The Wedding Buffet: Two Onset Clusters, One Menu
A wedding reception catered by a local restaurant served 62 guests on Saturday at 6 p.m. The buffet included glazed ham held in a chafing dish, rice pilaf, tossed salad, rolls, and cheesecake. By Monday, 33 guests had reported illness. Of 55 guests interviewed, food histories showed: 40 ate ham, 28 ill; 34 ate rice, 10 ill; 45 ate salad, 24 ill; 20 ate cheesecake, 8 ill. Twenty-four ill guests reported vomiting with onset 2.5 to 5 hours after the meal and no fever; nine reported diarrhea at 22 to 30 hours. The ham was sliced by a worker with a bandaged finger and held at about 120°F for three hours before service. No leftovers remain and the restaurant kept no retained samples.
1. Calculate food-specific attack rates. Which food is the leading vehicle, and what caution applies to your conclusion?
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Attack rates (ill / ate):
| Food | Ate | Ill | Attack rate |
|---|---|---|---|
| Ham | 40 | 28 | 70% |
| Salad | 45 | 24 | 53% |
| Cheesecake | 20 | 8 | 40% |
| Rice pilaf | 34 | 10 | 29% |
Overall attack rate: 33/55 = 60%.
Leading vehicle: ham - highest attack rate and the only food with a documented time/temperature abuse (held near 120°F, below the 135°F hot-holding minimum, for three hours) plus a possible handler contamination point (bandaged finger).
Caution: marginal attack rates are confounded because most guests ate several items. Salad looks elevated (53%) mainly because nearly everyone who ate ham also ate salad. To separate the effects you need two-by-two data: the attack rate among people who ate salad but not ham, and among people who ate ham but not salad. Without those cells, ham is the strongest candidate but salad cannot be excluded. Rice and cheesecake are less likely because their attack rates are near or below the overall rate.
2. What do the two onset clusters suggest, and can you conclude this was staphylococcal intoxication without laboratory results?
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Short cluster (24 people, vomiting at 2.5–5 h, no fever): this is the classic profile of a preformed toxin - staphylococcal or Bacillus cereus - absorbed quickly after ingestion. The ham held at 120°F for three hours is a plausible incubation period for toxin formation if it was contaminated after cooking.
Long cluster (9 people, diarrhea at 22–30 h): too late for a preformed toxin. This pattern fits an infection with a longer incubation (Salmonella, E. coli, Campylobacter) or a second exposure event.
Conclusion: do not declare Staphylococcus aureus on epidemiology alone. You have no food leftovers, no retained samples, and no clinical specimens. The epidemiology supports a toxin-mediated illness linked to the ham, but the second cluster means the outbreak may involve more than one agent or route. The correct statement to the facility and the public is that a foodborne illness cluster is under investigation with a leading hypothesis, not a confirmed cause. Offer stool and vomitus collection for anyone still symptomatic and request the restaurant's records.
3. List your immediate actions, in priority order, and identify the one action most often skipped.
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Priority actions:
- Notify the local health authority within the required timeframe; a cluster of this size is reportable in essentially every jurisdiction.
- Protect the public: stop serving the implicated process (ham held below 135°F); if the operation is still running, embargo or discard suspect product.
- Interview and collect specimens from ill guests and food workers; get food histories with times, not just food names.
- Exclude ill food workers from food handling until cleared; interview the bandaged-finger worker about the wound and hand hygiene.
- Environmental assessment: observe hot-holding, thermometer calibration, handwashing, and slicing practices.
Most often skipped: interviewing food workers and taking their specimens. Investigators focus on guests and menus, but the worker with the bandaged finger is the most likely contamination source and the only person who can explain the handling chain. Also frequently skipped: asking for the guest list and contact information before it goes cold.