Researchers have expanded knowledge of the United Kingdom’s involvement in a multi-country Listeria outbreak.

Twelve patients were detected in the UK between 2015 and 2018 and contaminated sweetcorn was recovered from the freezer of another person in 2019.

Overall, the outbreak included 54 cases of listeriosis in Australia, Finland, Sweden, Denmark, Austria, and the UK with 10 deaths. It was traced to a frozen vegetable factory in Hungary that in 2018 was owned by Greenyard.

In the UK, the outbreak strain was detected in a chicken and sweet corn sandwich filling from a manufacturer in June 2018. The producer added frozen sweet corn directly to fillings without additional cooking or blanching. Sampling was done during routine testing and was not part of the outbreak investigation. The company has now put in place additional controls, which include a heating step for frozen sweet corn before adding it to the sandwich filling.

Six strains recovered
The sweet corn was supplied by Greenyard Frozen UK, a distributor in England, that got frozen products from Greenyard in Hungary. The study, published in the International Journal of Food Microbiology, showed that sweet corn was more frequently contaminated than other frozen vegetables collected at Greenyard Frozen UK.

A total of 208 food and environmental samples were taken at the distributor’s two sites. Listeria monocytogenes was detected in 31 of 70 samples of frozen sweet corn and 5 percent of 79 other foods.

The outbreak strain was found in the frozen sweet corn, in frozen mixed vegetables and the factory environment. It was also recovered from frozen beans on retail sale in the first four months of 2019.

Five other Listeria monocytogenes strains and two other Listeria species were detected in samples from Greenyard Frozen UK’s premises. One of the Listeria monocytogenes strains in the factory, which was not the outbreak strain, was also recovered in sweet corn from the sandwich manufacturer, sweet corn tested in England in 2013 and 2016 and the blood of two listeriosis patients in England in June and November 2014 — a 69-year-old male and a 33-year-old pregnant female.

The number of listeriosis infections reported in the UK varied between 192 and 160 per year between 2013 and 2017. Another related study looked at the presence of Listeria and E. coli in frozen fruit and vegetables at retail and catering premises in England 2018 and 2019.

Two deaths in the UK and risk after recall
Of the 12 UK outbreak patients, ages were available for 11 of them. They ranged from 1 to 86 years old and apart from patients aged 1 and one aged 22, all the others were 58 or older. Eight were male and four were female.

Nine of them lived in England, four in the North East, one in the North West, two in the Midlands and two in London, and three were in Scotland. One illness occurred in 2015, two each in 2016 and 2017, and seven in 2018. Two people died.

Of eight cases, five reported consumption of frozen vegetables including sweet corn, four ate fresh mixed vegetables containing sweet corn, and two had canned sweet corn. The 1-year-old child consumed frozen sweet corn puréed with other vegetables. The outbreak strain was recovered from this food.

In February 2019, the outbreak strain was recovered from a 58-year-old male patient in London. This strain was also detected in an opened pack of frozen sweet corn from his freezer and an opened pack of cheese from his fridge. The sweet corn was a batch part of the international recall in summer 2018 but it was not clear when products were purchased. This shows consumers in the UK were still at risk from contaminated products after the recall, according to the report.

Researchers said findings highlight the risks of listeriosis from frozen sweet corn despite Listeria monocytogenes being unable to grow in it and potential for its misuse as a ready-to-eat (RTE) product.

“Frozen vegetables including sweet corn represent a hazard both as a source of cross-contamination in food production environments and through the potential for misuse as RTE food ingredients,” they said.

“Advice from the European Food Safety Authority on public health risks as well as sampling and environmental monitoring in processing plants will help to implement better HACCP-based food safety management systems and, it is hoped, preventing further outbreaks. Consumers should also be advised on the appropriate purchase, storage and cooking of frozen vegetables.”

Experts added that manufacturers and retailers should also clearly mark packaging with the need for cooking.