HS340 Unit 10 Epidemiology Case Report Example

Fried Rice Held Overnight: A Unit 10 Epidemiology Case Report on Vomiting After a Composite Graduation Banquet

[Student Name]

Purdue University Global

HS340: Epidemiology

Unit 10 Epidemiology Case Report

[Instructor Name]

[Date]

The banquet, the attendees and every count are composite; the review of the organism, the causation criteria and the field manual cited are real.

Summary

On a Friday evening in May, [312] people attended a graduation banquet catered by a local restaurant. By midnight, the college's on-call nurse had received a dozen calls about vomiting and stomach cramps. A retrospective cohort study of [240] attendees found that [64] met the case definition, with a median onset of two and a half hours after the meal. Illness was strongly associated with eating fried rice, risk ratio 5.8. Leftover rice grew Bacillus cereus at high counts. The rice had been cooked the afternoon before, cooled slowly at room temperature and reheated in a warming cabinet. No one was hospitalized.

Background

Bacillus cereus causes two forms of food poisoning. The emetic form, caused by a toxin produced in food before it is eaten, brings vomiting within about a half hour to six hours and is classically linked to cooked rice left at warm temperatures; the diarrheal form starts later and is linked to a wider range of foods (Bottone, 2010). The toxin survives reheating, so reheating rice that has been held too long does not make it safe.

Measurement

Case definition: an attendee who developed vomiting, or at least two of nausea, abdominal cramps and diarrhea, within 24 hours after the banquet.

The definition was written before the food history was analyzed, so that the choice of foods could not shape who counted as ill. It was broad enough to include people with mild illness who did not call anyone.

Case finding used the guest list supplied by the college. Health department staff called or emailed all [312] attendees over four days and reached [240], 76.9 percent. Each was asked about symptoms, onset time and every food and drink served.

Onset times ran from one to five hours after the meal began, with a median of two and a half hours. That short, tight distribution points to a toxin already present in the food rather than an infection, which would take a day or more to cause symptoms.

Design

Because the exposed group was a defined guest list, the investigation used a retrospective cohort design, comparing attack rates among people who ate and did not eat each item. Had the guest list been unknown, or the crowd too big to reach, cases would have been compared with well controls instead (Rasmussen & Goodman, 2019). A cohort has one advantage worth naming: it gives attack rates and risk ratios directly, which are easier for a restaurant owner or a college dean to understand than odds ratios.

Food-specific attack rates:

Fried rice: of [150] who ate it, [58] ill, 38.7 percent; of [90] who did not, [6] ill, 6.7 percent. Risk ratio 5.8. Grilled chicken: of [196] who ate it, [54] ill, 27.6 percent; of [44] who did not, [10] ill, 22.7 percent. Risk ratio 1.2. Green salad: of [171] who ate it, [44] ill, 25.7 percent; of [69] who did not, [20] ill, 29.0 percent. Risk ratio 0.9. Sheet cake: of [208] who ate it, [56] ill, 26.9 percent; of [32] who did not, [8] ill, 25.0 percent. Risk ratio 1.1.

Only the fried rice separates the ill from the well: almost everyone who got sick ate it, and the few who ate none of it rarely got sick, while every other food shows attack rates within a few points of each other.

For fried rice, the 95 percent confidence interval for the risk ratio, computed on the log scale, runs from 2.6 to 12.9, well above 1.

Laboratory and Environmental Findings

Leftover fried rice, refrigerated by the caterer after the event, grew Bacillus cereus at more than one hundred thousand colony-forming units per gram, a level associated with illness. Stool testing is not routinely used for this organism, since it can be found in healthy people, and none was done. The other leftover foods did not grow the organism at significant levels.

A visit to the restaurant found that the rice for [312] people had been cooked in four large batches at about 2 p.m. on Thursday, left in the pots on the counter until the evening shift ended, then refrigerated in deep containers. On Friday it was stir-fried with vegetables and held in a warming cabinet from 4 p.m. until service at 7 p.m. The cabinet's thermometer read below the safe hot-holding temperature when inspectors checked it.

Inference

Several lines of evidence point to the rice. The association is strong, a risk ratio of 5.8. People who ate two or more servings of rice had an attack rate of [51] percent against [31] percent for one serving, a gradient consistent with dose. The short incubation fits a preformed toxin. The organism was found in the food at a level known to cause illness, and the preparation history explains how it got there. These are several of the considerations Hill proposed for judging whether an association is causal: strength, a biological gradient, plausibility and coherence with other evidence (Hill, 1965).

The evidence has limits. A quarter of attendees were not reached, and those reached may have been more likely to have been ill. Memory of what was eaten at a buffet is imperfect. And the six ill people who reported eating no rice may have forgotten a spoonful or had an unrelated illness.

Control Measures and Recommendations

The restaurant agreed to cook rice the same day it is served, to cool any rice that must be held in shallow pans in the refrigerator within two hours, and to replace the faulty warming cabinet. The health department reinspected two weeks later and found the changes in place. Staff also checked whether the restaurant had catered other events that week from the same batches of rice, and found one small office lunch on the Friday; none of the eleven people at that lunch reported illness, which fits with their rice having been taken from the warming cabinet within an hour of reheating, before toxin levels could build further. A short notice to other caterers in the county reminded them that rice cooked ahead is a common source of this illness.

Conclusion

A point-source outbreak of emetic Bacillus cereus food poisoning followed a graduation banquet. A cohort study of attendees implicated fried rice, the laboratory confirmed the organism in leftovers, and the kitchen's practices explained why. The outbreak ended with the meal, and the restaurant changed how it cools and holds rice.

References

Bottone, E. J. (2010). Bacillus cereus, a volatile human pathogen. Clinical Microbiology Reviews, 23(2), 382-398. https://doi.org/10.1128/CMR.00073-09

Hill, A. B. (1965). The environment and disease: Association or causation? Proceedings of the Royal Society of Medicine, 58(5), 295-300. https://doi.org/10.1177/003591576505800503

Rasmussen, S. A., & Goodman, R. A. (Eds.). (2019). The CDC field epidemiology manual. Oxford University Press.