MPH 510 Module 4 Outbreak Investigation Example

Tracing the Source: Investigating a Salmonella Outbreak After a Community Dinner

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Master of Public Health Program, Aspen University

MPH 510: Epidemiology in Public Health

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Month Day, Year

Tracing the Source: Investigating a Salmonella Outbreak After a Community Dinner

Outbreak investigations are epidemiology in its most urgent form. Investigators must quickly identify who is ill, find the source and stop further cases, often before laboratory results are complete. This paper applies the standard steps of an outbreak investigation to a composite Salmonella outbreak following a church fundraising dinner and places the event in the context of national foodborne disease data.

The Burden of Foodborne Illness

According to national modeling, the 31 leading foodborne pathogens cause roughly 9.4 million domestically acquired illnesses a year, leading to about 56,000 hospital stays and some 1,350 deaths. Nontyphoidal Salmonella accounts for 11% of these illnesses but 35% of hospitalizations and 28% of deaths, making it the leading cause of both (Scallan et al., 2011). Worldwide, nontyphoidal Salmonella causes an estimated 93.8 million cases of gastroenteritis and 155,000 deaths a year (Majowicz et al., 2010).

Step One: Verify the Outbreak

On a Tuesday, the county health department received calls from two urgent care clinics reporting several adults with diarrhea, fever and abdominal cramps who had all attended a Saturday evening church dinner. A cluster of similar illness linked to one event, well above the expected background, confirmed an outbreak.

Step Two: Case Definition

Investigators defined a case as diarrhea on three or more occasions in one 24-hour span, beginning between Saturday evening and the following Thursday in a person who attended the dinner. Confirmed cases had Salmonella isolated from stool. A clear definition kept counting consistent and separated outbreak cases from unrelated illness.

Step Three: Case Finding

The church provided a list of 212 attendees who had bought tickets. Investigators reached 196 of them by phone within three days using a standard questionnaire on symptoms and foods eaten. Of the 196, 74 met the case definition, an overall attack rate of 38%. Stool samples were collected from 22 ill attendees.

Step Four: Describe the Outbreak

The epidemic curve showed onset times clustered between 12 and 48 hours after the dinner, with a median of 22 hours, rising sharply and falling off over two days. That shape, a single peak soon after a shared meal, indicates a point-source outbreak, and the timing fits the usual incubation period of Salmonella. Cases ranged from 6 to 84 years old; nine were hospitalized and none died.

Step Five: Test Hypotheses

Investigators compared attack rates among attendees who did and did not eat each food. The table shows the key results.

Interpreting the Table

Chicken salad stood out. Attendees who ate it had an attack rate of 61% compared with 7% among those who did not, a relative risk of 8.5, and 68 of the 74 cases, 92%, had eaten it. Other foods showed relative risks near 1, meaning no association. The six cases who did not recall eating chicken salad may reflect recall error or cross-contamination of serving utensils.

Step Six: Laboratory and Environmental Evidence

Salmonella Enteritidis grew from 19 of 22 stool samples, all with the same genetic fingerprint on whole genome sequencing. A leftover sample of chicken salad yielded the same strain. Interviews with volunteers revealed that the chicken was cooked Friday afternoon, cooled slowly in deep pans in a home refrigerator, mixed with mayonnaise Saturday morning and held at room temperature for about five hours before serving.

Step Seven: Control Measures

Because the dinner was a single event, the immediate risk ended when leftovers were discarded. The department advised ill people on hydration and when to seek care, excluded ill volunteers who handle food at church events until symptoms resolved and two negative stool tests were obtained, and reported the strain to the national molecular surveillance network to check for links to other outbreaks.

Step Eight: Communicate and Prevent

The department sent a summary to the church, clinics and state health department and offered free food safety training for volunteers at community events. The training focused on cooling cooked foods quickly in shallow containers, keeping cold foods at 41 degrees Fahrenheit or below and limiting time at room temperature.

National Outbreak Patterns

National outbreak surveillance for 2009 to 2015 recorded 5,760 foodborne outbreaks causing 100,939 illnesses, 5,699 hospitalizations and 145 deaths. Norovirus caused the most outbreaks, but outbreaks caused by Listeria, Salmonella and Shiga toxin-producing E. coli accounted for 82% of hospitalizations and deaths, and chicken was the food category linked to the most outbreak illnesses (Dewey-Mattia et al., 2018). The composite outbreak fits this pattern closely.

The Role of Molecular Surveillance

Whole genome sequencing has transformed outbreak detection. Isolates from ill people across the country are sequenced and compared, so cases that look unrelated, spread across states and weeks, can be linked to a common source. A local outbreak like this one may prove to be part of a larger event if the same strain appears elsewhere, for example from a contaminated batch of eggs or chicken.

Communication During the Investigation

Investigators kept the church pastor informed daily and agreed on a statement to attendees, which encouraged people to answer questions and submit stool samples. Clear, nonjudgmental communication with the group involved is essential to cooperation, particularly when volunteers fear blame. The final report emphasized system fixes, such as cooling practices, over individual fault.

Limits of the Investigation

Sixteen attendees could not be reached, and their exclusion could bias attack rates if their illness status differed. Food histories relied on memory three to five days after the event. People who ate small amounts of chicken salad may have reported not eating it. These limits did not change the conclusion, given the strength of the association and the matching laboratory results.

Timeliness

The investigation from first report to identified source took five days, fast enough to inform the public before the incubation window closed.

Conclusion

Following the standard steps, verifying the outbreak, defining cases, finding them, describing the pattern, testing hypotheses with attack rates, confirming with laboratory and environmental evidence and acting, led investigators from a handful of clinic reports to a single contaminated dish within days. The investigation shows how basic epidemiological tools protect communities and why food handling education remains essential.

References

Dewey-Mattia, D., Manikonda, K., Hall, A. J., Wise, M. E., & Crowe, S. J. (2018). Surveillance for foodborne disease outbreaks: United States, 2009-2015. MMWR Surveillance Summaries, 67(10), 1-11. https://doi.org/10.15585/mmwr.ss6710a1

Majowicz, S. E., Musto, J., Scallan, E., Angulo, F. J., Kirk, M., O'Brien, S. J., Jones, T. F., Fazil, A., & Hoekstra, R. M. (2010). The global burden of nontyphoidal Salmonella gastroenteritis. Clinical Infectious Diseases, 50(6), 882-889. https://doi.org/10.1086/650733

Scallan, E., Hoekstra, R. M., Angulo, F. J., Tauxe, R. V., Widdowson, M.-A., Roy, S. L., Jones, J. L., & Griffin, P. M. (2011). Foodborne illness acquired in the United States: Major pathogens. Emerging Infectious Diseases, 17(1), 7-15. https://doi.org/10.3201/eid1701.p11101