Much more than the fate of the multi-billion dollar fresh leafy greens industry is at stake in the wake of last fall's E. coli 0157:H7 outbreak. Increased daily consumption of fresh fruits and vegetables is a needed to improve the public's health.
With new food-safety practices in place and inspections beginning this month, this is a good time to consider possible causes and lessons learned and relearned.
The strain of E. coli O157 isolated in patients stricken during the September outbreak was found in multiple bags of Dole baby spinach. The fresh cut spinach that caused the outbreak had been grown and packed under contract by Natural Selection Foods, best known for its Earthbound organic produce.
Throughout the fall of 2006 and into 2007, the FDA and the California Department of Health Services carried out a mammoth investigation. Their final report included:
• None of the water, raw spinach, or finished product from the San Juan Bautista plant where the Dole spinach was processed tested positive for E. coli O157:H7.
• Several cases were traced to raw product processed during shift A on August 15, 2006. Records showed that spinach grown on four fields in Monterey and San Benito Counties were processed during that shift.
• E. coli O157 was found in samples collected near each of the four fields, but the exact strain was found only near the Paicines Ranch field.
• The field was leased to Mission Organics, and was transitioning from conventional to organic production.
The Response Team conclusion: "No definitive determination could be made regarding how E. coli O157:H7 pathogens contaminated spinach in this outbreak."
Possible Causes
Two, and possibly three failures triggered the 2006 outbreak. The first occurred in the field – the movement of E. coli O157:H7 on to a leafy green field in ample numbers, and late enough in the season to contaminate leafy greens at harvest.
A second failure occurred if the harvest crew missed signs indicating that wild pigs had been in the field.
The processing plant's washing procedures also failed to eliminate the E. coli O157 that entered the plant on contaminated raw spinach, and quality assurance failed to find the bacteria on finished product.
Cattle grazing on the nearby Paicines Ranch are the most likely source of the O157 bacteria involved in this outbreak. The spinach field on the Ranch is bordered to the north by a large cattle pasture that slopes upward from the spinach field.
Cattle manure is deposited on this pasture during winter-spring grazing, some of which was likely contaminated with E. coli O157. Spring runoff could have flushed manure and bacteria down onto nearby crops. E. coli O157 can persist for three or more months in the soil, thus spring runoff is a plausible cause.
Dust from the cattle pasture on the Ranch could have triggered the outbreak. Dust can carry E. coli O157 bacteria long distances. Summers in the Salinas Valley are dry and warm, and sometimes hot. A layer of dried manure was likely on the ground, in the corner of the field nearest the spinach field, where cattle may have gathered in the winter and spring would for water and salt. If farm activity in early August had disturbed areas where cattle had, or were congregating, dust would be stirred up and could have blown onto growing spinach, possibly carrying with it still-viable E. coli bacteria.
Sprinkler irrigation was likely applied daily, or every other day during summer, creating a perfect storm for O157 colonization – water to help adhere the dust to the growing leaves, coupled with moisture and heat to trigger bacterial proliferation.
Processing plant
Investigators concluded that the outbreak was likely caused by field contamination. There are many possible explanations of what happened in the plant:
• E. coli O157 levels were unusually high in some raw product and overwhelmed the system's ability to control all bacteria.
• Mechanical or human error caused a system malfunction.
• There was something unusual about the way the E. coli O157 was lodged on the leaf, because of the presence of a biofilm, or because the leaves were folded, or stuck together.
- Chuck Benbrook, Ph.D. is Chief Scientist, The Organic Center. His Critical Issues Report is at www.organic-center.org.