An Investigation of a Norovirus Outbreak Linked to Contaminated Vegetables in Mueang District, Chanthaburi Province, Thailand, December 2021–January 2022

Abstract

A norovirus outbreak in Mueang District, Chanthaburi Province, was notified on 29 Dec 2021. This study’s objectives were to describe epidemiological characteristics of the outbreak and identify possible sources of the ongoing outbreak. A descriptive study was performed by reviewing diarrhea cases and laboratory results from 1 Dec 2021 to 5 Jan 2022. Interviews were conducted with recent diarrheal cases who had onset during 3 to 5 Jan 2022. Polymerase chain reaction and genetic sequencing were used on patients and environmental specimens to identify genetic linkage. In the retrospective cohort analysis, a medical student cluster was investigated. A total of 675 diarrheal cases were found in Mueang District. Tha Chang Subdistrict had the highest morbidity rate at 0.95%. Out of 77 patient specimens, 30% (23/77) were confirmed norovirus. The median age of cases was 26 years old. Common symptoms included diarrhea (89%), nausea (67%), abdominal pain (67%), and fever (56%). No severe cases were reported. The interviews of recent diarrheal cases revealed that they visited four markets before symptom onset. Seventy percent (21/30) of the markets’ vegetable samples tested positive for norovirus. However, the samples of tap/drinking water, ice, and seafood tested negative. Patient and vegetable genotypes were matched as GII.4[P7]. Contaminated vegetables were suspected sources of the outbreak. The recommendation for early case detection and pre-cool season preventive measures are ensuring adequate residual chlorine level, promoting handwashing, and washing vegetables before consumption.