Norovirus Transfer during Chopping of Contaminated Romaine Lettuce

Introduction:  Human noroviruses are the leading cause of outbreaks of nonbacterial gastroenteritis in the U.S., and are often implicated in outbreaks associated with ready-to-eat foods, such as salads. Such foods may be extensively handled, and norovirus may be transferred readily between infected food handlers and food surfaces.

Purpose:  To investigate the transfer of noroviruses between lettuce, cutting board and knife used in fresh food preparation.

Methods:  Fresh Romaine lettuce was cut into 25 cm² pieces and inoculated with 25 µl MNV-1 (approx. 7 log PFU/leaf). After 10 min drying in a biosafety cabinet, 5-g piles of inoculated leaves were cut with a sterile stainless steel knife on a sterile polyvinyl cutting board. Sampling sponges soaked in Dulbecco’s Modified Eagle’s Media (DMEM) were used to swab blade and board surfaces for MNV-1 transferred from lettuce. In some cases, fresh, uninoculated Romaine lettuce was chopped using the contaminated knife and board, and MNV-1 that was transferred to the chopped lettuce was recovered by stomaching in DMEM and enumerated by plaque assay.

Results:  MNV-1 transfer was lower from contaminated lettuce to cutting board (P < 0.05; range from 0.6 to 6.2%) than that from a contaminated cutting board to lettuce (1.6 to 91%). This same trend was observed between a stainless steel knife and lettuce.  The transfer coefficient of MNV-1 was lower when contaminated lettuce was chopped on a wood board (P < 0.05; range from 0.1 to 2.5%) compared to a polyvinyl cutting board (0.6 to 6.2%).

Significance:  Norovirus transfers readily between surfaces common in the preparation of fresh produce preparation, but the transfer coefficient appears to be dependent on the surface type.  This research provides the scientific basis for the development of a risk assessment model for norovirus transfer during the fresh food preparation.