Duration of norovirus excretion and the longitudinal course of viral load in norovirus-infected elderly patients.

To prevent dissemination of norovirus in semiclosed environments such as aged-care facilities, it is important to know the period of infectivity in norovirus-infected individuals. We recruited 13 elderly patients aged 60-98 years with norovirus gastroenteritis (11 residents in aged-care facilities and two healthy adults) for this study, and measured the viral loads for norovirus in a total of 63 follow-up faecal samples using a real-time quantitative polymerase chain reaction assay. The average period of norovirus excretion was 14.3 days (range: 9-32 days; median: 13 days). All of the follow-up samples collected between 7 and 10 days after the onset of symptoms tested positive. Viral loads in samples collected between 14 and 18 days after the onset of symptoms were divided into three groups: those testing negative, those with <10(4) copies/g stool, and those with >10(4) copies/g stool. Stools from the group with <10(4) copies/g stool were found to be negative for norovirus up to 21-24 days after the onset of symptoms; however, the group with >10(4) copies/g stool showed prolonged norovirus excretion (up to 32 days) in stools. Although the period of infectivity of excreted viruses has not yet been clarified, these results suggest that careful attention should be taken for at least 14 days after the onset of symptoms and that the measurement of viral load in stools around 16 days after onset might be a useful method for following the course of viral shedding for each patient infected with norovirus.