Most of the food borne illness pathogens associated with fresh produce can be spread through faecal contamination. Such contamination events can be direct (e.g. directly applied to the crop) or indirect (e.g. applied to the crop via contaminated equipment). The technical term for a carrier of indirect contamination is a fomite. Fomites are inanimate objects that have become contaminated with micro-organisms. Fomites further spread contamination to people and objects which come into contact with them. Fomite is a very broad term and in an agricultural environment; it covers objects as diverse as ropes, tractors, worker's boots, knives, transport crates and field gates.
The risks associated with manure contamination of tractors and other equipment used for harvest are difficult to quantify, as not all manure contains food borne illness causing pathogens. There is almost no information in the published literature on the role of manure-contaminated fomites and the probability of pathogen transfer to food crops. However, common sense and a basic knowledge of microbiology mean that most growers acknowledge it's not a safe practice to drive a tractor through a field containing livestock (and hence fresh manure) immediately before harvesting a ready-to-eat food crop. Furthermore, most growers would acknowledge a requirement to clean a tractor unavoidably required to drive though such a hazard immediately before a harvest. The best approach is to prevent contamination of equipment in the first place and care should be taken in the storage of harvest equipment away from manures or livestock and wildlife.
Brackett (1999) believes that many growers assume that since raw produce is grown in the agricultural environment, it is already contaminated and therefore cleanliness of harvesting equipment is not of primary importance. However, Brackett also reports that an increased awareness of potential transfer of food borne pathogens from farm equipment to food crops has prompted some of the more progressive producers to begin formally cleaning and sanitizing their harvesting equipment and tools before use (Brackett, 1999).
Some fomites of relevance to fresh produce growers have been reported by Kaneko et al. (1999). Post harvest, fresh-cut produce were shown to be contaminated by knives and factory-based processing equipment acting as fomites (Kaneko et al. 1999). It is known that pathogens can persist on equipment surfaces for considerable times. For example Salmonella enterica Typhimurium can be recovered from clean stainless steel for a number of hours after contamination (Moore et al., 2007). The importance of equipment surfaces as points of contamination is underscored by 41 cases of Salmonella enterica Bovismorbificans infection that were traced to a cutting wheel on a lettuce shredder (Stafford et al. 2002). These observations demonstrate that contaminated equipment may continue to contaminate fresh produce for a few hours. It is important that equipment that comes into contact with the edible portion of the crop (e.g. harvesting equipment and cutting tools) is cleaned adequately and sanitised regularly. Cleaning and sanitising of tools and equipment surfaces is particularly important where crops are to be eaten uncooked, such as leafy salads or herbs. You can get advice on equipment cleaning and sanitising schedules, and suitable food-grade cleaning agents from many cleaning material suppliers. It is best practice to validate your cleaning and sanitising of equipment to ensure that acceptable levels of cleanliness and microbial kill are being achieved. Additional information on the validation of sanitation is available here.
Brackett,R.E. (1999) Incidence, contributing factors, and control of bacterial pathogens in produce. Postharvest Biology and Technology 15, 305-311.
Kaneko, K.-I., Hayashidani, H., Takahashi, K., Shiraki, Y., Limawongpranee, S. and Ogawa, M. (1999) Bacterial contamination in the environment of food factories processing ready-to-eat vegetables. J Food Prot 62, 800–804.
Moore, G., Blair, I.S. and McDowell, D.A. (2007) Recovery and transfer of Salmonella Typhimurium from four different domestic food contact surfaces. Journal of Food Protection. 70, 2273–2280
Stafford, R.J., McCall, B.J., Neill, A.S., Leon, D.S., Dorricott, G.J., Towner, C.D. and Micalizz, G.R. (2002) A statewide outbreak of Salmonella Bovismorbificans phage type 32 infection in Queensland. Comm Dis Intell 26.