Smoked fish products in England have a relatively low prevalence of Listeria but have been associated with illnesses, according to a study recently published.
Researchers encouraged manufacturers of smoked fish to carry out risk assessments to prevent outbreaks and recognize the microbiological risks associated with such products.
Samples of smoked fish products including salmon, mackerel, trout and herring were collected across 99 local authorities in England from November 2022 to March 2023.
Listeria monocytogenes was detected in 28 of 783 samples with a high rate in smoked herring. It was found more frequently in cold smoked fish than hot smoked samples.
Almost two-thirds of samples had a water activity that would not prevent the growth of Listeria monocytogenes.
According to the study, published in the Journal of Applied Microbiology, positive samples were produced by 15 manufacturing plants. Four positives were above 20 colony forming units per gram (CFU/g).
The majority of samples were taken from retail, while a few came from catering, wholesalers, and manufacturing premises. Listeria presence in smoked fish at retail is primarily caused by contamination during processing. Most samples were from production plants in the UK with 86 samples from Poland.
Investigation of historical isolates revealed that 11 strains were associated with specific manufacturers for up to 10 years.
Link to human illnesses
Other Listeria species were detected in 28 samples. They included Listeria welshimeri, Listeria seeligeri, Listeria innocua and Listeria grayi.
Of 28 isolates characterized, three strains were linked to human cases. Three products from one manufacturer where an outbreak strain was detected had use-by dates between February and April 2023. These dates were beyond those listed in recalls issued in December 2022 involving fish produced by St. James Smokehouse and sold at Lidl.
Samples were all below 20 CFU/g, meaning they were compliant with legislation. In total, 24 people fell sick between 2020 and 2024. Some cases occurred after the recall. It was not clear if this was because of the product being frozen by consumers or inefficient withdrawal at retail.
The two other strains from salmon were linked to one case each from 2021.
Listeria monocytogenes was detected more frequently in samples with elevated levels of Enterobacteriaceae. However, researchers said the predictive strength of Enterobacteriaceae counts for detection of Listeria should be viewed with caution.
“The increased likelihood of detecting Listeria monocytogenes in samples with elevated levels of Enterobacteriaceae may reflect a lapse in hygiene causing contamination with both organisms possibly through a common source,” they added.
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