Food-Borne Outbreak

Food-Borne Outbreak (Case Study #2)


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 sppand Vibrio cholerae. Food poisoning can also be caused by chemicals or heavy metals, such as copper, cadmium or zinc, or by shellfish toxins.

Foodborne Outbreak

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.

Foodborne Outbreak

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.


epidemic curve

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).