A large outbreak of Salmonellosis among kindergarteners in Northern Thailand, August 2016

Abstract

Background: On 19 August 2016, Chiang Rai Provincial Health Office reported a foodborne outbreak of more than 100 students in a kindergarten school. This investigation was initiated to describe the outbreak and determine the source of contamination.
Methods: Suspected case defined as person in school or outsource food provider of this school whose developed diarrhea and vomiting, or, diarrhea or vomiting with any of following symptoms, i.e., nausea, abdominal pain, fever and headache during 16 to 24 August 2016. We performed active case finding by individual interview and confirmed by laboratory testing for identifying causative agents. Environmental study was conducted by interview and walkthrough survey in cooking area and school. We performed laboratory testing of foods and environmental samples. Retrospective cohort study was conducted to identify risk factors.
Results: We interviewed all of 420 kindergarteners, 29 staffs of kindergarten and 3 food providers to identify cases. Total of 234 cases occurred between 18 and 20 August 2016. Overall attack rate was 51.7% and 46 cases (19.7%) were admitted with no death. Of the 25 cases with rectal swab culture, fifteen cases (60%) were confirmed with positive for Salmonella group E (Salmonella enterica serovar Weltevreden). Food for kindergarteners provided from one outsource food provider. Rice and pork with boiled egg at lunch of 18 August was suspected to be the source. Multivariate analysis revealed that rice consumption (OR = 8.01, 95% CI 1.11-57.82) and egg consumption (OR = 5.34, 95% CI 2.09–13.64) were incriminated. We found Salmonella group E in rice and pork. Even all foods were well-cooked but boiled eggs, left overnight and cut with bare hand, were highly probably contaminated and caused the outbreak after mixing with rice and pork.
Conclusions: Improper prepared boiled eggs from the outsource food providers was the most suspected source of this Salmonella outbreak. We recommended to improve personal hygiene practices and sanitary inspection of outsource food provider should be intensified.