Purpose: This investigation assessed the internalization and tissue distribution of HuNoV and its surrogate viruses, porcine sapovirus (SaV) and Tulane virus (TV), in leafy greens.
Methods: Lettuce and spinach plants (n=6/time point/plant) were inoculated with HuNoV GII.4 (108 GE/ml), SaV and TV (106 TCID50/ml) either through: (i) the seedlings’ roots with viruses tracked on days one and six in the roots and leaves; or (ii) the cut petiole of mature leaves with viruses tracked on days one and three in the central veins, lower and upper leaf lamina. Processed tissues were subjected to RNase treatment prior to RNA extraction and virus-specific RT-qPCR. The infectivity of SaV and TV was determined in tissue culture.
Results: In both lettuce and spinach: (i) HuNoV translocated from roots to leaves at similar RNA titers; whereas, TV and SaV were retained at significantly higher RNA titers in the roots; and (ii) HuNoV disseminated with no tissue preference inside the leaves, while SaV and TV RNA titers were detected at significantly higher titers in central veins as compared with the upper and lower leaf lamina. Although TV and SaV were detected at comparable RNA levels, infectious SaV was detectable only in roots and veins. Infectious TV was detectable in all tissues, suggesting a higher stability of TV compared with SaV.
Significance: HuNoV internalized through roots and cut leaves and disseminated similarly into various spinach and lettuce tissues, raising concerns of internal contamination through irrigation and/or wash water. The internalization patterns of SaV and TV did not mimic HuNoV; this limits their utility as surrogate viruses.