Foodborne outbreak investigation — DTM&H MCQ

Correct answer: C — Attack rate among exposed and unexposed groups

The correct answer is C, attack rate among exposed and unexposed groups. In a point-source foodborne outbreak with a defined cohort (all banquet attendees), the food-specific attack rate, the proportion of exposed and unexposed persons who became ill for each food item, is the standard measure used to identify the causative vehicle. Comparing 24/80 (30%) among chicken curry eaters with 4/40 (10%) among lentil eaters gives a risk ratio of 3, pointing strongly to the curry as the source. This retrospective cohort approach with two-by-two tables per food item is the recognised method in field outbreak investigation because the whole population at risk (all who attended) is known and can be classified by exposure and outcome. Why the other options are wrong: E. Case fatality ratio: this measures the proportion of cases who die, irrelevant here as the outbreak is described as watery diarrhoea with no mention of deaths. B. Incidence density using person-years: this is used for ongoing exposure over variable follow-up time in open cohorts, not a single point-source exposure event of a few hours' duration. D. Maternal mortality ratio: this is a specific indicator of pregnancy-related deaths, entirely unrelated to a diarrhoeal food poisoning outbreak. A. Basic reproduction number from national data: R0 describes transmissibility of an infectious agent through a population over generations of spread, not the risk from a single shared meal exposure. Key point: for a single common-meal outbreak with a known denominator, compare food-specific attack rates (exposed versus unexposed) to identify the implicated food.

Reference: UK Health Security Agency, outbreak investigation methodology using food-specific attack rates in retrospective cohort studies; example UK application: Salmonella Enteritidis outbreak, Suffolk, PMC516780, ncbi.nlm.nih.gov/pmc/articles/PMC516780/