Investigators trace 2016-17 Salmonella outbreak to dairy cows

A months-long US Salmonella outbreak in 2016 and 2017 linked to ground beef was likely caused by contaminated dairy cows, emphasizing the need for a "One Health" approach with better tracking to prevent illness, a report today in Morbidity and Mortality Weekly Report (MMWR) has found.

The report marks the first time the Centers for Disease Control and Prevention (CDC) has notified the public about this outbreak, which appears to be over. The CDC said it did not have specific enough information to issue an alert when the outbreak was ongoing.

"During this outbreak, there was not a specific brand or type of ground beef that health officials could tell people not to eat," CDC public affairs specialist Brittany Behm, MPH, told CIDRAP News.

"The ill people reported various forms of ground beef, including fresh, frozen, chubs, and pre-formed hamburger patties. Due to the lack of specific advice, CDC did not issue a public warning while the outbreak was ongoing. We are publishing the investigation results now to document the outbreak and provide information for consumers, public health officials, and the beef industry."

Alarms first raised in January 2017

CDC investigators first noticed a cluster of Salmonella Newport infections sharing an almost identical pulsed-field gel electrophoresis (PFGE) pattern in January 2017. Federal officials eventually confirmed 106 cases in 21 states—including 1 death and 42 hospitalizations—in people who had bacterial cultures taken from Oct 1, 2016, through Jul 31, 2017. CDC, US Department of Agriculture's Food Safety and Inspection Service (USDA-FSIS), and other researchers performed whole-genome sequencing (WGS) as part of outbreak investigations.

Almost three fourths of cases (72%) were in southwestern states, with Arizona (30 cases), California (25), New Mexico (14), and Texas (7) hardest hit. Dates of first symptoms ranged from Oct 4, 2016, through Jul 19, 2017.

Initial interviews with patients homed in on ground beef consumption as a common exposure. Among the 52 patients who ate ground beef at home, 31 (60%) reported that they bought it or maybe bought it from multiple locations of two national grocery store chains, while 21 (40%) said they bought it from 1 of 15 other grocery chains—illustrating the difficulty officials had in pinning down the outbreak source.

Officials tested isolates from opened, leftover samples of ground beef from three patients' homes. One of the three yielded the outbreak strain while the other two tested negative for Salmonella.

Detections in cattle

Scientists also isolated the outbreak strain from four New Mexico dairy cattle. One was collected from a miscarried calf in July 2016, one was isolated from the feces of a young calf in November 2016, one was identified by searching the USDA-APHIS veterinary lab database, and one was from a USDA-FSIS routine cattle fecal sample collected at a Texas slaughterhouse in December 2016.

For the slaughterhouse isolate, USDA-FSIS investigators determined the sample was from a dairy cow and were able identify the New Mexico farm of origin. Because of confidentiality practices, however, officials could not identify the farms of origin for the other three positive sample. In addition, none of the 11 patients with trace-back information ate ground beef produced at the Texas slaughterhouse.

The CDC's Behm explained, "Investigators were not able to link dairy cows to slaughter establishments that were the source of ground beef consumed by ill people because the movement of cows is not systematically tracked. This means there are incomplete, or sometimes missing, records linking cows from farms, to slaughter facilities, or to the ground beef.

"Only 10% of ill people had receipts or shopper card information to trace back the ground beef to an establishment. Many ill people also reported purchasing large packages of ground beef, removing it from its original packaging at home, and then freezing it in smaller amounts. The investigation highlighted the need for records to trace where contaminated food comes from."

New Mexico officials visited the dairy farm that was the source of the cow from the Texas slaughterhouse and noted no health-related concerns. "However," the report authors note, "this visit occurred late in the investigation, and conditions at the time of the visit might not have represented those present immediately before and during the outbreak. No samples from the environment or cows were collected during this visit."

Complex investigation

Federal and state officials in today's report noted several facets that made for a challenging outbreak investigation:

  • The PFGE pattern of the outbreak strain is fairly common, making it difficult to distinguish outbreak from non-outbreak cases
  • WGS helped experts better discriminate among strains and helped exclude 39 non-outbreak illnesses, but WGS is not performed in real time for Salmonella, which slows the response
  • Trace-back investigations did not converge on a single contaminated lot of ground beef
  • Low numbers of patients with receipts or credit card information, as noted above
  • The lack of systematic tracking from farms to slaughter and processing plants

Most US ground beef is from beef cattle, according to the MMWR report, but 18% is from dairy cows, which are sold for beef if they age or have reduced milk production. The authors say that Salmonella-contaminated dairy cows could have gone to multiple slaughterhouses or processing plants, resulting in multiple brands and lots of ground beef being contaminated.

The authors conclude, "Foodborne outbreak investigations could be enhanced by improvements in the traceability of cows from their originating farms or sale barns, through slaughter and processing establishments, to ground beef sold to consumers."

See also:

Apr 20 MMWR report

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