Passive wastewater surveillance of viral pathogens on cruise ships as a powerful tool for public health protection.

Wastewater-based epidemiology (WBE) provides a powerful, non-invasive tool for monitoring infectious diseases at the population level. While it has been widely implemented in urban settings, its application in mobile, confined, and operationally complex environments remains limited. Cruise ships are a major global travel sector, carrying tens of millions of passengers annually and can act as hubs for rapid viral transmission and transboundary spread, due to high population density, shared facilities, and continuous passenger interaction. Early detection of viral pathogens in such settings is critical for preventing widespread outbreaks. We conducted a four-day study on a passenger cruise ship, comparing passive and composite wastewater sampling approaches. Samples collected from multiple blackwater tanks were analyzed for eight clinically relevant viruses (SARS-CoV-2, IAV, IBV, RSV, HAV, HEV, NoV-GII, and AdV F40/41) using RT-qPCR and RT-ddPCR. SARS-CoV-2, IAV, and NoV-GII were consistently detected in both sampling types. Passive samplers yielded lower viral loads and detection frequencies for SARS-CoV-2 and IAV, but captured similar temporal trends to composite samples. For NoV-GII, performance between the two approaches was comparable, reflecting its high shedding levels. A strong positive correlation was observed between methods for SARS-CoV-2 detection dynamics (r = 0.99, p = 0.010). Passive sampling represents a cost-effective, operationally simple strategy for WBE on cruise ships. Its ability to reliably detect trends highlights its potential for early-warning surveillance in high-mobility environments, strengthening outbreak preparedness and supporting public health protection in globally connected travel networks.