the turkey out of the oven at 12:30. It looked brown, and felt hot throughout. He made gravy from the turkey drippings, and served it over potatoes. He then warmed up some beans that had been home-canned for him by his mother. The dinner started at 1:00 when the guests ate. Along with the turkey, potatoes, gravy, milk, and beans, Joe also served a gelatin salad and biscuits. People helped themselves to the pies on the counter all afternoon. Having learned the characteristics of the dinner, you then wanted to learn more about the people attending the dinner. You ask Joe for the names of the people who came to the party. You give these people a call on the phone, and ask them a list of questions including their experiences of illness since the party (symptoms, time of onset), what they ate at the party, when they ate, and whether they saw a doctor about their illness. Some of the data collected is attached to this exercise. Your objective is to use this data to characterize the outbreak. You can approach this exercise as a foodborne disease outbreak. 1
Subject Sex Ill 1 F Y 2 F Y 3 F Y 4 M Y 5 M Y 6 F Y 7 M Y 8 F Y Onset Sx Time Beans Gravy Milk Turkey Pie Vomiting Diarrhea Fever 2am N N N Y N Y Y N 10am Y Y N Y Y Y Y N 5pm Y N N Y N Y Y Y 5pm Y Y Y Y N Y Y Y 2pm Y Y Y Y Y Y Y Y 3pm Y Y N Y Y Y Y N 3pm N N N Y N N Y Y 11pm Y Y Y Y N Y Y Y 7pm Y Y N N Y N N Y 9 M Y 10 M N N N N N Y N N N 11 M N Y Y Y N N N N N 12 F N Y Y N N Y N N N 13 M N Y N Y Y Y N N N 14 M N Y Y N N N N N N 15 F N Y Y Y Y Y N N N TOTAL 12 10 6 10 8 7 8 6 2
Purpose: You are to determine which food served at Joe s Thanksgiving dinner caused the foodborne outbreak. Use the mathematical concepts taught in Module 4 to determine if there is a statistical association between a given exposure and onset of illness. Instructions: Read the information regarding the background setting of the outbreak. Then combine into groups to go through the exercise. Please answer the following questions in order after reading the notes section. tes: A 2x2 table is frequently used to summarize data to show the relationship between a specific food exposure and subsequent onset of disease. Each case or control is categorized into one of the four cells in the table. a b c d a= the number of individuals who are exposed and have the disease b= the number who are exposed and do not have the disease c= the number who are not exposed and have the disease d= the number who are not exposed and do not have disease An attack rate (AR) is the proportion of people in a well-defined population that develop the illness of interest during a limited time period. The greater the difference between attack rates for those exposed and those not exposed to a particular food the higher the probability that the food item caused the illness outbreak. AR exposed = a/(a+b) AR unexposed = c/(c+d) AR exposed - AR unexposed = Risk Difference 3
The relative risk (RR) is another way of comparing attack rates among exposed and unexposed persons. Use RR to measure the association between exposure and disease in a cohort study. RR= a/(a+b) c/(c+d) The odds ratio (OR) also estimates the magnitude of an association between exposure and disease but is used in case-control studies. The math calculation is slightly different because you are calculating the odds that someone is at risk of developing disease given an exposure in an unknown population instead of the rate of disease development in an exposed known population such as in a cohort study. Questions: OR= a/c = ad b/d bc 1. What type of a study would you perform in Joe s case in order to determine which food caused the illness? Why? 2. What is the attack rate of people who ate turkey that developed vomiting? Diarrhea? Fever? Vomiting: Diarrhea: Fever: 4
3. Fill in the 2x2 table for people who ate pie and became ill. to Pie 4. Fill in the 2x2 table for people who ate turkey and became ill. to Turkey 5. Calculate the relative risk for developing disease if the subject ate pie. What does this value mean? 6. Calculate the relative risk for developing disease if the subject ate Turkey. What does this value mean? 7. Which food item was more likely to cause illness- the Turkey or the pie? Why? 8. The p-value for the relative risk of the turkey is p=0.03. What does this mean? What if the p value is p=.65? 5