Nation’s ‘salad bowl’ not overrun with E. coli

UC DAVIS (US)—A strain of E. coli known to cause illness in humans appears to be present but rare in some wildlife species of California’s Central Coast region—an area often referred to as the nation’s “salad bowl.”

Researchers from the University of California, Davis, who are nearing completion of a massive field study to help identify potential sources of E. coli O157:H7 near Central Coast farms, report finding occasional infections in fecal samples of wildlife species common to the area, including cowbirds, coyotes, crows, mice, and feral pigs.

Farmers should continue to follow the “good agricultural practices,” a set of accepted, on-farm procedures designed to protect crops from contamination during production and harvest, the researchers say. They presented their findings May 24 during the annual meeting of the American Society for Microbiology in San Diego.

The study was spurred by a 2006 nationwide E. coli O157:H7 outbreak linked to fresh, bagged spinach grown in California, that resulted in 205 reported illnesses and three deaths.

“The study helps us better understand the possible risk of crop contamination from wildlife and allows us to compare that to the risk of contamination from other possible sources such as livestock and irrigation water,” explains lead study author Michele Jay-Russell, a veterinarian at UC Davis’ Western Institute for Food Safety and Security.

“We are sharing this data with the produce industry, regulators, and conservation groups to help improve prevention strategies that protect public health and preserve native wildlife populations and their habitats,” she says.

E. coli O157:H7 poses a serious human health threat, commonly causing abdominal cramps and diarrhea, sometimes bloody. Severe infections may require hospitalization and result in kidney damage and even death. People most at risk for serious complications include young children, the elderly, and those with compromised immune systems.

From 2008 through 2009, the team collected and tested 1,133 fecal samples from wild birds and mammals on 38 private properties in Monterey, San Benito, and San Luis Obispo counties in California. All three counties are home to farms that grow fresh spinach, lettuce, and other produce.

Laboratory tests revealed that E. coli O157:H7 was present in samples from two cowbirds, two coyotes, five crows, one deer mouse, and 10 feral pigs. Samples from deer, opossums, raccoons, skunks, ground squirrels, and other bird and mouse species all tested negative for the bacterium.

Robert Mandrell, principle investigator and research leader from the U.S. Department of Agriculture’s Agricultural Research Service, says the discovery of a low level of E. coli O157: H7 among Central Coast wildlife was somewhat surprising.

“The fact that we have identified two bird species with an incidence of E. coli O157:H7 of more than 3 percent, feral swine with about a 4 percent incidence and several coyotes and rodents that tested positive for O157:H7 suggests there are at least several sources of pathogen movement in this region,” Mandrell says.

“We have no evidence that the concentration of the pathogen was high in the feces of the animals that tested positive, so the significance of wildlife as a source of direct contamination associated with outbreaks remains unclear.”

Mandrell says the researchers are comparing the genetic makeup of the E. coli O157: H7 strains found in wildlife to that of strains isolated from other sources including cattle, soil and water.

They hope these comparisons will help them to better assess the movement of the bacteria in this agriculturally important region.

Following up on these findings, the study team is evaluating other potential wildlife sources of E. coli O157:H7, including amphibians and reptiles, and is conducting focused research to refine best practices that promote appropriate management to protect both food safety and the environment.

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