Epidemiological Investigation of Risk Factors for Microbial Contamination in Produce at the Preharvest Level
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In the United States, the proportion of outbreaks of microbial foodborne illnesses associated with fresh produce has increased over the past decades. A large proportion of these outbreaks have been caused by enteric pathogens, including Listeria monocytogenes, Salmonella, and Escherichia coli O157:H7. The overall objective of this dissertation was to study the risk factors for preharvest produce contamination with these three pathogens and generic Escherichia coli, as an indicator organism of fecal contamination, to improve control of foodborne illnesses associated with fresh produce. This objective was accomplished through three independent studies. The first study identified and characterized known risk factors for contamination of fruits and vegetables at the preharvest level with L. monocytogenes, Salmonella, and E. coli O157:H7 by conducting a systematic review. The review identified and evaluated 68 published research articles which indicated soil and irrigation water as the most important routes of produce contamination with pathogens. The review indicated the existence of solid evidence for several additional risk factors, including growing produce on clay-type soil, the use of contaminated or non-pH-stabilized manure fertilizer, and the use of spray irrigation with contaminated water, with a particular risk of contamination on the lower leaf surface. A total of 955 spinach samples were collected from 12 spinach farms in Colorado and Texas for the second and third study. The second study evaluated the effect of farm management and environmental factors on spinach contamination with generic E. coli at the preharvest level. The results indicated that spinach contamination was influenced by the time since last irrigation, the use of pond water for irrigation, workers’ personal hygiene, the use of the field prior to planting, and the proximity of a poultry farm. The third study evaluated the role of weather and landscape factors, in addition to the farm management and environmental factors, in occurrence of spinach contamination with generic E. coli at the preharvest level. The results indicated that spinach contamination was influenced not only by the amount of rain, but also by workers’ personal hygiene, the use of the spinach field prior to planting, and the use of manure fertilizer. In conclusion, the three studies have identified important risk factors for microbial contamination of produce at the preharvest. The control of several of these risk factors has already been the focus of the currently established Good Management Practices (GMP) in produce production. The novel findings suggest that the GMP may need to account for rainfall and improve workers’ personal hygiene in order to further reduce produce contamination with microorganisms.
Escherichia coli O157:H7
Park, Sangshin (2013). Epidemiological Investigation of Risk Factors for Microbial Contamination in Produce at the Preharvest Level. Doctoral dissertation, Texas A & M University. Available electronically from