Ecology & Epidemiology of EcO157:H7 in Produce Regions of Salinas, CA

NON-TECHNICAL SUMMARY: Numerous produce-associated outbreak investigations (tomatoes, lettuce, spinach, cantaloupe) over the last four years have resulted in findings suggestive of pre-harvest contamination. The recurring association between EcO157 outbreaks and lettuce emphasizes the urgent need to conduct this study and develop science-based recommendations to reduce risks at the farm level. We propose to employ innovative epidemiological approaches to unravel the mechanism(s) of in-field contamination of lettuce by EcO157 where contamination is occurring based on preliminary data from outbreak investigations and applied field studies (Table 1 and Figure 1). The Salinas Valley is often referred to as the "Salad Bowl of the Nation" for good reasons. More than three-fourths of the state's total production of salad greens come from the region, including the majority of the lettuce products for the U.S. market, which is valued at $1.3 billion. Thus, the identification of risk factors and recommendations for risk reduction that will result from our proposed study will have major impact for both the agricultural industry and public health.

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APPROACH: Our overall study design combines epidemiological and microbiological methods (Aim 1) followed by outreach and education activities to disseminate prevention and control information (Aim 2). We will conduct an in-depth longitudinal study that identifies the key biotic and abiotic processes that sufficiently load, then hydrologically link and disseminate, primary environmental reservoirs of EcO157 within and between lettuce fields, resulting in bacterial contamination of this raw agricultural commodity.<P>

Year 1 will be devoted to a preliminary watershed-scale survey that will to enroll cooperators, producers and landowners from upslope rangeland sites down through the watersheds to the irrigated fields used for lettuce production. This first year will allow us to identify key locations for sampling vertebrates, water, soil, and lettuce, allow for optimization and sensitivity/specificity calculations of analytical methods for the matrices to be tested, and to allow for installation and troubleshooting of automated sampling and data acquisition (climate) equipment.

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This first year of installation and optimization will be followed by two years of highly intensive longitudinal sampling across the entire watershed and irrigated field continuum for (a) vertebrate environmental loading of EcO157, (b) hydrological transport of EcO157 and commensal E. coli down through the watershed and within/between fields of produce via irrigation water, and (c) in-field monitoring of lettuce production, harvesting, and irrigation practices typical of this region of CA. For each node (vertebrate sources, water, soil, lettuce) of the system, we will collect a detailed set of covariates that will be used to identify critical control points, points of environmental amplification, and management practices that either elevate or decrease the risk of in-field contamination and dissemination of EcO157 on lettuce. Although we recognize that other potential sources could contribute to contamination (for example, compost, invertebrates such as flies, bioaerosols), our preliminary data from outbreak investigations and subsequent studies strongly suggest that one or more of these four nodes are the primary risk factors and routes of dissemination. We will use an isolation method we have modified to detect EcO157 in the various samples and matrices described. Multiple colonies will be subcultured for testing by a combination of Multi-Locus Variable number tandem repeat Analysis (MLVA) (24, 25) and Pulsed Field Gel Electrophorsis (PFGE) (26) to characterize isolates by genetic differences, determine molecular epidemiologic linkages among samples isolated or obtained from public health collaborators, and thus, pinpoint the mechanisms that link and disseminate vertebrate sources of EcO157 within and between fields of lettuce.