Norovirus trends and seasonality in Baltimore wastewater: Year-long insight

• First year long wastewater-based surveillance of norovirus GI and GII conducted in Baltimore, USA. • NoV-GII is more prevalent than NoV-GI, potentially the primary strain causing gastroenteritis. • A seasonal rise in viral concentrations was observed from late winter into spring. Norovirus (NoV), a leading cause of acute gastroenteritis worldwide, presents a persistent public health risk because of its higher environmental stability and pronounced seasonality. This study reports a year-long of wastewater-based epidemiology (WBE) surveillance of NoV genogroups I and genogroup GII in Baltimore based on total of 110 influent raw sewage samples collected from two major wastewater treatment plants between July 2022 and August 2023. Samples were concentrated by polyethylene glycol precipitation (PEG) and quantified by reverse transcription quantitative PCR (RT-qPCR). NoV-GII was detected in nearly all samples, 100 % at WWTP-A and 98.2 % at WWTP-B, with mean concentrations of 4.63 and 5.07 log₁₀ copies/L, respectively. NoV-GI had slightly lower detection rates (96.4 % at WWTP-A, 92.7 % at WWTP-B) and lower average concentrations (3.70 and 3.88 log 10 copies/L). At the same time, Cliff's delta suggested a high seasonal effect for NoV-GI at WWTP-B (δ= 0.594) and moderate effects elsewhere (δ= 0.467–0.499). Spearman's correlation analysis found statistically significant links between NoV-GII concentrations and temperature (ρ= 0.45, p < 0.001), salinity (ρ = 0.44, p < 0.001), and TDS (ρ = 0.28, p < 0.05) at WWTP-B. These associations were not observed at WWTP-A, which further demonstrated spatial heterogeneity in environmental drivers of viral presence. Collectively, the results demonstrate the utility of WBE for NoV surveillance and support for integrating environmental data to improve early-warning capabilities and public health interventions.