Unseasonal GI Norovirus Trends in the Eastern Upper Peninsula of Michigan: Insights from Wastewater Surveillance

Norovirus is the leading cause of acute gastroenteritis worldwide, responsible for up to 90% of viral gastroenteritis outbreaks and an estimated 10.6 billion USD in annual economic losses in the U.S. Despite its well-documented seasonality, wastewater surveillance in the Eastern Upper Peninsula of Michigan reveals persistent GI norovirus detection year-round, diverging from national clinical trends that consistently show far greater GII prevalence. To characterize norovirus dynamics in this region, 250 mL wastewater influent grab samples were collected once per week across 14 sites, concentrated using a PEG-based method, and analyzed via digital droplet PCR (ddPCR) for GI and GII concentrations. Across the study period, the rate of positive sites per month ranged from 57 to 100% for GI and 74 to 97% for GII, with mean positivity rates of 85.4% (GI) and 88.7% (GII), indicating that both genogroups were detected frequently at comparable levels. GI was more prevalent in winter and spring (December–May), whereas GII was more prevalent during spring and summer (March–August). Mean GI gene copies per 100 mL ranged from 12,898 (October) to 532,792 (February), while mean GII concentrations ranged from 29,806 (December) to 1,100,215 (May). These patterns contrast with national clinical data, where GI contributes to a small minority of reported norovirus cases. This study explores potential environmental and behavioral factors contributing to this regional pattern. GI norovirus demonstrates greater resistance to wastewater treatment and environmental stability, which may facilitate its persistence in the region. Additionally, congregate living settings, such as college campuses and correctional facilities, may contribute to sustained GI prevalence through foodborne transmission and asymptomatic viral shedding. Overall, these findings suggest that environmental and social factors influence norovirus seasonality and genogroup distribution in this region, underscoring the need for improved monitoring and expanded multi-site wastewater and epidemiological research to better understand norovirus persistence in similar communities.