RECORD ALL YOUR ANSWERS IN BOLD
Section1: Background
You have been asked to investigate a suspected food-borne illness outbreak at a local church ice cream social. There were 75 people who attended the church social and 46 of them became sick. The onset of illness in all cases was sudden, characterized chiefly by nausea, vomiting, diarrhea, and abdominal pain. None of the ill persons reported having an elevated temperature; all recovered within 24 to 30 hours. Approximately 20% of the ill persons visited physicians.
Section2: Outbreak?
An outbreak may be defined as an epidemic that is confined to a certain area. An epidemic is the occurrence of cases of an illness that clearly is in excess of normal expectancy. One way to determine if an illness is “more than expected” is to calculate the Attack Rate.
The Attack Rate is calculated by counting number of new cases (46 in this case) occurring during a short period of time, divided by the population at risk (75 in this case) at the beginning of the time period. This rate is multiplied by 100.
Question 1. What is the attack rate for this illness at the church ice cream social?
Question 2. Would this be considered an outbreak? Why/Why not?
Section3: Epidemic Curve
It is often very helpful to develop an “epi curve” for a suspected outbreak. An epi curve is constructed in a program like Excel in order to understand the magnitude and timing of the outbreak. The curve is essentially a histogram with time on the x-axis and case counts (number of sick people) on the y-axis. Look at the epi curve below for the Ice Cream Social and answer the following questions.
Question 3. If the church ice cream social was held between 5-7 pm. How many hours later (roughly on average) were people getting sick? In other words, what is the incubation period for this outbreak?
Question 4. Is this typical of a type of food-borne infection or perhaps a food-borne intoxication? Why?
Question 5. Which bacteria that we discussed in the lecture video (and textbook) could be a likely culprit here based on the incubation period?
Question 6. There are two outliers in this graph. One person got sick at 3pm BEFORE the ice cream social. What questions would you want to ask this individual if you were investigating the outbreak?
Question 7. What questions would you like to ask the person that was sick later the next day (10 am)?
Section4: Calculating the Relative Risk Ratio
Although you may be thinking that this is clearly an outbreak, and that most of the people who reported illness where sick relatively quickly after eating at the ice cream social, we still need to determine which food (or activity) made the participants sick.
One way to compare risks (of a certain exposure) is to calculate a Relative Risk Ratio. This ratio is a measurement of the risk of disease, relative to the exposure. In this case, the exposures are two different types of ice cream (Chocolate and Vanilla). The risk ratio is calculated using at 2X2 table (see below). Once this table is correctly constructed, you can compare the risk of disease to the exposure using the formula:
2 X 2 Table
Risk Disease status
Sick Not Sick
Ate Food
Did NOT eat Food
a= number of people who ate the food and DID get sick
b= The number of people who ate the food and DID NOT get sick
c= The number of people who did not eat the food, but DID get sick
d= The number of people who did not eat the food, and DID NOT get sick
Chocolate Ice Cream: There were 25 people who ate Chocolate Ice cream and got sick. These people (number) would go into the “a” box above. There were 23 people who ate Chocolate Ice Cream and did not get sick. This number (23) would go into the “b” box. There were 20 people that did not eat Chocolate Ice Cream and got sick. This number (20) would go into the “c” box. There were 7 people that did not eat chocolate ice cream, and they did not get sick. This number (7) would go into the “d” box. Once you have filled in the box, calculate the relative risk ratio (RR) using the above formula. You should get a number like 0.703. This number means that if you ate Chocolate Ice Cream, you were 0.7 times more likely to get sick than people who did not eat chocolate ice cream. In other words, you were less likely to get sick if you ate the chocolate ice cream. (A little confusing perhaps, but bear with us.)
If you risk ratio equals 1, then your risk of getting sick if you ate the food is the SAME as if you had not eaten the food. As your risk ratio increases above 1, the number is a value that reflects increased risk.
Your turn:
Calculate the Relative Risk Ratio for eating Vanilla Ice Cream using the below numbers:
# ate and sick (a) 43
# ate and not sick (b) 11
# sick and did not eat (c) 3
# no sickness or eat (d) 18
Question 8. What is the relative risk ratio for getting sick if you ate vanilla ice cream compared to if you had not eaten vanilla ice cream? Again, the answer is given by calculating the RR value (equation above) using the values in the above chart.
Question 9. Do you think Vanilla Ice Cream is responsible for the outbreak? Why/Why not?
Question 10. What kind of food safety recommendations would you give to the church members for future ice cream socials?
Bonus Question:
If vanilla ice cream is responsible for the outbreak, how might you be able to explain the three (3) people that got sick, but did not eat the vanilla ice cream (c)?
