Canteen Lunch Cluster: Chicken, Time, and Temperature
On a Tuesday in May, a school canteen in a mountain town served a single lunch to 240 students aged 11–14. Within 10 hours, 68 students developed vomiting and watery diarrhoea; 41 vomited within 3 hours of eating. No staff were ill. The canteen had no hot holding: roast chicken was cooked at 10:00, left covered on the counter until 13:00, then served cold in sandwiches. A second serving of rice was reheated at 12:30. The school nurse collected food histories from 233 students. Among 133 who ate chicken, 61 became ill; among 100 who did not, 7 became ill. Rice was eaten by 150 students, of whom 42 were ill; non-rice eaters had a similar rate. Hummus and salad showed no association. Two stool samples were requested, but the laboratory could only perform culture 48 hours later. The principal asks whether to close the canteen and what to tell parents.
1. Calculate the attack rate in chicken eaters and in non-eaters, and the attack rate ratio. What does this suggest about the vehicle, and what alternative explanations should you consider?
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Chicken eaters: 61/133 = 0.459, or 45.9%. Non-eaters: 7/100 = 0.070, or 7.0%. Attack rate ratio = 45.9/7.0 = 6.6. Students who ate chicken were about 6.6 times more likely to become ill. This is a strong association, and the rice data do not support rice as the vehicle: 42/150 = 28.0% among rice eaters versus 26/83 = 31.3% among non-eaters, a ratio below 1. The chicken is therefore the leading suspect.
Alternative explanations still need checking. Food histories were collected after the outbreak, so ill students may recall chicken consumption better than well students. Chicken eaters may also have differed in other ways, such as which serving line they used. However, the timing is supportive: a median incubation of roughly 4–6 hours and vomiting within 3 hours in 41 students point to a preformed toxin rather than invasive infection. The 3-hour holding time at room temperature is the critical failure. A single food item can explain the cluster, but the investigation should still record who prepared and handled the chicken and whether any handler had skin lesions.
2. Name the most likely agent and toxin, and state what specimen and test would best confirm it. Why is a 48-hour stool culture alone insufficient?
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The most likely agent is Staphylococcus aureus producing staphylococcal enterotoxin. The clues are the short incubation (many ill within 3 hours), prominent vomiting, and cooked chicken held for 3 hours without temperature control. S. aureus is carried on skin and in the nose; a handler with a hand lesion or poor hand hygiene can contaminate cooked food, and the toxin is heat-stable, so reheating does not reliably destroy it.
Confirmation should target the toxin and the food, not only the bacteria. Collect vomit or stool from ill students as early as possible and send food remnants, especially chicken, for staphylococcal enterotoxin testing. Swab the nares and any skin lesions of food handlers and compare isolates with food isolates by typing if available. A 48-hour stool culture alone is insufficient because the illness is toxin-mediated: culture may be negative by the time symptoms appear, and a positive culture would not prove that the strain produced enterotoxin. Culture also cannot detect preformed toxin in food. Bacillus cereus is a secondary possibility, but the emetic syndrome is classically linked to cooked rice, and the rice attack rate here is not elevated.
3. List immediate control measures and write three plain-language messages for parents.
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Immediate control: stop serving the implicated chicken and discard remaining product; clean and disinfect the kitchen, utensils and surfaces; exclude any food handler with vomiting, diarrhoea, or infected skin lesions until cleared; reinforce hand hygiene and use gloves; review hot holding and cold storage, with cooked food kept either above 60°C or below 5°C and never left at room temperature for hours; notify the local health authority and keep food samples and invoices; monitor students for dehydration and bloody stool. Closing the whole canteen may be unnecessary if the kitchen can be cleaned and safe service restored, but the decision should be made with the health authority.
Parent messages: (1) “If your child has vomiting or diarrhoea, give small frequent fluids and contact the clinic if they are drowsy, not passing urine, have blood in the stool, or cannot keep fluids down.” (2) “The likely source was chicken served at lunch; the canteen has stopped serving it and is being cleaned.” (3) “Most children recover within 1–2 days, but seek care early for young children, persistent vomiting, or high fever.” Avoid blaming individual staff before the investigation is complete.