Norovirus Detection in Ready-To-Eat Salads by Propidium Monoazide Real Time RT-PCR Assay

Ready-to-eat (RTE) salads have recently been associated with food-borne norovirus outbreaks, although these infections are mainly related to shellfish and berry consumption in the EU. A total of 135 bagged RTE vegetables were analyzed in order to investigate the occurrence of norovirus (NoV) genotype I (GI) and II (GII) RNA and to differentiate between infectious and non-infectious viruses by using propidium monoazide (PMAxx) coupled with the real time Reverse Transcription (RT) PCR method. Initially, the PMAxx real time RT-PCR assay was optimized on NoV GI and GII suspensions, and proved capable of detecting significant (p < 0.05) differences between infectious and inactivated viruses. Our analysis conducted on RTE salads samples showed the presence of norovirus GII in 74.8% of samples, of which 37.6% were infectious. The samples tested for viral contamination came from only two RTE vegetable-processing plants. The findings in this study could also be due to virally-contaminated water used in food production, processing, or preparation. This study stresses the need for effective real-time RT-PCR tools capable of qualitative and quantitative detection of NoV RNA, as well as being able to measure virus infectivity, for risk assessment, which is crucial in several public health measures and food regulations.