Occurrence of Salmonella in Canals Delivering Irrigation Waters used for Produce Production

Abstract

With the increase in produce consumption in recent years, the risk of foodborne illness increases as well. It is estimated that 1,940 reported cases of foodborne illnesses were associated with the consumption of produce in the United States in 2015. Salmonella enterica is one of the top bacterial foodborne pathogens of concern and causes gastroenteritis estimated to have caused about 1,300 reported produce-borne outbreaks in the US/year. One of the ways Salmonella can travel to produce fields and contaminate produce is via irrigation water. The objective of this study was to detect the presence of Salmonella and indicator organisms of fecal contamination including Escherichia coli, coliforms, and coliphages in constructed canal systems which deliver irrigation waters for produce production. A total of 355 irrigation water samples were collected from various irrigation canals in Southern Arizona from January 2017 to August 2017. All samples were tested for general water quality or physiochemical parameter and analyzed using enrichment media, selective media, and confirmational methods for the presence of Salmonella enterica. This analysis yielded 11.6% (41/355) positive isolates with an average concentration of 4.10 MPN/100 ml. E. coli and coliforms were detected via IDEXX Colilert ®kit and IDEXX Quanti-Tray/2000® resulting in 97% (344/355) and 100% (355/355) positive samples, respectively. F+ and somatic coliphages were detected using the FastPhage™ MPN Quanti-tray method and resulted in 35.4% (62/175) and 60.5% (127/210) positive samples, respectively. The results of this study indicate that Salmonella and fecal indicator organisms were found at detectable levels in irrigation waters used in crop production. This implies the potential to cause foodborne illnesses, if the crops are contaminated with irrigation water that contains pathogens. A quantitative microbial risk assessment analysis was performed, using a Monte Carlo simulation and the exponential dose response model, to determine the spread of Salmonella to lettuce from irrigation water. The probability of causing infection annually was estimated as 2.1x10-7 and 5.1 x 10-5 for concentrations of 4.1MPN/100 ml and 1000MPN/100 ml at day 0 following the irrigation event, respectively. This is lower than the EPA standard of 1 in 10,000 per year, making this concentration of Salmonella in irrigation canal waters relatively safe for produce consumers. However, additional sampling and data would help further determine potential risks of irrigation water and contamination potentials.