6.9: Hypothetical Case Study- Foodborne Illness Outbreak
Attack rates are also used to help identify the source of an outbreak from a specific source, such as in a foodborne illness investigation. Let’s pretend you are a public health officer investigating an outbreak of foodborne illness associated with a single meal (point source) at a U.S. Army basic training location. Several recruits experienced vomiting and diarrhea within a few hours of breakfast. Although samples have been sent to the lab for testing, you won’t get the results for 3-5 days. In order to prevent further illness among the recruits, you can use epidemiological methods (like calculating attack rates) to determine which food item was likely the source of contamination. After interviewing all of the recruits, you come up with the following data on two suspected food items:
Breakfast food items: scrambled eggs and sausage.
Number of recruits who ate scrambled eggs and became ill: 186/400 attack rate = 46.5%
Number of recruits who ate sausage and became ill: 140/400 attack rate = 35%
Number of recruits who didn’t eat scrambled eggs and became ill: 4/400 attack rate = 1%
Number of recruits who didn’t eat sausage and became ill: 54/400 attack rate = 13.5%
Next, you’ll calculate a new statistic: the risk ratio. This is the mathematical probability of becoming ill after eating each food vs. becoming ill after not eating each food. Your results are as follows:
Scrambled eggs risk ratio: 46.5/1 = 46.5
Sausage risk ratio: 35/13.5 =2.59
At this point you could draw a reasonable conclusion that the most likely source of the food poisoning was the scrambled eggs, since the risk ratio of becoming ill after eating scrambled eggs is 46.5, compared to the risk ratio of eating the sausage which was only 2.59. You would probably still do further investigation by taking a sample of the scrambled eggs and testing it for a microorganism, interviewing the chefs and cooking staff on their preparation methods, and finding out the source of the eggs used and perhaps testing that source as well. But you don’t even need to know the pathogenic cause of the outbreak to advise the kitchen staff to discontinue serving the scrambled eggs until the exact cause is identified and removed.