Fresh produce, most notably lettuce, spinach, radishes and alfalfa sprouts, has become one of the major sources of E. coli O157:H7 outbreaks in the US, accounting for at least 38 reported E. coli outbreaks since 1982.
E. coli is not naturally found on fresh fruits or vegetables, but it is naturally found in the digestive tract of animals. On farms, animals may be raised close to the fields where fresh produce is grown. Because of this close proximity of animals to the produce, animal feces directly from farm animals has been found to be one source of E. coli contamination. Another source of contamination is the use of raw manure which contains the feces of animals. Water, including both irrigation and flood water, has been found to be a source of contamination of produce. Research suggests that dust from livestock or a livestock operation may be a source of E. coli contamination (1).
Contamination would not be a problem if E. coli was only viable for a few days in the field, but it can last at least one month and up to two months (2). So if contaminated manure is laid down less than 120 days before harvest, contaminated produced could be sold to the consumer. Since the time lettuce, spinach and sprouts need from planting to harvest is much less than 120 days (30-90 days), these crops are at risk of being contaminated with E. coli if raw manure is used as fertilizer. Low temperatures, clay soil, close proximity to roots and cattle feces in the soil all can increase the longevity of E. coli in the soli. E. coli O157:H7 has been isolated from carrots 168 days after contaminated manure was spread, so even 120 days may not be enough time for produce to be E. coli free if treated with manure (1).
Once produce is harvested it may be rinsed or washed before sale to consumers, but it would be unwise to assume that this washing is enough to remove all pathogens. Before consumption, consumers need to thoroughly wash all fruits and vegetables, being sure to wash every single part of the fruit or vegetable. Even a small amount of E. coli, only 100-200 individual E. coli O157:H7 bacterium, can cause illness.
It is especially difficult to remove E. coli bacterium from sprouts. Therefore, young children, the elderly and people with compromised immune systems are advised to avoid fresh sprouts.
Sources:
1. Cooley M, Carychao D, Crawford-Miksza L, Jay MT, Myers C, et al. 2007. Incidence and Tracking of Escherichia coli O157:H7 in a Major Produce Production Region in California. PLoS ONE 2 11: e1159.
2. Nicholson FA, Groves SJ, Chambers BJ. 2005. Pathogen survival during livestock manure storage and following land application. Bioresource Technol. 96:135–143.
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