2. Key risk factors affecting pathogen infection and spread

The risk factors for produce contamination are in the supply chain, both at the pre-harvest (in the field) and post-harvest stages. During the pre-harvest phase, pathogen populations can establish themselves on growing crops. The risk can be amplified after harvest either by further direct contamination, or by proliferation of existing pathogen populations during processing and post-harvest handling procedures.

Water is likely to be an important source of contamination in the field. Possible sources are run-off from nearby animal pastures and irrigation from a contaminated source. The risk associated with using water from a range of sources that vary in microbiological quality for irrigation of produce has been assessed and the need for improved guidelines has been recognized. While studies have found no internalization of E. coli O157:H7 in spinach plants through contaminated soil, uptake and internalization was found in spinach leaves after contaminated water was dropped on the leaves. This suggests a lower likelihood of transmitting pathogens from contaminated water through drip irrigation versus a higher likelihood through overhead sprinkler systems. However, irrigation is not the only reported route of contamination linked to water. Use of water in post-harvest processing has also played a role. An outbreak of infections with Salmonella serovar Newport was linked to consumption of mangoes treated with a process involving hot water aimed at preventing importation of fruit flies. Pathogens may be transferred to the environment by application of inadequately composted or raw animal manures or sewage. The faeces of wild animals may also be a source. In the US outbreak of E. coli O157 infections associated with pre-packaged spinach, trace-back and environmental investigations determined that one ranch in California’s Salinas Valley was the likely source of the outbreak. The patterns produced by pulsed-field gel electrophoresis (PFGE) and multilocus variable number tandem repeat analysis (MLVA) of the strains involved in the outbreak matched those from isolates recovered from local feral swine and cattle faeces. However, the manner in which the spinach became contaminated was not determined. Insects are also a possible source of contamination. In laboratory conditions, contaminated flies have been shown to directly transfer bacteria to plant leaves or fruits. Studies have implicated flies in contamination of leaves by E. coli O157:H7. Large numbers of flies belonging to the Muscidae and Calliphoridae families which were found in production fields adjacent to rangeland habitats occupied by cattle were shown to carry E. coli O157:H. Proposed sources of contamination of fruit used for juice have included the use of fallen fruit that has been in contact with contaminated soil, water, sewage or manure, use of contaminated water in washing or processing of fruit, and contamination at the point of consumption. Post harvesting processes, ranging from storage and rinsing to cutting, are also possible sources of contamination. Cut surfaces of leaves are a specific target for pathogenic bacteria such as Salmonella, which show a specific tropism towards them, and cutting melons may carry pathogens from the rind onto the edible part of the fruit where bacteria may multiply if the cut melon is not refrigerated. The use of inadequately decontaminated water in hydrocoolers, which are used to store and process large quantities of fresh produce, can lead to contamination of an entire lot.

Other multiple factors that have likely contributed to the emergence of produce as a source of enteric illness are:

• Changes in the produce industry:
• Intensification and centralization of production;
• Wider distribution of produce over longer distances;
• Introduction of minimally processed produce;
• Increased importation of fresh produce;
• Changes in consumer habits:
• Increased consumption of meals outside the home;
• Increased popularity of salad bars;
• Increased consumption of fresh fruits and vegetables, and fresh fruit juices;
• Increased size of at-risk population (elderly, immunocompromised people);
• Enhanced epidemiological surveillance;
• Improved methods for detection, identification and tracking of pathogens;
• Emerging pathogens with low infectious dose.