A report from the Centers of Disease Control and Prevention (CDC) has traced an outbreak of Salmonella enterica serotype Newport to dairy cattle and indicates a need for better traceability system.
From October 2016–July 2017 an outbreak of Salmonella Newport was sourced to ground beef by the CDC. The protracted outbreak infected 106 people in 21 states, hospitalizing 42 and leading to one death. The majority of illnesses, approximately 72%, occurred in the southwest with the following states having high infection rates:
- Arizona, 30 cases
- California, 25 cases
- New Mexico, 14 cases
- Texas, 7 cases
Interviews at that time conducted by the CDC on 65 patients determined a common exposure was attributed to ground beef. Further interviews determined 80% of patients interviewed had eaten ground beef at home in the week prior to the illness. This was double the infection rate from home cooked ground beef in a survey of people who had foodborne illnesses in 2006-2007.
Of the 52 people surveyed who indicated they had eaten ground beef at home, 31 respondents (60%) from multiple locations of two national grocery chains. The remaining people bought beef from 15 different grocery chains.
When asked about ground beef preparation, 12 (36%) of 33 patients reported that they definitely or possibly undercooked it.
FIGURE 1. Infections with the outbreak strain of Salmonella Newport (n = 106), by state of residence — 21 states, October 2016–July 2017
USDA’s Food Safety and Inspection Service traced back the outbreak using shopper card records or receipts from 11 people. Leftover ground beef samples from three homes of ill patients were tested and only one yielded any Salmonella.
Eventually the investigation was traced to four dairy cattle in New Mexico. Isolates of the strain of Salmonella were detected in the following ways:
- Spontaneously aborted fetus (July 7, 2016)
- Feces from a young calf (Nov. 14, 2016)
- Cattle of unknown age collected because of infection (Nov. 23, 2016)
- Routine sampling at slaughter facility in Texas; cow traced to New Mexico (Dec. 19, 2016)
The farm or farms of origin have not been released due to confidentiality practices. None of the 11 patients who were interviewed by USDA FSIS had eaten ground beef from the Texas beef packer.
Lab investigations using whole genome high-quality single nucleotide polymorphism (SNP) analysis indicates that 106 infected people all had a Salmonella isolates that were genetically related to the four infected dairy cattle and the contaminated beef sample. No antibiotic resistance was detected either.
This outbreak was difficult for USDA-FSIS to conduct a traceback investigation because there was no common source of ground beef or processor identified. No recall was requested or public warning was issued for this reason.
State officials from New Mexico visited the dairy where the cull cow originated that tested positive at the Texas plant and they saw no reason for concern. The report notes that conditions at the farm could have changed as the visit was late in the investigation. No samples were taken from the farm at that time.
The report concludes that dairy cows were the likely source in this outbreak and notes that 18% of ground beef comes from dairy cows. It could be possible that cattle from other states might have been a cause for the outbreak as well.
Tracing back the outbreak was difficult for investigators because there was no identification system for cattle to determine farm of origin once they entered the packing system. Another difficulty was that only 10% of ill patients had receipts or shopper card information available.
The report advises a four point “farm to fork” plan to help prevent future outbreaks:
- Farms can implement good management practices for cattle health, including vaccination, biosecurity (e.g., controlling movement of persons and animals on farms, keeping a closed herd [so that no animals on the farm are purchased, loaned to other farms, or have contact with other animals], planning introduction of new animals and quarantining them, and performing microbiologic testing of animals), and cleaning and disinfection measures to decrease Salmonella burden in animals and the environments in which they reside, reducing the likelihood that Salmonella will enter beef slaughter/processing establishments.
- Slaughter/processing establishments are required to maintain Hazard Analysis and Critical Control Points systems to reduce Salmonella contamination as well as slaughter and sanitary dressing procedures to prevent carcass contamination.
- Although Salmonella is not considered an adulterant in not-ready-to eat (NRTE) meat products, USDA-FSIS likely will consider the product to be adulterated when NRTE meat products are associated with an outbreak.
- Consumers are advised to cook ground beef to 160°F (71°C) as measured by a food thermometer to destroy any bacteria that might be present. Consumers are also advised to wash hands, utensils, and surfaces often; separate and not cross-contaminate foods; and refrigerate foods promptly and properly.
The full report is available below: