Although cattle manure has been pinpointed as a likely source of contaminated spinach that sickened nearly 200 people, scientists and food safety experts are continuing to probe the Salinas Valley's environment for clues as to how the lethal bacteria spread to the fields and tainted so much food.
On Thursday, federal and state investigators announced that they have matched E. coli O157:H7 in feces from an unidentified cattle ranch to the bacteria found in sick people as well as bagged spinach they ate. Spinach fields surround the sprawling ranch, which covers thousands of acres.
Food safety experts are not surprised by the genetic match to cattle, since they had long suspected they were the source. Yet many mysteries remain, and until they are solved, the safety of the region's leafy greens cannot be assured.
Because spinach has a short growing cycle, about 30 days, the tainted August crop probably was not the first of the season from those fields. So, scientists want to know what was unusual about that crop.
How did the pathogen move from cows to crops? Large numbers of wild pigs frequent the ranch and might have fed on manure and excreted it in fields or tracked it there.
Runoff or spring flooding could have transported feces from the ranch, but the suspect cattle pastures are downhill from spinach fields. How did it spread to so much spinach?
Other sources can't be ruled out, officials said, given the eight previous outbreaks that have been traced back to other Salinas Valley lettuce and spinach fields over the last decade.
Kevin Reilly, deputy director of the state Department of Health Services' prevention services division, said 13 people from his agency and the FDA were taking samples in four fields Friday, focusing on the cattle ranch and trying to find out "precisely how the contamination could have reached the actual spinach on the fields."
"We're talking about detective work.... We don't have that definitive evidence yet but we have a number of clues," he said.
Food experts have long known that leafy vegetables are susceptible to pathogens, hosting more fecal bacteria than other produce. In laboratories, pathogens live for weeks on leafy greens, thriving even as the vegetables wilt. And in soil, they survive for months.
Given the right set of circumstances -- particularly warm temperatures -- pathogens thrive. If the host, the bacterium and a favorable environment -- the "disease triangle" -- coexist, the conditions are ripe for an epidemic.
"Something was out of balance, if you will," said UC Davis food pathologist Trevor Suslow. "It could be egregious contamination but it is equally possible that it was some initial contamination and then the spinach was exposed to conditions that let it multiply. O157:H7 grows very, very well on spinach, unfortunately, and the warmer the temperature, the faster it grows."
Alejandro Castillo, a Texas A&M University associate professor of food microbiology, said the virulence of the epidemic has puzzled him. Far more people were hospitalized, many with kidney failure, than during any previous O157:H7 outbreak. Three people died.
"How in heaven can you get so much produce contaminated?" Castillo said. "We need to figure out the mechanism of transport to the spinach fields. If it's a short distance, wind can transport it, floods, rain, whatever. Even people walking from one side to another."
Whether it was animal hoofs, human feet, creeks or something else that brought it to the fields, Castillo and Suslow suspect that water -- irrigation water, floodwaters, runoff or underground flow -- spread it.
"One way or another I imagine that water could ultimately play a role -- if not causing the initial contamination, then spreading it," Suslow said.
Castillo said that "something, such as the irrigation system, magnified the effect," spreading it from leaf to leaf during growth, harvesting or processing.
In the Salinas Valley, sprinklers are used on spinach, which Castillo calls risky. "They are smaller plants, close to the soil; the water can transport it," he said. "Spray irrigation should be eliminated from the crops when the plants are already growing."
Suslow, who has long studied the region's crops, began tracing back the chronology to uncover what may have caused the pathogen to multiply. He noticed that in mid-July, as green rosettes sprouted in spinach fields, record-breaking temperatures smothered the normally cool coastal valley for days.
In Hollister, temperatures exceeded 100 degrees and broke records from July 22-25, according to National Weather Service data. A normal July day there peaks at 81 degrees. Gilroy was even worse, with 10 consecutive days reaching 99 degrees or more, peaking at 112 -- 23 degrees above its normal high.
Less than two months later, those crops were responsible for sickening people in 26 states.