An outbreak of food poisoning likely caused by Bacillus cereus at a secondary school in Mukono District, Uganda, July 2023

On July 20, 2023, the Ministry of Health was notified of a suspected food poisoning outbreak at a secondary school in Mukono District, Uganda, following an alert about students with sudden onset of abdominal pain and diarrhoea. We investigated to determine the scope, cause and risk factors of the outbreak, and recommended measures to prevent future outbreaks. We defined a suspected case as a student or staff member at the school with sudden onset of abdominal pain, diarrhoea or vomiting between 19-20 July 2023. We found cases by reviewing school and health facility records and created a line list. We interviewed respondents (both cases and non-cases) using a structured questionnaire and conducted descriptive epidemiology and environmental assessments. We tested our hypothesis using a cohort of all boarding students (boarders), in the school reportedly consisting of 300 boarders and 900 day scholars. Among 267 cases, 13% were day scholars and 87% were boarders. The commonest symptoms included abdominal pain (100%) and diarrhoea (92%). The epidemic curve indicated a point-source outbreak with onsets occurring 6-36 h after eating school lunch or dinner. Cases peaked within 18 h from the first meal consumed on July 19, 2023, and dropped to baseline the following day, within 26 h from symptom onset of the first cases. Posho (boiled cornmeal) and beans from the same bags were cooked and consumed on preceding days with no illness reported, suggesting that the food was contaminated during preparation on that day. Nearly all 299 cohort members ate the food for lunch and/or dinner on July 19. None of the 4 students who did not eat either meal became ill compared to 229/295 (78%) students who ate either lunch (RR = 7.50, 95%CI 0.52-108), dinner (RR = 7.68, 95%CI 0.55-107) or lunch plus dinner(RR = 7.76, 95%CI 0.56-108). Attack rates were 80% for those who ate only lunch, 77% only dinner, and 78% for those who ate both. Bacillus cereus was isolated from samples of leftover cooked food using both automated microbiological culture-based tests and molecular diagnostics. This was a point-source outbreak likely caused by Bacillus cereus in food prepared at the school; the mechanism of contamination was unknown. Proper food handling, preparation, and storage were emphasized to prevent repeat occurrences.