Vegetable growers are always on the defense against plant pathogens and diseases. But what happens when the crop itself is harboring a pathogen that causes human illness? Defending against human foodborne illness requires knowledge of the risks and a plan to prevent contamination in the field.

Outbreaks of Shiga toxin-producing E. coli (STEC) infections were at one time primarily associated with undercooked ground beef, leading to severe, debilitating illnesses and death. Unfortunately, these STEC infections, along with infections from Salmonella and Listeria species, have recently been associated with field-grown vegetable crops, most notably leafy greens from the Salinas Valley in California and the Yuma Valley in Arizona.

Manure runoff from nearby concentrated animal feeding operations (CAFOs) or irrigation water contamination are common suspects in moving human pathogens into nearby vegetable fields, causing outbreaks of foodborne illnesses. While these pathways of infection are a real threat, direct infection of crops by wildlife has been considered as a potential threat.

Do growers need to be concerned about wildlife presence on the farm introducing E. coli or other pathogens and contaminating their produce? What wildlife carry these pathogens? Does the surrounding environment play a role? What control measures can be taken to prevent issues?

Wildlife & Disease

The Center for Produce Safety recently hosted a presentation, “The Risk of Wildlife to Produce Safety.” Michele Jay-Russell, a veterinarian and research biologist at the Western Center for Food Safety, spoke of the research conducted to quantify the threat from wildlife. Alan Franklin of the USDA National Wildlife Research Center addressed the question of risk assessment and abatement on the farm.

CAFOs do pose a risk to nearby crop fields – but can wildlife transmit pathogens from CAFOs to nearby produce? According to Jay-Russell, rodents were found to stay within a 100-foot radius of the CAFO, while birds fly over fields regularly and could pose a threat via droppings, particularly if large flocks gather.

Amphibians and reptiles pose little risk of STEC infection; wading birds are also of little concern. Although wild dogs and coyotes were thought to be a problem in some fields where outbreaks had occurred, a study determined that they had no incidence of STEC pathogens, although salmonella was prevalent in the canine population.

Clinically relevant STEC infections in birds are rare, as are salmonella infections, Jay-Russell said. But situations where birds aggregate in large flocks are potential pathways for foodborne pathogens due to the increased amount of fecal material generated in a given area.

Finding means of preventing or eliminating these flocks near crop production fields can reduce pathogen risk. Scouting for visual signs of fecal contamination, done by trained crew members directly ahead of harvesting equipment so contaminated areas of fields are left unharvested, is an important step in preventing human illness.

The transmission of foodborne human pathogens by wildlife is a “low-probability/high-consequence” event, Jay-Russell explained. When such contamination does occur, “other things are happening to actually result in international and multi-state outbreaks” of illness.

Amplification and resulting cross-contamination occur due to there being no “kill step” for leafy greens or other produce eaten in raw form. The pathogens can attach to edible parts of the plant, and the infectious dose of these illness-causing pathogens is low.

Assessing Wildlife Risk

For STEC infections, domestic rock pigeons are the primary animal of concern. For salmonella, blackbirds and gulls are the most likely culprits.

Host animals can either serve as reservoirs for a pathogen, meaning they carry infection from a pathogen over their lifetime and can transmit it, or they are bridges, meaning they simply carry the pathogen from one location to another and are only infectious for a short period of time.

“Once you have maintenance hosts in your agricultural system, then the source doesn’t matter anymore,” Franklin said. “They have become the source.”

The abundance of the host, as well as the visitation rate to the crop area, are important in disease spread. The risk of pathogens being present increases as host numbers or host time in the field increases. The surrounding landscape is an important factor in this equation. The number of CAFOs in the surrounding area, the type of land use surrounding the farm and other qualities such as elevation (with higher elevations possibly being at reduced risk due to greater solar radiation), the presence or absence of riparian areas and the number of humans nearby can all play a role in the risk of wildlife transmission of human pathogens.

Perhaps counter-intuitively, it’s not the rural agricultural lands surrounded with wildlife habitat that have been found to be the most at risk for pathogens. Instead, it is ag land in mixed-use areas, where wildlife habitat is scarce and animals are in closer contact with livestock, humans and built infrastructure.

Much of the wildlife habitat in the U.S. is on privately owned land. Farmland plays an important role in maintaining habitat, and conservation programs in the Farm Bill assist farmers in establishing these important ecosystems. Wildlife risk “can be co-managed with conservation goals,” Jay-Russell emphasized. “Focus on a real problem and not trying to tackle every animal out there. It’s going to be different in every location, even every season, and year-to-year.”

Wildlife in general is not a threat to crop safety. Instead, certain types of wildlife are most likely to case issues of concern, and can be targeted. Identifying target species for a given pathogen is important.

Control of Wildlife

Lethal removal of wildlife is not recommended in most situations, especially with native species of animals. If the farm is near wildlife population sources, the animals will continue to move onto the land unless there are tools in place to prevent further intrusion.

Removing wildlife habitat can actually increase pathogen presence. When non-crop vegetation is removed, wildlife intrusion into crops has been seen to increase. Studies have shown a greater abundance of birds in crop fields if natural habitat is scarce than when the birds have areas other than crop fields suitable for their needs.

Preventing wildlife from entering crop fields can be done with fences, falconry or other barrier methods. Sonic nets, which rely on sound waves to disrupt birds’ normal vocalization patterns, are barrier methods shown to be effective in reducing bird intrusion. Reductions of 50% – 80% in bird visitations have been seen with this technology.

Guard dogs have been shown to decrease wildlife intrusion into crops. Breed differences were found to exist, with those needing more human guidance and attention being less effective than breeds which can work independently.

Adaptive management, where a problem is identified, the actual risk is assessed and various methods to reduce the risk are strategized, implemented, monitored and assessed, is the best tool for managing wildlife concerns, Franklin advised. A systems approach on a regional scale, with multiple tools and quantitative measures, may be the best option for obtaining results on a meaningful scale.

“The role of wildlife as bridge and maintenance hosts is still really poorly understood,” Franklin said. “Adaptive management … is flexible for competing objectives, such as maintaining wildlife habitat and reducing wildlife intrusion to crop fields.”