Potential Use of Wastewater from Passenger Ships to Estimate the Import Rate of COVID-19 and Other Viruses Across Maritime International Boundaries

The worldwide spread of SARS-CoV-2 and the resulting COVID-19 pandemic has been driven by international travel. This has led to the desire to develop surveillance approaches which can estimate the rate of import of pathogenic organisms across international borders. The aim of this study was to investigate the use of wastewater-based approaches for the surveillance of viral pathogens on commercial short-haul (3.5 h transit time) roll-on/roll-off passenger/freight ferries operating between the UK and the Republic of Ireland. Samples of toilet-derived wastewater (blackwater) were collected from two commercial ships over a 4-week period and analysed for SARS-CoV-2, influenza, enterovirus, norovirus, the faecal-marker virus crAssphage and a range of physical and chemical indicators of wastewater quality. A small proportion of the wastewater samples were positive for SARS-CoV-2 (8% of the total), consistent with theoretical predictions of detection frequency (4-15% of the total) based on the national COVID-19 Infection Survey and defecation behaviour. In addition, norovirus and enterovirus were detected in wastewater at higher frequency (32 and 24% of samples positive, respectively). No influenza A/B viruses or enterovirus D68 were detected throughout the study period. We conclude that testing of wastewater from ships that cross international maritime boundaries may provide a cost-effective and relatively unbiased method to estimate the flow of infected individuals between countries. The approach is also readily applicable for the surveillance of other disease-causing agents and antimicrobial resistance (AMR) genes.