Prevalence of Shiga toxin-producing Escherichia coli and Salmonella in native pecan orchards as influenced by waiting periods between grazing and harvest.

Animals (grazing, working, or intrusion) in produce production areas may present a potential contamination source of foodborne pathogens on produce. Cattle grazing on native pecan production orchards, a common practice in the Southern United States, provides a great opportunity to study the impact of grazing practice and waiting periods on contamination rates of foodborne pathogens of tree nuts. Therefore, the objective of this study was to determine the prevalence of Salmonella spp. and Shiga toxin-producing Escherichia coli (STEC), in native pecan production orchards as influenced by waiting periods between grazing cattle and pecan harvest. Soil (10 g), cattle feces (10 g), and in-shell pecans (25 g) were sampled from five cattle-grazed orchards in areas with cattle removed two or four months prior to harvest and not removed. Five non-grazing orchards were sampled at harvest for comparison. Detection and isolation of the pathogens were performed by enrichment, selective isolation, and multiplex PCR. Statistical analyses were performed using contingency tables with Pearson's chi-squared test. The prevalence of STEC (36%) and Salmonella (29%) in cattle-grazed orchards was significantly higher compared to non-grazed orchards (13%; 7%). STEC prevalence in cattle-grazed orchards was higher (38%) in areas with cattle at harvest than in fenced areas where cattle were removed two (29%) and four (27%) months prior to harvest. Salmonella prevalence was similar in areas without fencing (31%), and areas with cattle removed at two (22%) and four months prior to harvest (30%). However, there were not significant differences ( P ≤ 0.05) in contamination rates between waiting periods for either pathogen, suggesting a limited impact of waiting periods on reducing the risk of contamination.