Epidemiological Analysis of Norovirus Stool Testing in Food Handlers

We conducted an epidemiological analysis of 534,565 norovirus (NoV) stool tests commissioned by food business operators between September 2018 and August 2023. The aims were to clarify the implementation status of NoV testing, determine the prevalence of NoV carriers among food handlers, and evaluate the time required to achieve a negative test result. Approximately 83% of the tests were conducted during the testing period recommended by the Ministry of Health, Labour and Welfare (October to March), with the highest number performed in December. Although no significant differences in monthly testing volumes were observed among industry categories, substantial variation was found between individual businesses. The overall NoV positive rate was 0.90%; however, among businesses conducting testing throughout the year, the positive rate was lower at 0.33%. Monthly positive rates reflected both testing volume and seasonal trends in infectious gastroenteritis, with higher rates observed from April to June. By genotype, GII accounted for 82.8% of positive cases, GI for 16.4%, and mixed GI/GII infections for 0.7%. Detection of GII followed the seasonal pattern of infectious gastroenteritis, whereas GI was detected throughout the year. The positive rate declined during the COVID-19 pandemic. Among NoV-positive individuals, the mean duration of viral shedding was approximately 10 days, and the mean time to achieve a negative result was approximately 15 days; the longest observed duration of positivity was 60 days. Some individuals became positive again after initially testing negative, with re-positivity occurring more frequently within 10 days after a negative result. These findings provide scientific evidence to support the development of more effective and practical NoV control measures tailored to the operational realities of food business settings.