A System Model for Understanding the Role of Animal Feces as a Route of Contamination of Leafy Greens before Harvest

Abstract

The majority of foodborne outbreaks in the United States associated with the consumption of leafy greens contaminated with Escherichia coli O157:H7 have been reported during the period of July to November. A dynamic system model consisting of subsystems and inputs to the system (soil, irrigation, cattle, wild pig, and rainfall) simulating a hypothetical farm was developed. The model assumed two crops of lettuce in a year and simulated planting, irrigation, harvesting, ground preparation for the new crop, contamination of soil and plants, and survival of E. coli O157:H7. As predicted by the baseline model for crops harvested in different months from conventional fields, an estimated 13 out of 257 (5.05%) first crops harvested in July would have at least one plant with at least 1 CFU of E. coli O157:H7. Predictions indicate that no first crops would be contaminated with at least 1 CFU of E. coli O157:H7 for other months (April to June). The maximum E. coli O157:H7 concentration in a plant was higher in the second crop (27.10 CFU) than in the first crop (9.82 CFU). For the second crop, the probabilities of having at least one plant with at least 1 CFU of E. coli O157:H7 in a crop were predicted as 15/228 (6.6%), 5/333 (1.5%), 14/324 (4.3%), and 6/115 (5.2%) in August, September, October, and November, respectively. For organic fields, the probabilities of having at least one plant with Ն1 CFU of E. coli O157:H7 in a crop (3.45%) were predicted to be higher than those for the conventional fields (2.15%).IMPORTANCE This study is the first attempt toward developing a mathematical system model to understand the pathway of E. coli O157:H7 in the production of leafy greens. Results of the presented system model indicate that the seasonality of outbreaks of E. coli O157:H7-associated contamination of leafy greens was in good agreement with the prevalence of this pathogen in cattle and wild pig feces in a major leafy greens-producing region in California. On the basis of comparisons among the results of different scenarios, it can be recommended that the concentration of E. coli O157:H7 in leafy greens can be reduced considerably if contamination of soil with wild pig and cattle feces is mitigated.KEYWORDS leafy greens, Escherichia coli O157:H7, outbreaks, system model, animal feces T he "Dietary Guidelines for Americans, 2010" recommends that adults and children eat a variety of fruits and vegetables in order to lower the risk of chronic diseases and to achieve and maintain a healthy weight (1, 2). While leafy vegetables are an important part of a healthy and nutritious diet, they are usually consumed raw; thus, any leafy vegetables with contaminated pathogens have the potential to cause food-

Postharvest Supply Chain with Microbial Travelers: a Farm-to-Retail Microbial Simulation and Visualization Framework

“…Foodborne illness associated with contaminated fresh produce can be mitigated by understanding and controlling key supply chain factors. A number of existing risk assessment models have focused on on-farm interventions to describe the impact of season and field temperature, animal intrusion, and soil amendments on contamination of various fresh produce commodities, lettuce and leafy greens in particular (20)(21)(22). In this study, a detailed and mechanistic model of dynamic microbial behavior was developed to provide a tool specific to the unique postharvest practices in fresh produce supply chains.…”

“…The systems biology approach has been utilized for studying outbreaks of foodborne illness in order to obtain an understanding of the biology in an often complex, intricate network (11,12). For example, risk assessment models have been conducted in various food matrices and production chains, such as fresh pork (13), leafy greens (14)(15)(16), cheese (17), poultry (18), and berries (19), as well as on-farm preharvest contamination pathways (20)(21)(22). While these detailed and mechanistic models allow for both conceptual understanding of contamination dynamics during the entire production process and estimations of the impact of the burden to public health, they are often developed for specific microorganisms in specific food matrices.…”

Multi‐Exposure Pathway Model to Compare Escherichia coli O157 Risks and Interventions

“…The fate of fecal‐borne E. coli O157 has implications for environmental, veterinary, and public health outcomes that can be captured through comprehensive QMRAs. A number of existing water‐body impairment and human health risk assessment models describe the fate of generic and pathogenic E. coli , both in mixed‐use environments, water systems, and along the farm‐to‐fork continuum . However, with exception, these either consider a single route of exposure or are coupled to existing complex, site‐specific resource (e.g., hydrological or soil erosion) models.…”

“…A two‐thirds reduction in the number of contaminated LGUs at harvest resulted in a 54% risk reduction. However, this module did not consider splashing of surface water or soil onto the LGUs, as performed elsewhere, or internalization of E. coli O157 through the root system. It is unclear what impact, if any, irrigation technique has on pathogen internalization—a contamination phenomenon that has yet to be fully characterized, and a noted knowledge gap.…”

Enterohaemorrhagic and other Shiga toxin-producingEscherichia coli(STEC): Where are we now regarding diagnostics and control strategies?

“…For example, the outbreak in Germany and France in 2011, involving about 4000 cases with over 50 deaths, was epidemiologically linked to sprouting seeds(Frank et al, 2011). Fresh products are frequently grown in soils fertilized or contaminated with animal waste and/or irrigated with brown water, which can lead to plant contamination on harvesting(Mishra, Pang, Buchanan, Schaffner, & Pradhan, 2017). STEC is capable of an endophytic lifestyle(Wright et al, 2013) enabling attachment to, and survival in, plant material and even protection from post-harvest disinfection processes.…”