A recent report from the Food and Drug Administration has found that a sheep farm was likely a contributing factor in the contamination of onions.

The finding came when investigators searched for the cause of a 2020 outbreak of Salmonella Newport foodborne infections associated with red onions from the Southern San Joaquin Valley and Imperial Valley in California.

“The outbreak, which caused 1,127 reported domestic illnesses and 515 reported Canadian cases, is the largest Salmonella outbreak in over a decade,” according to the research report. “This outbreak is also remarkable because the food vehicle, whole red onions, is a raw agricultural commodity that had not been previously associated with a foodborne illness outbreak.”

The specific outbreak strain of Salmonella Newport was not found in most subsamples tested, 11 subsamples — 10 water and one sediment — collected near one of the growing fields identified in the traceback were positive for Salmonella Newport, representing a total of three different strains.

“Although a conclusive root cause could not be identified, several potential contributing factors to the 2020 red onion outbreak were identified, including a leading hypothesis that contaminated irrigation water used in a growing field in Holtville, California, may have led to contamination of the onions. While our investigation did not occur during any harvesting activities, visual observations of the implicated red onion growing fields suggested several plausible opportunities for contamination, including irrigation water and sheep grazing on adjacent land. . .” 

The investigation did not occur while packing activities were ongoing. However, visual observations and records review of packing house practices confirmed numerous opportunities for the spread of foodborne pathogens such as Salmonella, including signs of animal and pest intrusion as well as food contact surfaces that had not been inspected, maintained, cleaned, or sanitized as frequently as necessary to protect against the contamination of produce, according to the FDA’s report.

Thomson International Inc., the producer of the onions, cooperated with the FDA throughout the investigation and is continuing to engage with the FDA on the agency’s findings and recommendations.

Notably, Salmonella isolates from two sediment subsamples and two water subsamples collected during this investigation were found to be genetically related by WGS to clinical isolates from 2016 and 2018 foodborne illness outbreaks, Salmonella Muenchen and Salmonella Montevideo, respectively, associated with consumption of sprouts. 

This may indicate human pathogen persistence and distribution in this growing region — a concentrated area of seed for sprouting production — which could pose a risk of contamination for any produce commodity. FDA issued an assignment to follow up with the associated firms. Sprouts are not a food vehicle of interest in the 2020 Salmonella Newport foodborne illness outbreak.

“We urge growers to conduct risk assessments that include evaluation of hazards that may be associated with adjacent and nearby land uses — especially relating to the presence of livestock and wildlife and the potential for runoff into growing fields or water sources — and implement risk mitigation strategies wher appropriate,” the FDA researchers said.