Survival of infectious human norovirus in water and on leafy greens

In the US, the majority of foodborne illnesses (58%) are caused by human norovirus (HuNoV). Lettuce and other leafy greens are most often implicated in the HuNoV outbreaks. Lettuce is the most popular salad vegetable and is almost exclusively used fresh with minimal chlorine washes postharvest. The presence of HuNoV RNA in river water used for irrigation has been demonstrated in numerous studies worldwide. However, in the US, under the current Produce Safety Rule, agricultural water applied to produce is not required to be tested for viruses, thus overlooking a potential threat to the produce industry. The produce rule requires agricultural water to be tested for generic E. coli which may not predict the levels of HuNoVs. Furthermore, it is not evident whether current lettuce chlorine washes is effective against HuNoV. Because HuNoV was difficult to grow in cell culture, limited data exist on the actual infectious viruses in irrigation water, or on pre-and postharvest lettuce washed with chlorine. However, recently a new cell culture system for HuNoV based on human intestinal enteriods (HIE) derived from 3D cultures of intestinal stem cells has been reported. Therefore, the objectives of this proposal are to determine (i) the survival of infectious HuNoV in irrigation water and it’s percent die-off in relation to that of E. coli, (ii) the survival of infectious virus on lettuce under preharvest conditions and (iii) and on post-harvest lettuce following chlorine washes. Irrigation water will be sampled from multiple produce growing regions and used to set microcosms in which HuNoV will be spiked with or without generic E. coli. The microcosms will be incubated under conditions similar to pre-harvest environment and sampled weekly for 2 months. In addition, green house grown lettuce will be spiked with HuNoV and incubated for 2 weeks. Lettuce samples (whole and shredded) spiked with the virus will be subjected to chlorine washes (100 and 200 ppm) for 2 min. Wash water will also be spiked with the virus and chlorinated at 100 and 200 ppm for 2 min to assess whether infectious HuNoV will be inactivated. The infectivity of HuNoV and its RNA titers will be determined using HIE and real-time qPCR as well as the physiochemical and microbiological quality of irrigation and wash water. Knowledge generated from this proposal is expected to benefit the produce industry in better assessing the risks associated with water and food transmission of infectious HuNoV.