Norovirus Outbreak and Use of Molecular Methods as a Diagnostic Tool: Cross-Sectional Study From January 2022 to April 2025

Acute diarrhea is the third leading cause of mortality worldwide in children under 5 years, with approximately 1.7 billion cases per year, according to the World Health Organization. Norovirus stands out as one of the main causes, responsible for 18–20% of all diarrhea cases, with an estimated 685 million cases and more than 200,000 deaths annually. To describe the main etiological agents of acute diarrhea identified by molecular panel and to characterize the norovirus outbreak in the first quarter of 2025. Observational, retrospective study from January 1, 2022, to April 30, 2025, using the FilmArray GI®/bioMérieux panel, which detects 22 gastrointestinal pathogens in a single stool sample, performed in a tertiary hospital in São Paulo. Between January 1, 2022, and April 30, 2025, 22,923 tests were performed, with pathogen detection in 36% of samples. The five most frequent agents were: enteropathogenic Escherichia coli (17%), Clostridioides difficile (12%), norovirus GI/GII (11%), enteroaggregative E. coli (9%), and rotavirus (6%). During the norovirus outbreak in the first quarter of 2025, positivity increased to 21.53%, with 48% of cases in the 20–59-year age group and no sex difference (males 51%, females 49%). Epidemiological studies provide crucial information on etiologic agents of acute diarrhea using molecular biology tools, which are important for outbreak surveillance and for monitoring pathogen–age group associations to guide optimal therapeutic management.