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Saturday, December 3, 2022

Is your spinach safe to eat? Most birds do not pose food safety threats to produce, new study finds

The study found that wild birds living near livestock feedlots present the greatest risk of transmitting pathogens to crops

By MARGO ROSENBAUM — science@theaggie.org

From dinosaurs to sea otters, animals have always interested Daniel Karp, but in the third grade, Karp finally settled on birds as his primary passion. In the time he spent outdoors as a young avid birdwatcher, he watched the natural spaces and species he loved disappear, swallowed up by residential and commercial developments, ultimately leading him to the world of conservation biology.

As a young scientist, Karp learned he could have a career that combined his love of birds with his other interests: research, teaching and conservation. Now an assistant professor in the Department of Wildlife, Fish and Conservation Biology at UC Davis, he studies connections between wildlife and agriculture in his attempt to save the wildlife species he has loved since he was a kid.

Ever since the 2006 E. coli outbreak among bagged spinach in the California Central Coast, Karp has been particularly passionate about studying wildlife and their potential conflicts with food safety. The originating strain of that outbreak was found in the feces of wild pigs, increasing pressure for growers to keep wildlife away from their farms.

Like Karp, Elissa Olimpi has been absorbed in the world of wildlife ever since she was young, and grew up catching salamanders and watching insects — well before she ever knew she could do it as a career. In her previous role as a postdoctoral researcher at UC Davis, Olimpi studied birds alongside Karp and the threats they pose to food safety. And now, she is a postdoctoral researcher at Virginia Tech studying the impacts of birds on agriculture. 

Olivia Smith, a postdoctoral researcher at Michigan State University, had a similar conservation origin story as Olimpi and Karp. Today, Smith also studies ways to sustainably manage agricultural systems to understand “the good and the bad of birds,” as Smith puts it, and “how to tip the scale” to minimize harm to birds and maximize the benefits they provide. 

To contribute to the ever-growing body of work searching for ways that wildlife and farming can coexist, Olimpi and Karp partnered up with Smith, who was at the University of Georgia at the time, to understand the food safety risks specific bird species pose to crops along the West Coast. 

“If you don’t really know which birds are risky, it’s hard to provide advice and identify efficient management strategies for growers,” Olimpi said.

Their resulting study, published in the journal Ecological Applications, found that most birds pose little risk of pathogenic spread to crops, Smith said. 

The researchers collected 11,000 different tests of pathogenic E. coli, salmonella and Campylobacter. Among the top two pathogens — E. coli and Salmonella — less than 5% of birds in the study tested positive. Campylobacter presented a slightly greater concern: about 8% of birds tested positive. 

“The good news was for the Salmonella and the E. coli, we find they’re really, really rare in wild birds,” Karp said.

In addition to the tests for bacteria, the researchers conducted about 1,500 bird surveys across 350 fresh produce fields along the West Coast, and collected more than 1,200 fecal samples from fields. By modeling the prevalence of pathogens in feces and the likelihood of bird species to defecate on crops, the researchers constructed a “risk gradient” of bird species’ impact on these products, Smith said. 

Starlings, blackbirds and other birds that flock in large numbers and forage on the ground near cattle feedlots present the highest threats for transmission of foodborne pathogens to crops. Insect-eating birds, such as swallows and bluebirds, are less of a risk; they spend most of their time in the tree canopy and away from livestock.

The pathogens are spread to the produce through bird feces. The study focused on leafy greens that are eaten uncooked. The best way to keep yourself safe? Wash your vegetables, Karp said.

The only foodborne disease outbreak that has ever been conclusively attributed to birds was a Campylobacter outbreak in 2008. The outbreak in Alaska was spread from sandhill cranes to raw peas, according to Karp.

Because the study shows that many birds pose little risk of pathogenic spread to crops, the findings have significant management implications for both growers and conservationists, Smith said. 

Farms surrounded by natural habitats tend to have a greater diversity of birds, and will oftentimes promote beneficial insect-eating species to live in the area, Smith said. Farms surrounded by natural habitat were at a lower risk of being infected by pathogens from birds in comparison to those near feedlots.

“By providing other high quality habitat around farms and in agricultural landscapes, that’s one of the most powerful strategies for reducing food safety risks from this,” Olimpi said. 

As a result, bird management practices differ depending on the farm’s location. 

“If you’re a farmer and you’re right next to a feedlot, you probably want to try to keep the birds out,” Smith said. “But if you’re a farmer and you’re in a pretty forested area, the birds are probably on the safer side.”

Growers can encourage insect-eating birds on farms as a natural insecticide, providing free pest control services and necessitating fewer toxic pesticides. Relying on more sustainable farming practices can be beneficial to both the environment and growers, as these practices can result in higher returns for growers and lower levels of toxic substances.

“You can manage the farm to try to promote the insectivorous birds that can help farmers and then try to keep away these species that are highly associated with feedlots,” Smith said.

Nest boxes are one method of promoting certain bird species to farms. Specifically sized holes on the boxes allows for insect-eating birds to inhabit them, while keeping the unwanted species out, according to Olimpi. 

To keep riskier birds away from farms, growers can use a number of methods that range in cost and effectiveness, such as fencing or netting off produce, using sound cannons or setting up streamers and scarecrows. But, keeping birds away from farmland is almost impossible and comes at a high cost. The best way to prevent certain species from coming on farms is to stop removing natural habitat around farms, Olimpi said. 

While removing habitat has been a method of keeping wildlife away from farms in the past, growers often are not the ones that want to remove the habitat — it is the buyers, Karp said. Buyers want the food they purchase to be as safe as possible, for liability reasons.

“The growers are sort of caught in the middle,” Karp said. “They often will totally buy into the kinds of research that we’re doing, but at the same time, they’re sort of powerless to do anything else because of the pressure that they’re feeling.”

Karp said he sympathizes with the growers, who are in a “really tight spot” and “should never be painted as the bad guys” in these situations. Many growers are wonderful land stewards, he said.

“They’re just caught in this really impossible situation right now,” Karp said. 

Through further study, researchers like Smith, Olimpi and Karp hope to provide improved bird management strategies for farmers. While this field of study may seem most relevant to growers and conservationists, Olimpi said that everyone who eats produce grown on farms should have a vested interest in this work: “at least as a consumer, right?”

Additionally, people place a lot of cultural value on birds, such as the bald eagle and seahawk. Agricultural intensification is one reason why certain bird species are declining, Smith said. Without good management practices, future generations may not have the chance to see the same species that Smith, Olimpi and Karp loved — the wildlife that drew them to the field of conservation biology. 

“We want to figure out how to manage these farms to produce food that’s safe for people to eat, but healthy and preserving biodiversity,” Smith said. 

Written by: Margo Rosenbaum — science@theaggie.org