Samples of cattle manure on pastures surrounding a spinach field have tested positive for the same strain of E. coli bacteria that killed at least three people and sickened nearly 200 others -- the first direct evidence linking a Salinas Valley farm to the outbreak that has spanned 26 states and one Canadian province. The pasture is part of an unidentified beef cattle ranch that also leases its fields to spinach growers. Fences on the cattle operation had been penetrated by wild pigs, and disease detectives are trying to determine if feral swine might have played a role in spreading the bacteria from pasture to spinach field. Tests at the California Department of Health Services laboratory in Richmond found that E. coli O157:H7 bacteria detected in three cow pies produced the same genetic fingerprints as the strain found in human victims and in the bags of spinach they had purchased. "This is a significant finding,'' said Kevin Reilly, deputy director of prevention services for the California Department of Health Services, during a telephone press conference Thursday afternoon. healthren.com Out of nine outbreaks of E. coli food poisoning traced to spinach or lettuce from the Salinas Valley since 1995, this is the first in which investigators have been able to link the bacteria strain that caused the illness to a farm where greens were grown. Reilly stressed that the test results do not prove that the manure was responsible for the outbreak and that investigators are continuing to look at other potential sources. But the state has reduced from nine to four the number of farms under scrutiny in the investigation of the spinach outbreak. Fields located in Santa Clara County are no longer under suspicion, but samples taken from a field in each of the four unidentified farms in San Benito and Monterey counties are being examined. State and federal officials have steadfastly refused to identify which farms they are investigating. Investigators say fresh produce is no longer being grown on any of the four remaining fields that grew spinach linked to the outbreak. healthren.com

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The E. coli illnesses first came to light on Sept. 14, when the FDA pieced together reports of 49 illnesses in eight states -- all caused by the same genetic strain of O157:H7. All victims had eaten spinach traced to Natural Selection Foods, which had bagged the fresh produce at its San Juan Bautista plant. Using plant records, the investigators then focused on nine farms that had supplied spinach to a batch processed by Natural Selection for the Dole Baby Spinach label. Since then, they have taken more than 650 specimens from soil, water and manure on the nine farms.

Health officials exploring other possible routes of transmission have not ruled out contamination of irrigation water, improper farmworker sanitation, or bacteria spread through tainted fertilizer or dirty farm implements.

Reilly said that the farm where the suspect manure was found did not fully adhere to the voluntary guidelines used by growers to keep fresh leafy greens safe from contamination. For example, although no evidence indicates that the beef cattle had strayed onto the spinach fields, Reilly said there is concern about the proximity of cattle to spinach fields and that fences on the farm had failed to keep wildlife from trudging over pasture and fields.

"On this ranch there is a very large population of wild boar,'' said Reilly. There was evidence that the pigs had torn through fencing or burrowed under it, he said. "We don't know if that is the source (of contamination of the fields), but it is a potential source.''

The E.-coli-positive cow pies were taken some distance away, between half a mile and a mile from the field that produced spinach suspected of sickening consumers.

Reilly said it was not uncommon for ranches in San Benito and Monterey counties to have spinach fields adjacent to cattle operations. In this case, "the field is, frankly, surrounded by pasture,'' Reilly said.

Federal regulators are concerned about the practice of raising cattle near fields that grow salad greens. "The relationship of farm animals to produce is certainly something to take under consideration,'' said Dr. Robert Brackett, branch director of the Center for Food Safety and Applied Nutrition at the Food and Drug Administration. As the industry scrambles to develop stricter guidelines -- and as government agencies face political pressure to make the rules mandatory -- standards such as minimum distance, upslope and down slope between pasture and fields are likely to be established.

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The American bout of E. coli illnesses prompted Mexico on Monday to ban imports of lettuce from the United States, setting off an urgent round of discussions between the two governments. The ban was imposed in response to a decision by the Nunes Co. of Salinas to recall more than 8,000 cartons of leafy green lettuce Sunday after detecting E. coli bacteria in water used to irrigate it. Subsequent tests by the company have shown that the bacteria detected were not from the deadly O157:H7 strain. No illnesses have been linked to the lettuce.

The Monterey County Farm Bureau on Thursday condemned the Mexican ban. "There is no evidence that lettuce shipped from California to Mexico is unsafe,'' the bureau said in a prepared statement.

"We are working with the Mexican government to address this,'' said FDA food safety branch director Brackett.

Despite narrowing the search for the culprit in the spinach outbreak, Brackett warned that the FDA remains concerned about the potential for another crop to be contaminated. "No farm should feel they are 'off the hook,' " he said. "It is absolutely essential that farms throughout the country are doing everything they can to make sure this does not happen again.''