Integrating wastewater surveillance of norovirus epidemics with recreational water risk assessment for enhanced urban swimming safety

Noroviruses are the leading cause of hand-borne viral gastroenteritis worldwide, but recreational water exposure is an important transmission route to consider. This study integrates wastewater-based epidemiology with quantitative microbial risk assessment to enhance urban contamination knowledge and swimming safety in the Greater Paris region. Raw wastewater from 8 wastewater treatment plants and up to 16 Paris sewerage network sites were sampled from November 2021 to March 2025, totaling over 3000 composite samples. Surface waters from the Seine and Marne rivers, crossing the region, were collected monthly (122 samples of 20 L) from January 2017 to March 2025. Norovirus GI (NVGI) and GII (NVGII) were quantified using RT-qPCR and digital RT-PCR. Both genogroups were detected in 100% of samples; NVGII consistently showed higher median concentrations. NVGII concentrations showed moderate to strong temporal correlation with acute gastroenteritis incidence, exceeding that of NVGI. Pronounced seasonal patterns were observed; winter peaks and summer lows coincided with clinical surveillance data. Genotyping revealed dominance of genotypes II.4 and II.17 during epidemic phases, with a shift from II.4 to II.17 observed since winter 2024. The surface water/wastewater median ratio ranged from 0.017 to 0.035, confirming the contribution of treated effluent to contamination independent of rainfall. QMRA demonstrated higher risk in the Marne river than in the Seine river even during summer bathing periods. Compliance with bacterial indicator standards did not significantly reduce norovirus-associated gastroenteritis risk. This integrated surveillance demonstrates that wastewater monitoring provides essential complementary information for recreational water management, beyond the limitations of traditional bacterial indicators.