Evaluation of Human and Cattle Viruses as Indicators of Fecal Contamination in Irrigation Water

Abstract

Irrigation creates a pathway through which enteric pathogens can reach the edible portion of food crops and potentially create a risk of illness for the consumer. To reduce the risk of illness, the U.S. Food and Drug Administration’s standards for irrigation water safety currently monitor Escherichia coli levels in water. However, the presence of bacterial indicators such as E. coli may not fully represent the pathogenic diversity of fecal contamination. This study assessed the occurrence of viruses in irrigation water and determined the risk posed by select viral pathogens in irrigation water applied to fresh produce. Pepper mild mottle virus, Aichivirus, bovine polyomavirus, and bovine adenovirus were evaluated as potential indicators of fecal contamination in irrigation waters from Arizona and California and their appropriateness relative to the traditional bacterial indicator E. coli was determined. Correlation analyses were performed between indicator organisms (i.e., total coliforms, E. coli, and the four viral indicator species) and select viral and bacterial pathogens (human adenoviruses, enteroviruses, Salmonella, and E. coli). Based on Pearson correlation analyses, no significant correlations were observed between any indicators or pathogens, thus highlighting both the current safety of regional irrigation waters and potentially the need to assess other indicators of fecal contamination. Quantitative microbial risk assessment was applied to the observed levels of the viral pathogens, adenoviruses and enteroviruses, to predict the risk associated with the consumption of raw fresh lettuce irrigated with these waters. In the analyzed scenarios of the consumption of raw lettuce, the mean predicted risk was below the acceptable limits of risk of infection resulting from exposure to a water source. Improved understanding of the appropriateness of fecal indicators in irrigation water and the risk posed by exposure to irrigation water via fresh produce consumption can help inform management and regulatory decisions, thereby improving the quality and safety of food crops irrigated with these waters.