Under Field Conditions, Distance is Significantly Associated with the Amount of Escherichia coli That Transfers from Wildlife Feces to Preharvest Lettuce during Foliar Irrigation

Introduction: Previous studies have (i) found that splash during rain or irrigation events can transfer microbes from the surrounding environment (e.g., wildlife feces) to preharvest produce, and (ii) identified wildlife as potential vectors for the introduction of foodborne pathogens into farm environments. However, only two peer-reviewed studies have examined the transfer of pathogens from wildlife feces to field-grown produce. While both studies found a significant relationship between distance and Escherichia coli transfer, the studies only sampled produce that was close to the feces (e.g., <1 m).

Purpose: This study was conducted to provide additional data to refine our understanding of the impact of distance on E. coli splash.

Methods: Two trials were conducted; for each trial rabbit feces inoculated with a three-strain cocktail of nonpathogenic E. coli were placed in a lettuce field 2.5 hours before irrigation. Following irrigation, the E. coli concentration on lettuce, between 0.0 and 5.2 m from the feces, was determined. For analysis, data were combined with a subset of data from a previously published study.

Results: Thirty nine percent (38 of 97) lettuce heads had detectable levels (≥1 MPN) of E. coli. While none of the 39 lettuce heads ≥2 m from the closest feces had detectable levels of E. coli, 38 of 58 heads that were <2 m from the feces tested positive for E. coli. Linear regression showed that, for lettuce <2 m from the feces, the E. coli concentration decreased by 1.78 (95% Confidence Interval=-2.52, -1.05; P<0.0001) log MPN for each meter increase in the lettuce-feces distance.

Significance: Establishing a no-harvest buffer around in-field feces may reduce the risk of harvesting contaminated produce. As such, this study provided data that can be used in quantitative risk assessments to develop effective risk management strategies following wildlife intrusion.