In Fall 2006, four separate outbreaks of foodborne illness associated with the consumption of fresh produce occurred in the United States. In follow-up investigations, spinach, lettuce, and tomatoes were identified as the vehicles of illness. Epidemiologic investigations subsequently focused on finding the specific growing regions using traceback records. While the areas most likely involved in the outbreaks have been identified, the specific mode of contamination remains unconfirmed. Suspected risk factors in these cases include: proximity of irrigation wells and surface waterways exposed to faeces from cattle and wildlife; exposure in fields to wild animals and their waste materials; and improperly composted animal manure used as fertilizer. Difficulty in deciphering these and other on-farm routes of contamination is due to the sporadic nature of these events. Hence, evidence to support these contamination modes is based largely on experimental studies in the laboratory and field. Still at issue is the relevance of internalization of pathogens, whether this occurs through the roots and plant vascular tissues of vegetables and fruits or through plant surfaces into cracks and crevices. Potential for these events, conditions under which the events occur, and pathogen survival following these events, are questions that still need to be answered. Answers to these questions will ultimately affect the type of interventions needed for application postharvest. Currently, many chemical and biological interventions can reduce surface pathogens and minimize cross-contamination, however, they are largely ineffective on internalized pathogens. In the event internalization is a significant route of contamination in the field, physical interventions (irradiation and high pressure) may be needed to minimize risk. Ultimately, risk assessment studies will be useful tools in developing risk management strategies for the produce industry.
