A Food Poisoning Outbreak in a School from Progressive Contamination of Norovirus, Amnat Charoen Province, Thailand, February 2025

Abstract

On 13 Feb 2025, students and staff at School A in Hua Taphan District, Amnat Charoen Province, Thailand, developed gastroenteritis symptoms. An investigation was conducted to confirm the diagnosis, describe outbreak characteristics, identify possible sources and risk factors, and provide recommendations. A suspected case was an individual present at the school between 11–24 Feb 2025 who experienced three or more loose or liquid stools within 24 hours or at least one episode of vomiting. Data was collected via questionnaires and hospital records. Rectal swabs and stool samples were tested, and the environment was assessed. A retrospective cohort study was conducted using multivariable Poisson regression with robust error variance to calculate adjusted relative risks (ARR). The attack rate was 30.2% (142/470). Common symptoms were nausea (85%), abdominal pain (79%), and vomiting (76%). The epidemic curve indicated a point-source exposure followed by secondary transmission. Norovirus was detected in 71.4% of clinical samples, including one asymptomatic food handler. The chicken rice meal served on 11 February, had the highest ARR of 4.37 (95% confidence interval (CI) 1.12–17.10). Students served later (grades 4–6) had a significantly higher risk compared to the early serving group (ARR 2.17, 95% CI 1.37–3.43), suggesting progressive contamination. The epidemiological and laboratory results suggested that the chicken rice meal, was likely contaminated by an infected asymptomatic food handler, which was the source of the norovirus outbreak. The study recommendations included improving food-handling practices, strengthening hand hygiene infrastructure, and ensuring regular maintenance of the water treatment system.