A major study has found the presence of E. coli 0157:H7, the strain that was linked to four deaths and a massive spinach recall in 2006, is uncommon in California’s largest vegetable growing region.

“Even though there’s some risk, these outbreaks are very rare when you look at the number of servings of salad products,” said Michele Jay-Russell, a veterinarian at the Western Institute for Food Safety and Security at the University of California-Davis and the lead author of the study.

Over a two-year period, a research team collected and tested more than 1,130 fecal samples from wild birds and mammals at nearly 40 farms in Monterey, San Benito and San Luis Obispo counties, a region often called California’s salad bowl.

The bacterium was found in 20 samples from cowbirds (2), crows (5), coyotes (2), feral pigs (10) and one deer mouse. Samples from deer, opossums, raccoons, skunks, squirrels and other bird and mouse species tested negative, according to the study.

A total of 200 samples came from feral pigs, with 5% testing positive for the E. coli strain. Feral pigs were commonly found in the area where the spinach in the 2006 outbreak was grown, although a 6-month investigation into the cause of the outbreak didn’t establish a definitive link.

An earlier Centers for Disease Control and Prevention study found about 15% of feral pigs tested from the area were positive for that E. coli strain, as were 34% of cattle. One of the four farms implicated in the outbreak was about a mile from a cattle lot.

The recent study’s findings suggest grower-shippers should attempt to keep fecal matter out of raw produce fields, but do not indicate extreme measures are necessary, Jay-Russell said.

“To try to eradicate or remove large populations of wildlife would really make no sense,” she said. “They’re going to occur in natural habitats along the Central Coast.”

The research team, however, recommends grower-shippers act quickly if wildlife is present in large numbers, Jay-Russell said.

“A single animal presents minimal risk, but as larger populations have access to the field, that’s more of a concern,” she said.

The researchers recommend that growers in the region continue to follow good agricultural practices to protect crops from contamination during production and harvest.

There is no consensus among the scientific community, Jay-Russell said, whether chemical washes at the packing stage will prevent the bacterium from reaching the homes of consumers.

“There is controversy on whether E. coli can adhere to leaves despite the wash,” she said.

Consumers can play an important role in minimizing the presence of E. coli by maintaining the cold chain, Jay-Russell said.

“There have been studies that found E. coli 0157:H7, when allowed to get above refrigeration temperatures, will start to grow again,” she said. “Once those numbers go up, the risk increases.”

Consumers also can exacerbate the problem when trying to prevent possible pathogen contamination, she said.

“If a product is triple-washed, we don’t recommend another wash in the home, because it opens the possibility of cross contamination,” Jay-Russell said.

The study, funded by the U.S. Department of Agriculture and the Food and Drug Administration, was conducted by microbiologists and epidemiologists from the institute, the USDA’s Agricultural Research Service and Western Regional Research Center and Wildlife Services, the Western Center for Food Safety at UC-Davis and the University of California Cooperative Extension.

The study is nearly complete, but further research is planned, Jay-Russell said.

“We’ve had great cooperation from the growers,” she said. “They’ve been very important in trying to piece together what the risk factors are.”

The findings were presented May 24 at the annual meeting of the American Society for Microbiology in San Diego.