Earlier this week the UK issued a safety alert about the presence in smoked salmon of the bacteria, Listeria. This alert was precipitated by reports of 14 cases of Listeria gut infection in the past 2 years, with 8 since the start of 2022; 3 of those infected had died. These cases had in common that they had eaten contaminated ‘ready to eat smoked fish’, predominantly smoked salmon. The UK Health Protection Agency recommended that ‘vulnerable’ people – which included women who are pregnant, those with a weakened immune system including those who are on steroids and “those aged over 65” – should heat any raw smoked fish before eating. Is this just scare mongering or is the advice reasonable?
What is Listeria
- Listeria is a bacterium first identified as a cause of human infection around 100 years ago
- There are many different species of Listeria but mainly it is one, Listeria Monocytogenes, that is relevant to humans
- Listeria can grow in many foods and when we eat a contaminated food, Listeria can lead to infection called Listeriosis
- The infection varies in severity from being asymptomatic, to a mild gastro-enteritis, to a severe infection that spreads from the gut to other sites within the body
- Severe infections are very rare but can exceptionally be fatal
- As stated above in the recent alert, only some groups of the population are at risk of getting severe infection eg pregnant women and those with weakened immune systems
- (Pregnancy is associated with a reduction in the level of immunity so the mother’s immune system doesn’t ‘reject’ the foetus)
What foods can carry Listeria?
- Many raw animal products can carry Listeria, these include:
- Unpasteurised milk
- Cheeses made from unpasteurised milk, especially soft brie/camembert-type cheese
- Cold smoked fish (smoking doesn’t kill Listeria)
- Cured meats
- But the fact that a food carries Listeria doesn’t mean that it will cause infection, there needs to be a sufficient concentration of the bacteria
- Food regulators will therefore test batches of foods such as those above and identify not only if Listeria is present, but if the concentration is higher than acceptable
- They take samples of the salmon and try and grow Listeria in the lab
- Concentration is based on counting the number of colonies of the bacteria per gram of food stuff.
- Below 10 colonies per gram is acceptable
- Between 10-100 colonies per gram is a ‘grey zone’
- Above 100 colonies per gram is unacceptable and the batch is withdrawn from sale
How common is it that food like smoked salmon is contaminated?
- It is impossible to ensure that cold smoked fish is consistently free from Listeria
- The answer to “how common does contamination occur?” is, of course, that it varies!
- I found a research paper from 20 years ago that had tested a large number of samples selected from 19 batches of smoked salmon packaged in Europe, North America, and Australasia
- Listeria was detected in at least 10% of the samples collected in the large majority (16/19) batches
- However, as I mentioned above, that does not mean that the concentration of Listeria was high enough to make people ill
- The risk depends on the processing plant, how the salmon is prepared, packaged, and stored
- Accurate data rely on food safety authorities to have the testing regimes in place to monitor the levels
Risk factors for Listeria in smoked salmon
- Listeria can survive in fridge temperature eg 40
- Indeed can continue to multiply at this temperature, but won’t multiply in freezer
- Hence the longer the salmon is stored, even in the fridge, if Listeria is present, the higher the concentration
- Risk not reduced by how salty is the fish or when vacuum packing is used
- It is only heating by cooking that destroys Listeria
What is the incubation period for getting ill?
- The incubation periods is the time from ingestion of a contaminated food to becoming ill
- What is interesting about Listeria is that, unlike what we know about other bacteria contaminated food when infection happens very soon after ingestion, the incubation period following Listeria contaminated food can be much longer
- The figure below shows the results from one study of 37 cases of the range of the incubation period
- As you can see, in most patients, it is just a few days but in some it can be a month or longer
- Indeed cases with illness starting 3 months after the presumed exposure have been reported
- With such a long exposure it can be very difficult to link an illness to what was eaten so long ago
- Although modern investigation techniques identify the specific DNA ‘signature’ of the Listeria in each patient which can then be linked to a specific batch of salmon
Why the sudden concern?
- Outbreaks of Listeria from cold smoked salmon are not new
- It is difficult to get accurate data because many cases are presumably under-reported
- The number of cases in the current outbreak is similar to other outbreaks in the past
- I can’t find evidence that the risk of smoked salmon being contaminated during its production and handling has increased (or gone down!)
- The overall number of reported cases relative to the millions who eat smoked salmon is tiny
- The recent publicity is timely as it reminds us that:
- many raw foods can be a source of bacterial infection in general
- cold smoked salmon is potentially a risk for Listeria in particular
- The risk of getting seriously ill is tiny, has probably not changed and should not be a worry
- We do need to be vigilant, and for smoked salmon in particular, do not keep beyond its use by date (this message is my main reason for writing this blog)
- As with cheeses from unpasteurised milk, given that smoked salmon is not one of life’s essentials, it should be avoided by pregnant women and those who have impaired immunity (but this is not new advice although not often publicised)
- Nothing to stop enjoying it cooked with scrambled eggs!
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