Feedlots likely contribute to the contamination of fresh produce in nearby fields, according to a study initiated after a deadly E. Coli outbreak linked to romaine lettuce.

The 2018 outbreak sickened 210 people, with 96 requiring hospitalization. Five of the patients died. The outbreak, which was linked to romaine lettuce, was the impetus of the research project, which included the Food and Drug Administration, the University of Arizona, the Wellton-Mohawk Irrigation and Drainage District, local growers, industry groups, and others. The study in Yuma County, AZ, involved 12 percent of the Wellton-Mohawk Irrigation and Drainage District’s agricultural production area.

A preliminary report on the study, which ran from 2019 through the beginning of this year, included 100 sample collections from 7,000 acres. The researchers collected samples from 55 sites, resulting in more than 5,000 unique samples and 15,000 individual tests for the detection of generic E. coli, Salmonella, and Shiga toxin-producing E. coli (STEC), including E. coli O157:H7. 

The test results showed that dust from a large feedlot — 80,000 head of cattle — contaminates irrigation water and plants.

“. . . air, water, and lettuce leaf microbiome analysis demonstrated deposition of dust from cattle pens to the nearby water and land, suggesting that dust from CAFOs (concentrated animal feeding operations) may play a role in STEC transmission in this part of the region. These findings indicate that STEC can survive in the air and that dust can act as a transfer mechanism for both pathogens and indicator organisms (e.g., generic E. coli) from adjacent and nearby land to water, soil, and plant tissue,” according to the report.

“. . . The research team repeatedly observed that generic E. coli concentrations, STEC prevalence, and isolation frequency increased as irrigation canal water flowed past an adjacent livestock and compost operation. In addition, these changes in water quality occurred absent other explanations such as surface run-off or other direct contamination, which indicates that airborne disposition of dust from a nearby CAFO was potentially a factor in the contamination of the irrigation water”

The researchers reported that they did not observe similar findings from samples obtained concurrently from a nearby irrigation canal that flows south of the feedlot and compost operation.

In addition to water and plant samples, the researchers tested fecal matter from wildlife, including various mammals and more than 40 bird species. Produce growers have historically maintained that wildlife, especially birds, has contributed significantly to the contamination of crops. However, the scientists found that was not the case.

“. . . birds and other wildlife do not appear to be significant sources of STEC or E. coli O157:H7 in or around the part of the Southwest growing region evaluated,” according to the researchers.

The researchers used whole genome sequencing, often referred to as DNA fingerprinting, to test the samples they collected. They found more than 40 different STEC serotypes from about 500 different samples. The pathogens were found in water, sediment, and plant tissues, and strains were matched from air samples. That proved that bacteria in air from feedlots can transfer to other locations and surfaces.

In some areas of the country wher produce is grown in open fields, producers have voluntarily imposed buffer zones between their crops and feedlots. The distances range from 400 feet to 1,000 feet. Some observers have historically suggested that those distances are not enough to impact the problem of air-born pathogens from feedlots. The report from the recent study supports those observations but does not suggest a specific buffer distance.

“Distance played an important factor in the likelihood of STEC being detected in collected airborne dust, with the percentage of positive samples declining steadily as air sampling moved incrementally away from concentrated animal operations,” researchers reported.

As for recommendations, the researchers said better communication and cooperation among produce growers, feedlot owners, regulators agencies, and trade organizations is needed “to foster productive dialogue among diverse stakeholders to improve food safety.”