What lies beneath? Hepatitis E virus surveillance in Belgian wastewater suggests hidden trends in community circulation.

Hepatitis E virus (HEV) is a major cause of acute viral hepatitis worldwide. Eight genotypes have been identified so far, with genotype 3 (gt3) predominating in high-income countries and commonly linked to eating undercooked pork. In Belgium, an estimated 54,000 annual HEV infections contrast sharply with only 25-117 clinically reported cases, revealing a critical surveillance blind spot. To evaluate the potential to complement clinical surveillance, we retrospectively analysed municipal wastewater weekly for HEV (presumed genotype 3) using digital PCR during 2022-2023. Sampling covered four Antwerp wastewater treatment plants (WWTPs) in 2022 (population of approximately 503,243 inhabitants) and 13 WWTPs across Flanders and Brussels in 2023, serving a population of approximately 2.6 million inhabitants (≈23% of Belgium's population). Viral signals were normalised to pepper mild mottle virus (PMMoV) as a proxy for faecal load and aggregated across sites to describe temporal trends. HEV gt3 RNA was detectable year-round in 2022 and 2023, but amplitudes and timing varied by catchment. In 2022, the wastewater HEV RNA signal showed an early-summer surge (May-June). In 2023, it rose from late May to a July-August peak with a second, broader elevation in late October-November. Weekly clinical case data identified at the Belgian National Reference Centre did not align with wastewater signals, suggesting that current surveillance captures only symptomatic or diagnosed infections. Since wastewater surveillance reveals sustained, seasonally patterned HEV gt3 circulation and captures asymptomatic infections missed by clinical surveillance, we advocate for further wastewater-based surveillance to complement traditional epidemiological monitoring.