Developing buffer zone distances between sheep grazing operations and vegetable crops to maximize food safety.

Integrated livestock and crop operations are beneficial to producers of both products. Crop residues are an important source of food for livestock, however domestic and wild animals represent a potential source of food borne pathogens. Recent outbreaks of human infection with E. coli O157:H7 and other bacteria linked to consumption of California produce have raised concerns that sheep and other ruminants may elevate levels of pathogens within the soil, which have the potential of being transmitted to produce fields via aerosols. The California Leafy Green Products Handler Marketing Agreement (LGMA) of January 2010 lists sheep as one office mammalian species as "Animals of Significant Risk" and any intrusion by such animals requires a detailed food safety assessment prior to harvest."Buffer zones" between the crop production fields and livestock operations are important in order to prevent the potential transmission of pathogens from animals to crops. Currently, there is a paucity of information related to appropriate combinations of time and distance between livestock operations and crop systems, particularly in terms of pathogen survival in animal feces, soil, and aerosols, as well as the pathogen movements through wind, water or flies. The LGMA suggests that a distance of 400 ft exist between a concentrated animal feeding operation and the edge of a crop and 30 ftfor grazing lands/domestic animals, but recognize a lack of science on which to base this recommendation,This proposed research aims 1) to investigate factors associated with the survival of bacterial pathogens from initial deposition as feces, to presence in soil beforehand after irrigation events, to presence of pathogens in dust generated in fields with active livestock grazing, and 2) to investigate the distance over which pathogens can be transferred by aerosolized particles.

Concerns are raised with all animal production systems and their wastes where there is an increased potential for microbial pathogens to enter the aerosol environment. Thus, an understanding of relative risks can only be put into perspective with a clear knowledge of the actual levels of the various organisms at realistic distances from production systems. The present study aims to address the issue of transfer and survival of key organisms in the surrounding natural aerosol environment under normal grazing conditions to assess an overall pattern of transmission. . Currently, there is a paucity of information related to appropriate combinations of time and distance between livestock operations and crop systems, particularly in terms of pathogen survival in animal feces, soil, and aerosols, as well as the pathogen movements through wind, water or flies. While the California Leafy Green Products Handler Marketing Agreement (LGMA) has suggested distances in their guidelines, they recognize that additional quantitative scientific data would help with their recommendations. This research project will be directed by UC Davis, with collaboration from UC Cooperative Extension, Imperial County, and the California Wool Growers Association. This project has as its primary objective to identify factors associated with the survival of bacterial pathogens from initial deposition as feces, to presence in soil before and after irrigation events, to presence of pathogens in dust generated in fields with active livestock grazing, and to investigate the distance over which viable pathogens can be transferred by aerosolized particles.

1. Estimate the survival of E. coli O157:H7, commensal E. coli, and Salmonella spp. in sheep feces and soil where sheep graze (before and after irrigation). 

2. Estimate the potential distance travelled by viable bacteria in aerosols generated by sheep grazing activity. 

3. Based on pathogen survival and aerosol distance information, generate practical management guideline for growers to set scientifically valid “buffer zones” between livestock operations and crop production areas.

Integrated livestock and crop operations are beneficial to producers of both products. Crop residues are an important source of food for livestock, however domestic and wild animals represent a potential source of foodborne pathogens. Recent outbreaks of human infection with E. coli O157:H7 and other bacteria linked to consumption of California produce have raised concerns that sheep and other ruminants may elevate levels of pathogens within the soil, which have the potential of being transmitted to produce fields via aerosols. The California Leafy Green Products Handler Marketing Agreement (LGMA) of January 2012 lists sheep as one of five mammalian species as “Animals of Significant Risk” and any intrusion by such animals requires a detailed food safety assessment prior to harvest. “Buffer zones” between the crop production fields and livestock operations are important in order to prevent the potential transmission of pathogens from animals to crops. We detected E. coli O157:H7 in 1.8% of fecal samples and 0.4% of soil samples. Salmonella spp. was found in 0.8% of fecal samples and 0.4% of soil samples. These results indicate that fresh sheep feces are low (but not zero) risk products for leafy green producers. Soil where sheep have been actively grazing is even lower (but still not zero) risk. For these reasons, the LGMA recommendation of a food safety assessment following intrusion by sheep is warranted. The PFGE patterns indicate that unique strains of E. coli O157:H7 exist in this population, and this information can be useful if investigations are necessary. Airborne transmission of bacteria was also assessed in this project. The greatest number of bacteria was recovered at a distance of 2m from the edge of a field with sheep grazing. This difference was not significant, however. Given the low prevalence of pathogenic bacteria in either feces or soil, we were not able to detect these pathogens in air samples. Future studies that compared the genotypes of bacteria recovered by air sampling with bacteria from the sheep would be worthwhile. Based on our findings, we believe that the LGMA recommended buffer distance of 30 ft (9m) between grazing livestock and crops is justified, and will provide a more than adequate distance to ensure protection from potential contamination by grazing sheep.