Risky Business–Norovirus and Treated Wastewater

Noroviruses are a principal cause of gastroenteritis from many exposure routes, including recreational and shellfish-harvesting waters. Since 2008, data from clinical trials and outbreak studies have become available, allowing illness as a consequence of exposure to this virus to be used as an end-point in sewage-related health risk assessments in New Zealand (and abroad). The authors of a recent (2013) clinical trial inferred that the human median infectious dose is considerably higher than previous estimates. This finding has the potential to require revision of previous risk assessments. However the potential role of aggregation of these viruses in inflating infectious doses was not accounted for. When fitting dose-response curves to data from Norovirus clinical trials, the potential for virus adsorption onto storage matrices must be addressed. Standard dilution-series dose calculations assume uniform mixing of non-aggregated, non-adsorbed particles. "Low" doses derived in this manner may therefore actually be "No" doses. The possible consequences of including aggregation is explored. Research into the potential for sewage treatment processes to promote aggregation would be of benefit to sewage-related QMRAs. I conclude that the Norovirus dose-response relationships used in QMRAs to date remain valid.