Picnic Outbreak: Attack Rates and a Negative Stool Culture
Forty people attend a Saturday church picnic. The potato salad was prepared Friday night, kept overnight in a car trunk, set out at 11 a.m. in 29 °C shade, and served until 4 p.m. Within 2–5 hours of eating, 12 of the 18 people who ate the potato salad and 3 of the 22 people who did not eat it developed sudden vomiting, abdominal cramps, and watery diarrhea. None had fever, none had bloody stools, and all were well within 24 hours. No other food item was associated with illness. Stool cultures from six symptomatic patients show "no growth" at 48 hours, and the health department asks the church for an explanation.
1. Calculate the attack rates and the relative risk for the potato salad. How strong is the association, and what do the three illnesses among non-eaters tell you?
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Attack rate, exposed: 12 ÷ 18 = 0.67 (67%) Attack rate, unexposed: 3 ÷ 22 = 0.136 (14%) Relative risk: 0.67 ÷ 0.136 ≈ 4.9 Attributable risk: 67% − 14% = 53 percentage points
An RR near 5 is a strong association and, with a plausible time–temperature history, supports the potato salad as the vehicle. But the three cases among people who did not eat it are real and need an explanation - two are plausible:
- Cross-contamination: shared tongs, spoons, hands, or a contaminated serving surface passed the organism/toxin to other foods (e.g., sliced tomatoes, bread).
- Misclassification: "did not eat the salad" may include people who ate a few bites or whose food touched the salad on the plate.
Judgment: do not discard the hypothesis because of three discordant cases; instead interview them about what they actually handled and ate. Also note that with only 40 people, the confidence interval around RR ≈ 4.9 is wide (roughly 1.5–16), so the size of the risk is imprecise even though the direction is convincing.
2. Given the short incubation, vomiting predominance, absence of fever or blood, rapid recovery, and negative stool cultures, what mechanism and organism class fit best? Explain the negative cultures and state whether antibiotics are indicated.
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Mechanism - preformed toxin, not infection.
- Incubation 2–5 h is too short for most invasive bacteria (Salmonella/Campylobacter/Shiga-toxin E. coli typically 1–3 days).
- Vomiting predominance, no fever, no blood, recovery <24 h all point to a toxin already present in the food acting on the gut (or on the vomiting center).
- Most consistent agents: Staphylococcus aureus enterotoxin (classically 2–6 h, intense vomiting, handled protein/starch salads) or Bacillus cereus emetic toxin (classically reheated rice, 1–6 h).
Why cultures are negative: the illness comes from toxin, not from colonizing the patient. The organism may have been killed during reheating or outcompeted after the toxin was made, and stool culture detects organisms, not preformed toxin. A negative culture is therefore expected and does not weaken the diagnosis (confirming would require detecting toxin or the organism in the leftover food, which was likely discarded).
Antibiotics are not indicated. They do not neutralize preformed toxin, do not shorten a self-limited 24-hour illness, and carry resistance and C. difficile risk. Treatment is oral rehydration; IV fluids only if vomiting prevents intake; antiemetics are optional.
3. Using the time–temperature history, identify the single control measure that would have prevented most of these cases, and support it with an estimate of bacterial multiplication.
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The salad spent roughly 5 hours at 29 °C - squarely inside the 4–60 °C danger zone, where S. aureus can double in about 30 minutes under ideal conditions.
Estimate: 5 h ÷ 0.5 h per generation = ~10 generations → 2¹⁰ ≈ 1,000-fold increase in organisms (and a proportional increase in toxin) from the moment it left refrigeration until it was served. Even a modest inoculum from a food handler's hands becomes a large one by early afternoon.
Highest-yield control: hold the salad below 5 °C or above 60 °C, or discard it after 2 hours in the danger zone; transport it in a cooler rather than a car trunk, and set out only what will be eaten within 2 hours, refilling from the cooler.
Secondary controls: exclude workers with hand lesions or recent vomiting/diarrhea from food handling, use gloves/tongs, and avoid bare-hand contact with ready-to-eat salads. Note the asymmetry - reheating leftovers later would kill organisms but not inactivate the heat-stable staphylococcal enterotoxin, which is why prevention has to be temperature control at the picnic, not cooking afterward.