Food-Borne Outbreak (Case Study #2) Background An outbreak (epidemic) of gastroenteritis occurred in Greenport, a suburban neighborhood, on the evening of April 28. A total of 108 people went to the emergency departments of the three local hospitals during that evening. No more cases were reported afterward. These patients com- plained of headache, fever, nausea, vomiting and diarrhea. The disease was severe enough in 32 patients to require hospitalization for rehydration. Gastroenteritis outbreaks like this are usually caused by the consumption of a contaminated meal. Meal contamination can often be caused by pathogenic viruses or bacteria. However, acute outbreaks are more often produced by toxins from bacteria such as Staphylococcus spp., Clostridium perfringens, Salmonella spp. and Vibrio cholerae. Food poisoning can also be caused by chemicals or heavy metals, such as copper, cadmium or zinc, or by shellfish toxins. Outbreak Investigation The local health department was notified of a potential food-borne outbreak of gastroenteritis in Greenport, and the epidemic team, including a medical epidemiologist, a microbiology techni- cian and a nurse, visited the local hospitals to interview the attending physicians, the patients and some of their relatives. Some stool samples were obtained from patients for microbiologic identification of the causative agent. The epidemic team knew that these types of outbreak usu- ally occur in a very short time period that lasts no more than a few hours or one to two days after people ingest a contaminated meal. Epidemic investigators gather data to define the distribution of the disease by time (onset time and epidemic curve), place (potential places where the implicated meal was served, such as cafe- terias, restaurants and picnics) and person (the distribution of the disease by age, gender and food items eaten). The findings of the initial investigation included the following information. The distribution of the disease by person (age and gender) was found as follows: Gastroenteritis Outbreak Findings by Person, Case Distribution by Age and Gender Females Males Total Age Group No. % Female (in age group) No. % Male (in age group) No. % 0-5 1 1 6-10 52 42 11 and older 8 4 Total by Gender Please calculate the totals for each column and row and their corresponding percentages to try to determine if there are any important differences by age or by gender. Such a task is carried out to investigate if there are any high-risk groups and if the age and gender distribution can give some clues about the source of the outbreak. Interpret your findings. The epidemic curve above shows the onset time of illness in the 108 patients involved in the outbreak. The epidemic team studied the curve and recognized that this was a typical single (point) source acute outbreak. The team also could see that the onset of symptoms in all patients occurred during a six-hour period. Given the symptoms mentioned above and the epidemic curve, the epidemic team concluded that this type of epidemic usually corresponds to intoxication or food poisoning and that the potentially implicated meal was probably served and consumed within a period of a few hours before the onset of the symptoms. Therefore the epidemic team investigated the places where affected persons, their relatives and neighbors ate that day (April 28). The following table shows the team's findings. Note: Often, during interviews, more ill persons will emerge (who did not seek medical attention and may/may not be part of this outbreak). Gastroenteritis Outbreak Findings by Place People PeopleWho did WhoAttackNotAttackRelative PlaceAttendedNo. IllRate (%)AttendNo. IllRate (%)Risk Local204 8166 6 cafeteria Local30510 83 3 restaurant Lions Club163 4272 8 luncheon Elementary2769434114 school cafeteria Please calculate the attack rates (multiply your answer by 100 to get the AR%) by place to try to determine where the contaminated meal was served. For each place compare attack rates (AR) for those who attended with attack rates for those who did not, by using the relative risk (i.e., RR = AR in attendees/AR in nonattendees). Round all answers to the hundredths (i.e, 2.23 vs 2.2). Relative Risk is not a percentage, so only use % to denote ARs. Interpret your findings. (Hint: see explanation in textbook on how to interpret RRs). Once the implicated place was determined, the investigation centered on the food. The following table includes the food items served in that place on April 28: Gastroenteritis Outbreak Findings by Person Ate the Food ItemDid Not Eat the Food Item FoodNo.AttackNo.AttackRelative ItemPeopleNo. IllRatePeople No. IllRateRisk Beef 160 8 240 16 burritos Cheese 254 12 141 20 burger Tossed 88 56 292 12 salad Baked 205 11 179 21 potato Fruit 103 63 198 9 cocktail Ice cream 230 15 157 19 Important note: None of the kitchen personnel were ill. The names of the kitchen personnel and their participation in the food preparation are as follows: Manuel prepared the beef burritos and the potatoes, John prepared the salad and the fruit, Sally served all dishes except the ice cream, and Jane prepared the cheeseburgers and served the ice cream. The ice cream was a commercial brand and was bought at a nearby supermarket. Please calculate the attack rates per 100 (noted as a percentage) by food item to try to deter- mine the one that was probably contaminated. Compare attack rates (AR) for those who ate the food item with attack rates for those who did not eat the food item, by using the relative risk (i.e., RR = AR in those who ate the food/AR in those who did not eat the food). Round your answers to the hundredths (i.e, 2.23 vs 2.2). Relative Risk is not a percentage, so only use % to denote ARs. Interpret your findings. Given that the epidemic team worked fast enough and the implicated meal(s) was (were) identified before all food leftovers were discarded, food samples from some meal leftovers were taken to the laboratory. In addition, stool samples were taken from the kitchen personnel who prepared or handled each different food item. The laboratory confirmed that Salmonella toxin was present in some of the food samples and that one of the kitchen personnel of that place had the same Salmonella species. Furthermore, the Salmonella species found in the food and the kitchen worker was the same species found in stool samples of the patients. Please discuss these findings and identify the kitchen worker possibly responsible for the outbreak. Food-Borne Outbreak Quiz One hundred and seventy-five people attended a charity luncheon in which two different menus were served. A total of 82 attendees reported symptoms including nausea, vomiting, abdominal cramps and diarrhea. Most of the 82 patients who became sick reported symptoms about sixhours after the beginning of the luncheon. The symptoms lasted from four to seven hours. The dishes that were served included baked chicken, meat loaf, mashed potatoes and green beans. The table below shows the data collected from this investigation. Charity Luncheon Outbreak Data Ate Food ItemDid Not Eat Food Item Food Attack Attack Relative Item Ill Not Ill Total Rate Ill Not Ill Total Rate Risk Meat 2 45 47 74 46 120 loaf Baked 79 39 118 5 47 52 chicken Mashed 4 56 60 40 70 110 potatoes Green 58 67 125 3 39 42 beans Round attack rates (%) and RRs to the hundredths. 1.Complete the table. There were two menus people could choose from (each with a meat and a vegetable). From the RRs, list the two food items included in each of the two menus served. 2.To determine which food item(s) are responsible, is it enough to just calculate the ARs for those who ate the food items? Why or why not? (Give a thorough response) 3.From the RRs, identify the food item you believe is most responsible for the outbreak.