No one has become ill from eating contaminated spinach since Sept. 25. "All evidence points to this outbreak having concluded," said Dr. Kevin Reilly of the California Department of Health Services.
State and federal investigators have narrowed their focus to the ranch, where boar trampled fences that had hemmed in a spinach field.
Samples taken from a wild pig, as well as from stream water and cattle on the ranch, have tested positive for the same strain of E. coli implicated in the outbreak, Reilly said.
Still, investigators continue to look at three other ranches in the areas in seeking the source of the contaminated fresh spinach.
"We are not saying this is the source at this point," Reilly said of the ranch.
The outbreak sickened 204 people in 26 states and one Canadian province, he said.
Wild pigs are one "real clear vehicle" that could explain how E. coli spread from cattle on the ranch to the spinach field less than a mile away, Reilly said. The pigs could have tracked the bacteria into the field or spread it through their droppings, he said.
Investigators also are looking at runoff, flooding, irrigation water, fertilizer and other wildlife, including deer, as possible sources.
Investigators first recovered the same strain of bacteria earlier this month from three cattle manure specimens collected on the ranch. On Thursday, Reilly said the strain had been isolated from six other samples collected on the ranch, including from cattle.
The finds mark the first time that investigators have identified a possible source for any of the multiple E. coli outbreaks linked to the heavily agricultural area.
Reilly refused to give a location for the ranch, other than to say it's in a valley in the area of San Benito and Monterey counties.
Investigators have taken roughly 750 samples from the four ranches. They've found generic E. coli on all four ranches — the bug is commonly found in cattle — but turned up the particular strain involved in the outbreak on only one.
"We have no evidence to suggest people should not be eating spinach from other places, except from these four ranches," said Jack Guzewich, of the Food and Drug Administration's Center for Food Safety and Applied Nutrition.