Seasonal occurrence and abundance of norovirus in pre- and postharvest lettuce samples in Nanjing, China

To assess the contamination of lettuce with norovirus in Nanjing city (China), two hundred and eighty one lettuce samples were collected between 2017 and 2018 from production sites (preharvest) and retail outlets (postharvest) and analyzed. The presence of norovirus was confirmed in 26.9% (71/264) of lettuce samples, with norovirus genogroup I detected more frequently (20.1%) than norovirus genogroup II. Norovirus was detected less frequently in lettuce samples from supermarkets (4.9%) than in samples from greengrocers or farmer markets (36.5%) and was detected both in field-grown (28.9%) and in hydroponic lettuce samples (30%). The norovirus occurrence and levels varied markedly by seasons. The norovirus was more frequently detected in the cold season, with evident correlations between norovirus contamination and ambient temperatures. Interestingly, the norovirus occurrence in loose lettuce was significantly higher (p < 0.05) than in packaged lettuce, highlighting that norovirus contamination might have occurred during lettuce production. The data of this study add to the currently limited body of information on norovirus occurrence in fresh produce in China, highlighting the need for implementation of effective food safety monitoring and management.