Foodborne outbreak study design — DTM&H MCQ

Correct answer: A — Retrospective cohort study comparing attack rates by food exposure

The correct answer is A, retrospective cohort study comparing attack rates by food exposure. Here the entire population at risk (all 40 guests) is fully enumerated, with menu exposure data available for both ill and well attendees, which is exactly the setting in which a retrospective cohort design is the method of choice. Each guest is classified by exposure (specific foods eaten) and outcome (ill or not), allowing calculation of food-specific attack rates and relative risk to identify the vehicle. This mirrors standard UK outbreak practice for point-source foodborne events with a defined guest list, as reflected in UKHSA wedding-associated gastroenteritis investigations that used a cohort design with attack rate and relative risk once the whole exposed population was known. Why the other options are wrong: B. Randomised trial of antibiotics: this is acute self-limiting gastroenteritis requiring source identification, not a treatment question; randomising guests to antibiotics is unethical and irrelevant to finding the causative food. C. Ecological study using district rainfall: ecological studies use group-level, not individual-level, exposure and outcome data, so they cannot link any single guest's food intake to their illness status. E. Cross-sectional survey of nearby villages: this samples an unrelated population with no exposure link to the wedding, so it cannot establish a food-illness association within the affected group. D. Case-control study recruiting neighbourhood controls: case-control design is reserved for outbreaks where the exposed population cannot be fully enumerated; here the full guest list already exists, making a cohort study more efficient and less prone to selection bias. Key point: when the entire exposed population is enumerable with exposure data for all, use a retrospective cohort study; reserve case-control design for outbreaks where the population at risk cannot be fully listed.

Reference: Health Knowledge (UK Faculty of Public Health resource), 'The steps in outbreak investigation including the use of relevant epidemiological methods', Public Health Textbook: https://www.healthknowledge.org.uk/public-health-textbook/disease-causation-diagnostic/2g-communicable-disease/outbreak-investigation