Purpose: The objective of this study was to assess the survival and stability of human noroviruses and surrogates including murine norovirus (MNV) and Tulane virus (TV), on foliar surfaces of spinach plants in preharvest growth conditions.
Methods: Mature spinach plants were spray inoculated with human norovirus of both genogroups I and II (NoV GI or NoV G2), MNV and TV (approximately 4 log genomic copies) using an airbrush to deposit virus onto the entire foliar surface of the plant including the stem, adaxial and abaxial surfaces of the leaves. Spinach plants were grown in a biocontrol chamber at optimal conditions of 18 °C, 65% humidity and a light intensity of 150 µmol/m2s for a photoperiod of 15 h and 9 h dark period for up to 21 days. Samples were analyzed for virus presence on days 0, 1, 3, 5, 7, 14 and 21 by qPCR for all viruses and plaque assay for MNV and TV. Controls included virus inoculated HBSS and non-inoculated spinach plants.
Results: MNV and TV were infectious through day 14 at titers of 2.89 ± 0.36 and 2.77 ± 0.15 log PFU, respectively, but were not detected after 21 days on leaves. NoV G2 was detected by qPCR on leaves through day 14 at a titer of 2.91 ± 0.70 log genomic copies, and virus was not detected on day 21. MNV and NoV titers decreased significantly by day 3 (P ≤ 0.05), but titers did not decrease significantly between days 3, 5, 7 and 14 (P > 0.05). Control plants remained negative.
Significance: These results suggest noroviruses can survive for up to two weeks on foliar surfaces of spinach in environmental conditions. Norovirus surrogates, MNV and TV, showed similar survival in this study to human noroviruses. This understanding of the behavior of enteric viruses on spinach leaves can be used to develop produce growers guidelines and to assess risk under certain growing conditions.