An outbreak of E. coli O157 in England that affected more than 20 people was caused by a milk pasteurization failure, according to researchers.

In November 2019, a number of Shiga toxin-producing E. coli (STEC) O157:H7 infections were detected in South Yorkshire.

A total of 21 confirmed cases were linked to the outbreak, of which 11 were female, and 12 were either younger than the age of 15 or more than 65. Symptom onset dates ranged from Nov. 1 to 28, 2019. There were another five possible cases, said the study published in the journal Epidemiology and Infection.

Twelve patients were treated in hospital, and three developed hemolytic uremic syndrome (HUS). HUS is a severe complication associated with E. coli infection that causes kidney failure.

Investigations established consumption of milk from a local dairy as a common exposure. The majority of cases reported doorstep delivery of milk from the same farm. A sample of pasteurized milk from Darwin’s Dairy failed the phosphatase test, indicating contamination by unpasteurized, raw, milk. An inspection of the pasteurizer revealed a damaged seal on the flow divert valve.

Milk pasteurization fault
Results of the phosphatase test showed the milk had failed the pasteurization process.

“It is concerning that without the phosphatase test result it was unlikely that the epidemiological evidence alone would have been deemed sufficient to enable the incident management team to provide a case for suspending production,” said researchers.

The dairy was a small family run farm producer of pasteurized cow’s milk and cream products. Milk was supplied via doorstep delivery rounds and other local food retailers and caterers.

A visit from environmental health officers on November 21 revealed there was a lack of processing records available to inspect. In 2021, Darwin’s Dairy pleaded guilty to food safety and hygiene offences.

The pasteurizer at the farm was tested by an independent engineer in March 2019, with no operating faults found. The next check was not scheduled until March 2020 but it was arranged for an engineer to check the equipment.

This found a damaged rubber seal on the flow divert valve of the pasteurizer. The purpose of this valve is to divert milk not adequately heat treated back into the pasteurizer to be heated again. The faulty seal resulted in pasteurized milk being contaminated by unpasteurized milk leaking through the broken seal.

Outbreak strain found in cattle
The business recalled milk and cream products and stopped production while the pasteurizer had been repaired to ensure any milk was safe for consumption. The dairy restarted production and distribution on Dec. 17, 2019. 

The local authority continued to test milk samples and results showed sporadic unsatisfactory levels of Enterobacteriaceae until March 2020. These results point to poor hygiene or food handling practices. They are not enough to halt production or start a recall.

The outbreak strain was not detected in milk but it was found in six fecal samples from cattle on the farm. There was a reluctance by the company to accept this as evidence of a link between the outbreak cases and milk as the vehicle of infection. Questions were raised as to why only a few customers reported symptoms.

“Failure to detect the organism in a food vehicle does not provide evidence that the implicated vehicle is not the source of an infection. The detection of the pathogen in food samples should not be considered necessary to prove a link when the epidemiological data provide good evidence of an association,” said researchers.

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