A report into an outbreak of Salmonella linked to minced meat from dairy cows in the United States was slowed because investigators struggled to trace its source.

One person died and 106 people were infected due to outbreak of Salmonella Newport which struck 21 states between October 1, 2016 and July 31, 2017. Consumption of ground beef was quickly identified as a common factor in the infection, but from there investigators ran into problems.

The report published last week by the Centers for Disease Control and Prevention (CDC)said that progress was hampered because “product traceback did not converge on a single contaminated lot of ground beef”.

More than half of the 52 patients who ate ground beef at home said that they had purchased it or may have purchased it from multiple locations of the same two national grocery chains.

only 11 of the 106 victims retained documentation that could help the CDC, such as receipts or shopper card records. Approximately 20 ground beef suppliers belonging to at least 10 corporations were identified; 10 of the 11 records traced back to five slaughter/processing establishments belonging to one company (A), seven of 11 traced back to five slaughter/processing establishments belonging to another (B), and four of 1 to two belonging to a third (C).

Investigators struggled to link slaughterhouses and processing plants that were identified during their work with the farms housing dairy cows they had found to be contaminated with the outbreak strain in Arizona, California, Texas, New Mexico, and Wisconsin.

According to the report, tracing back cows at slaughter/processing establishments to the farm from which they originated was problematic because cows were not systematically tracked from farm to slaughter/processing establishments. 

It concluded: “Foodbourne 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.”

According to the CDC, Salmonella causes around 1.2 million illnesses, 23,000 hospitalisations, and 450 deaths in the United States every year. Food is the source for about one million of these illnesses.