Ranchers were taking steps to keep livestock droppings from infecting produce long before the recent outbreak, Byrne said. These include designating watering holes for cattle so they don't drink from creeks that flow through farms and creating "buffer zones" that keep the animals away from water that could later be used to irrigate crops.
"As producers of food and as consumers ourselves, the beef industry is very concerned about the safety of the food supply," Byrne said. "Our responsibility doesn't necessarily end at the fence line."
E. coli typically comes from human and animal waste. The evidence so far indicates the spinach farmer, who also has not been identified, didn't adhere to voluntary guidelines for vegetables meant to be consumed raw, said Kevin Reilly, deputy director of prevention services for the California Department of Health Services.
While it's not uncommon for cattle to graze near fields of vegetables in the intensely cultivated valley, this field was "frankly, surrounded by pasture," Reilly said.
One theory is that wild animals may have carried the bacteria by tracking it from the pasture to the spinach field or by eating it and then excreting it themselves. Investigators found evidence that wild boars had gotten through fencing meant to protect the spinach.
But Doug Powell, scientific director of the Food Safety Network at Kansas State University, says the size of the outbreak points to flood waters rather than animals being the carrier. If that's the case, then it could have been prevented by proper water quality testing, he said.
Either way, keeping livestock away from spinach and lettuce seems the best way to prevent future outbreaks, said Joseph Pezzini, vice president of operations for Ocean Mist Farms in nearby Castroville.
"That interface is something that needs to be looked at more closely," he said. "How compatible are they, and for them to be compatible, how do you operate safely from both ends?"
But many experts cautioned against focusing exclusively on the coexistence of cattle and vegetables.
"It would be a mistake to think that all outbreaks were a result of this kind of proximity," said Trevor Suslow, a research in the plant sciences department at the University of California, Davis. "It's an important finding, but that isn't the end of the story. There are other sources for the pathogen to arrive at the product."