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Feral in the Fields: Food Safety Risks from Wildlife

Last month’s announcement  by the Oregon Health Authority confirmed that deer droppings were the source of E. coli O157:H7 contamination in strawberry fields linked to 15 human illnesses, including one death.  These findings are not unprecedented because undercooked venison is a recognized vehicle of transmission for E. coli O157:H7.  Indeed, the first outbreak of deer meat-associated E. coli O157:H7 was described in 1995 among Oregon residents.

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Prior to the strawberry outbreak, free-roaming wild animals were investigated as a potential source of fresh produce contamination during several notable outbreaks.  In 1996, deer intrusion into apple orchards in California with subsequent fecal contamination of dropped apples was identified as a possible contributing factor in a multi-state E. coli O157:H7 outbreak associated with unpasteurized apple juice.

In 2006, a nationwide outbreak of E. coli O157:H7 was traced to baby spinach grown on a single ranch in California, where a large population of feral pigs shared pasture with a grass-fed beef cattle herd; feces from cattle and feral pigs tested positive for the spinach outbreak strain.  Two years later in Alaska, public health officials investigating a campylobacteriosis outbreak linked to raw peas found the outbreak strain in fecal material from a large population of sandhill cranes feeding in the pea fields.  

These outbreaks illustrate the potential for wildlife to carry foodborne pathogens and cause illness through ingestion of contaminated fresh produce.  However, how significant is this food safety risk?

Small Levels of Contamination May Represent Serious Risks


Domestic cattle and other ruminants such as goats and sheep are considered the primary reservoirs of E. coli O157:H7, but the pathogen has been isolated from wild and feral animals.  

In general, researchers in different parts of the country have found low background levels of E. coli O157:H7 in deer and other wildlife species, although there are still many regional data gaps.

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In comparison, Salmonella and Campylobacter carriage is more common, especially among wild bird populations including species that live commensally with humans (e.g., gulls, pigeons).

Due to the low infectious dose of E. coli O157:H7 and other foodborne pathogens, even a small level of contamination in the field may represent a serious human health risk, especially for vulnerable populations including the young, elderly, and persons with compromised immune systems.  

For example, an initial contamination event in the field (e.g., animal droppings), may later amplify into a more serious problem as a result of downstream failures during processing, transport, handling, and storage of the raw product.  A recently published modeling study predicted that even slight temperature abuse during handling and storage of spinach contaminated with a low level of E. coli O157:H7 in the field could have resulted in enough growth of the bacteria to explain the 2006 outbreak.

Balancing Food Safety and Environmental Stewardship Goals 

The produce industry and FDA have developed and implemented voluntary good agriculture practices (GAPs) to protect raw produce from contamination with foodborne pathogens.  Good agriculture practice programs may be commodity specific (e.g., leafy greens, tomatoes, melons, etc.), and are designed to address all of the known potential sources of contamination at the farm and harvesting levels including water, farm workers, wildlife, domestic animals, and soil amendments (compost).

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Following the 2006 spinach outbreak, the leafy greens industry in California and Arizona  created a Leafy Greens Marketing Agreement (LGMA).  Approximately 99% of leafy-green handlers are members of the LGMA, which mandates government audits to ensure that farmers follow accepted food-safety practices for lettuce, spinach and other leafy greens.  A series of detailed science-based metrics are used as guidelines for GAPs that must be followed by marketing agreement signatories. 

The wildlife component of GAP programs generally involves conducting pre-season and pre-harvest environmental risk assessments, and monitoring for animal intrusion during growth and harvest.  The crop is destroyed if it becomes contaminated with fecal material.  

Some food safety practices have led to potential conflicts with conservation and water quality programs.  For example, fencing to prevent deer or feral pig intrusions, and removal of water bodies near the crop fields to reduce wildlife attraction, are practices that have been cited as detrimental to environmental stewardship goals.  

To address this challenge, the concept of co-management has emerged from the food safety, agriculture, and conservation communities.  Co-management embraces the One Health Initiative by promoting mitigation of food safety risks while balancing environmental protection goals on America’s farms.

This summer, the California Leafy Greens Marketing Association (LGMA) announced approval of the addition of co-management to their accepted food safety practices.  Co-management is defined by the LGMA as “an approach to conserving soil, water, air, wildlife and other natural resources while simultaneously minimizing microbiological hazards associated with food production.”  

While protecting the public health must always remain the first priority in fresh produce production, the co-management approach represents a proactive and positive step forward in managing food safety risks from wildlife.  The implementation of co-management is also an example of successful collaboration between agriculture, conservation groups, industry, public health, researchers, wildlife agencies, and landowners.

What Consumers Can Do

Fresh fruits and vegetables are a nutritious and enjoyable part of a healthy diet.  However, produce is usually grown outside, thus food safety practices should be from the farm to the table.  

Consumers can take the following steps with their fresh produce to prevent foodborne illness:

Purchase produce from a reputable source with appropriate permits and inspections

At the supermarket, keep fruits and vegetables separate from meat, poultry, and fish to prevent cross-contamination

Store fruits and vegetables in the refrigerator in a produce drawer separate from meat, poultry, or fish juices or drippings

Thoroughly wash produce preparation areas in the kitchen with soapy water including the sink, cutting boards, and knives.

Wash your hands with hot, soapy water for at least 20 seconds before and after handling fresh fruits and vegetables

Always wash fresh fruits and vegetables under running water (do not use soap or chemicals) including those that come from a farmers’ market, roadside stand, or your home garden

For more information, visit UC Davis Food Safety:  http://ucfoodsafety.ucdavis.edu/UC_Publications/UC_Food_Safety_in_the_Kitchen_Publications/

Michele Jay-Russell, DVM, PhD is a researcher at the Western Institute for Food Safety and Security, University of California, Davis.  Prior to joining the university, she worked as an epidemiologist with the California Department of Public Health.


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Image of feral pig courtesy USDA Wildlife Services

© Food Safety News
  • Robyn Browne

    I thought the multi-state apple juice outbreak began in a Washington orchard, not in California.

  • mrothschild

    Robyn: The first illnesses in the Odwalla outbreak were detected and reported in Washington state but the suspect orchards were in California.

  • http://www.thecompletepatient.com dgumpert

    What I’d like to know is how did these pathogens contaminate wild animals like deer and birds. I’m going to guess that CAFO dairies and cattle operations have bred the pathogens, and then they spread through the CAFO ponds that are created. If so, why aren’t we doing more to reduce pathogens in CAFOs, aside from giving the cattle lots of antibiotics, which then leads to the spread of antibiotic resistant pathogens?

  • Mary Rothschild

    Robyn: The first illnesses in the Odwalla outbreak were detected and reported in Washington state but the suspect orchards were in California.

  • http://www.thecompletepatient.com David Gumpert

    What I’d like to know is how did these pathogens contaminate wild animals like deer and birds. I’m going to guess that CAFO dairies and cattle operations have bred the pathogens, and then they spread through the CAFO ponds that are created. If so, why aren’t we doing more to reduce pathogens in CAFOs, aside from giving the cattle lots of antibiotics, which then leads to the spread of antibiotic resistant pathogens?

  • Kim

    E coli naturally inhabit human and other warm blooded animals- mammals and birds… When the meat animals become excited/stressed before slaughter, more of the bacteria is shed in their manure so thats why it’s important to keep hides from contacting the surface of the carcass at hide removal.one needs to be careful using fresh manure in the garden that hasn’t been composted where the heat that accures during composting can kill the bacteria. Google E Coli O1:57H7 to learn more