CPS Rapid Response - Yuma Valley

Growers are faced with a myriad of options related to water treatment and/or sanitization with very little guidance on the requirements needed for successful implementation of the treatment option. With limited guidance, water treatment decisions are likely to be unsuccessful and expend both excess time and money without the ultimate outcome of reduction in generic E. coli, Total Coliform bacteria, and potential pathogen loading within a water source and thus little to no reduction in microbiological risk. Additionally, there may be regional and site-specific considerations important for effective water treatment. Grower guidance is needed on water treatment best management practices and monitoring strategies to ensure adequate treatment, attainment of regulatory limits, and ultimately the protection of public health. Research will focus on comparative evaluation of three agricultural water treatment options (PAA, Calcium Hypochlorite, and UV Light) in four produce growing regions of the Southwest (AZ and TX). The overall goal of this proposal is to develop scientific data, which allow produce growers to better manage their use of agriculture water treatments.

Recent metrics changes to the Arizona and California Leafy Greens Marketing Agreement(s) (LGMA) now require growers utilizing surface water for overhead irrigation, to treat their water within 21 days of harvest. For many producers, this is the first time that water quality data may indicate the need for antimicrobial treatment of agricultural water as a corrective action before irrigation can be applied safely. Exacerbating these challenges, growers are faced with a myriad of options related to antimicrobial water treatment with very little guidance on the most appropriate treatment option for their ranch, or the requirements needed for successful implementation. With limited guidance, water treatment decisions are likely to be unsuccessful and expend both excess time and money without the ultimate outcome of eliminating generic E. coli (non-detect per 100mL), and reducing Total Coliform bacteria (< 99 MPN/100mL). Unsuccessful treatments will likely lead to little or no reduction in potential pathogen loading in an agricultural water source and thus little to no reduction in microbiological risk. CPS-funded studies conducted by Dr. Rock characterizing microbial quality of water used to irrigate fresh produce in the Southwest found that foodborne pathogens are present in surface waters (Rock and Gerba, 2014) and that treatment options can be highly variable (Rock, 2019). Grower guidance is needed on antimicrobial agricultural water treatment options available to industry and monitoring strategies to ensure successful treatment and ultimately the protection of public health. Over the course of one growing season, the research team will evaluate the efficacy of three antimicrobial treatments (Peroxyacetic Acid - PAA, Calcium Hypochlorite, and Ultra Violet Light) across four produce growing regions of the Southwest; Yuma, AZ; Maricopa, AZ; Edinburg, TX; and Uvalde, TX. The overall goal of this proposal is to develop scientific data, which will allow produce growers to better manage their use of antimicrobial agriculture water treatments in the Southwest.

The overall objective of this rapid response research effort was to capitalize on the unique opportunity to study romaine production under real world conditions within close proximity to potentially implicated fields from the 2018 outbreak of E. coli O157:H7 in romaine lettuce. This venture allowed the research and extension team to generate new knowledge that is useful to industry in order to bridge the gap created by the recent outbreak. The team focused on the areas of water treatment, persistence of microbial contamination on romaine, as well as sampling protocols appropriate for the detection of indicators and pathogens in commercial scale agriculture.

Overall, our data indicated that both sanitizers evaluated, PAA and Sodium Hypochlorite, in the rapid response study were effective at reducing microbial loading in irrigation water. However, it should be noted that when larger sample volumes were assayed beyond the 100ml standardized metric, water samples that were assumed to be negative for bacteria resulted in positive detections. This indicates that while the 100ml sample volume is useful for its ease in assessment, that industry should not assume 100% efficiency with water treatment and be aware that bacteria still may persist in treated water. This finding is also exemplified in the results of detectable bacteria on raw-product and soil samples collected post treatment. With respect to the simulation and evaluation of contamination events and the impact of commercial scale harvest on bacteria transfer, results indicate that harvest practices do aid in contamination transfer. Both the animal intrusion and aerial application contamination events resulted in pervasive contamination and shed light on the likelihood or improbability of these types of contamination being the cause of the 2018 Outbreak. Surprisingly, furrow irrigation contamination simulations were detectable through harvest and point to the ability of furrow contaminated water to transfer to harvestable romaine lettuce. Resulting best management practices include raised bed height, omission of harvest at the head of the field, as well as improved or increased frequency in knife cleaning practices. While current industry standards used for pre-harvest assessment, S and Z, seem to be sufficient for detecting pervasive contamination, research indicated that they could be improved upon. While pre-harvest assessment has been the “go to” to understand field level contamination, results indicating the impact of harvest practices on the spread of contamination warrants a closer look at post-harvest sampling as a mechanism to enhance our capacity to detect contaminated product prior to distribution into the marketplace. Finally, with respect to the ability of bacteria to survive on “freeze-damaged” romaine, or data indicates an increased propensity for bacteria to be recovered from damaged product. This result was seen at both high and low inoculum concentrations. Our data also indicates that variety type had minimal impact on our ability to recover bacteria. Overall, this data shows proof-of-concept that “freeze damaged” romaine may have the ability to harbor bacteria for extended periods of time and may have played a role in the 2018 Outbreak.