A probabilistic model of norovirus disease burden associated with greywater irrigation of home-produced lettuce in Melbourne, Australia

The reuse of domestic greywater has become common in Australia, especially during periods of extreme drought. Greywater is typically used in a raw, untreated form, primarily for landscape irrigation, but more than a quarter of greywater users irrigate vegetable gardens with the water, despite government advice against this practice. Greywater can be contaminated with enteric pathogens and may therefore pose a health risk if irrigated produce is consumed raw. A quantitative microbial risk assessment (QMRA) model was constructed to estimate the norovirus disease burden associated with consumption of greywater-irrigated lettuce. The annual disease burdens (95th percentile; DALYs per person) attributed to greywater irrigation ranged from 2 ? 10-8 to 5 ? 10-4, depending on the source of greywater and the existence of produce washing within households. Accounting for the prevalence of produce-washing behaviours across Melbourne, the model predicted annual disease burdens ranging from 4 ? 10-9 for bathroom water use only to 3 ? 10-6 for laundry water use only, and accounting for the proportionate use of each greywater type, the annual disease burden was 2 ? 10-6. We recommend the preferential use of bathroom water over laundry water where possible as this would reduce the annual burden of disease to align with the current Australian recycled water guidelines, which recommend a threshold of 10-6 DALYs per person. It is also important to consider other exposure pathways, particularly considering the high secondary attack rate of norovirus, as it is highly likely that the estimated norovirus disease burden associated with greywater irrigation of vegetables is negligible relative to household contact with an infected individual.