Pregnant women and those with weakened immune systems have been advised against eating ready-to-eat (RTE) cold-smoked or cured fish.

The advice comes from a risk assessment by the Food Standards Agency (FSA) and Food Standards Scotland (FSS) that shows these groups are at a higher risk of severe illness from listeriosis. The risk assessment was prompted by an outbreak of Listeria infections linked to RTE cold-smoked fish.

Past advice listed smoked fish as a food of concern with the recommendation that care should be taken during pregnancy and it should only be eaten by vulnerable groups after being thoroughly cooked.

The FSA, FSS, and UK Health Security Agency (UKHSA) have been investigating the ongoing outbreak since October 2020. There have been 19 linked cases of listeriosis in England and Scotland and four people have died, this is up from the 15 cases and three deaths reported several months ago. Most patients are above 65 years old but one was a pregnant woman. 

In December 2022, Lidl recalled Deluxe Oak Smoked Scottish Louch Trout and Lighthouse Bay Smoked Trout Trimmings, produced by St James Smokehouse. Product testing found Listeria monocytogenes that matched the outbreak strain but levels were below permitted limits.

Potential for serious illness
In RTE foods that can support growth of Listeria, which include smoked fish, the bacterium must not be present in 25-grams of sample when leaving production plants, or businesses must show the products will not exceed the limit of 100 colony Forming Units per gram (CFU/g) during their shelf life.

As the risk of serious illness from listeriosis increases with age, FSA and FSS are also advising that people older than 65 should be aware of the potential danger posed by products such as smoked salmon or trout and gravlax. Neither the cold-smoking process nor refrigeration kill Listeria.

The assessment found that while the risk of contracting listeriosis in higher-risk individuals from cold-smoked fish is low, severity of illness is high. This means there is the potential for serious illness, hospitalization, and death among higher risk groups.  

Uncertainties included the difficulty in estimating the infectious dose for Listeria monocytogenes and how it varies between different vulnerable groups; the long incubation period which can make attribution to a food vehicle difficult; the initial level of contamination and how it multiplies through the food chain; and consumer behavior around use-by dates and temperature abuse.

Professor Robin May, FSA chief scientific adviser, said: “Our risk assessment shows that there is still an ongoing risk to health associated with eating cold-smoked fish for specific groups of vulnerable people, including pregnant women and individuals with impaired immunity. In light of the risk assessment, we are advising that these consumers avoid ready-to-eat cold-smoked and cured fish products.” 

Those with weakened immune systems are people with certain underlying conditions such as cancer, diabetes or liver and kidney disease.

Fish processing details
Cold-smoked fish such as smoked salmon or trout, and cured fish such as gravlax, have not been fully cooked during the production process to kill any Listeria that may be present. Cold-smoked fish is normally labeled as smoked fish on packaging, so processing type is unclear. RTE cold-smoked fish typically comes in thin slices, and can be eaten cold. It may also be found in sushi.   

Smoked fish products that have been heat-treated during production, such as tinned smoked fish, are safe for consumption without further cooking. These products are subjected to a high temperature during processing, which is sufficient to kill Listeria.

Gauri Godbole, consultant microbiologist at UKHSA, said: “While smoked fish has a higher risk of carrying Listeria, the overall risk to the population is very low. However, some people are more likely to get a serious infection including those who are pregnant and those with weakened immune systems. The risk also increases with age. Those who are more vulnerable can be at risk of severe illness such as meningitis and life-threatening sepsis. Listeriosis in pregnancy can cause very serious illness in mothers and their babies.”

UKHSA has identified 31 patients with microbiological and epidemiological links to eight smoked fish incidents between 2015 and June 2023, with eight deaths and three pregnancy associated patients.