Quantitative Microbial Risk Assessment (QMRA) models with 10,000 Monte Carlo simulations were applied to ascertain the risks of rotavirus and Ascaris infections for farmers using different irrigation water qualities and consumers of lettuce irrigated with the different water qualities after allowing post-harvest handling. A tolerable risk (TR) of infection of 7.7 × 10−4 and 1 × 10−2 per person per year were used for rotavirus and Ascaris respectively. The risk of Ascaris infection was within a magnitude of 10−2 for farmers accidentally ingesting drain or stream irrigation water; ∼100 for farmers accidentally ingesting farm soil and 100 for farmers ingesting any of the irrigation waters and contaminated soil. There was a very low risk (10−5) of Ascaris infection for farmers using pipe−water. For consumers, the annual risks of Ascaris and rotavirus infections were 100 and 10−3 for drain and stream irrigated lettuce respectively with slight increases for rotavirus infections along the post-harvest handling chain. Pipe irrigated lettuce recorded a rotavirus infection of 10−4 with no changes due to post harvest handling. The assessment identified on-farm soil contamination as the most significant health hazard.

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