Salmonella cases in the UK are expected to rise until the common source is identified, something that is notoriously difficult to establish, experts have warned.
There have been 474 confirmed cases and one death linked to the current outbreak, according to the UK Health Security Agency (UKHSA), with the latest investigations suggesting it is associated with eating imported eggs in cafes and restaurants.
About 1.6bn eggs are imported to the UK every year, according to the British Egg Industry Council, with the number having risen 60% since 2021.
The link to eggs consumed outside of the home is consistent with imported eggs commonly being used in catering, according to Paul Wigley, a professor of animal microbial ecosystems at the University of Bristol.
Wigley said: “Bought in bulk these eggs are cheaper than UK lion mark eggs and so can end up being used in schools and nursing homes along with cafes, restaurants and takeaways. The eggs may be mixed batches from different batches, but the clustering of cases suggests there may be a single source.”
He added: “As far as controls and the future, it’s difficult to judge, though identification of the source would help in being able to put specific controls on a producer or country in place. Avoiding runny eggs outside the home, unless lion mark eggs, is perhaps the only step an individual can make.”
Although previous salmonella outbreaks in the UK have been linked to eggs imported from European countries such as Poland, the current source of this outbreak has yet to be identified. For some experts, this makes the current surge in cases particularly worrying.
Prof Chris Elliott of the school of biological sciences at Queen’s University Belfast, said: “All we have been told is that the likelihood is that it’s been imported.
“There is a huge amount of difficulty in tracing the origins of something like this, particularly when it’s food service. Within the UK’s retail industry, they really know their supply chains extremely well. But with food service, less so.”
British lion-stamped eggs are from UK hens vaccinated against salmonella. Photograph: Geoffrey Swaine/Shutterstock
He added that the number of cases would keep rising until the source was identified, which could take “not days, but weeks”.
Elliott added: “My expectation is those numbers will continue to rise until they can not only track the source, but right back to the country of production. It is fiendishly difficult to track the source of a food and ingredient, so I certainly wouldn’t be critical of the agencies. They’re under immense pressure, and the worst thing that can happen is that they feel pressurised to say that it came from Poland or it came from Romania when they’re not 100% sure about that.”
Prof Brad Spiller, the head of medical microbiology at Cardiff University, said the fact that this particular outbreak was linked to a common source was “probably the most concerning aspect” of the cases.
He said: “It is only once we have a better idea of which countries are involved, we can begin to track back the source.
“This is how they identified that both of the previous outbreaks seen in recent years that were happening simultaneously, and were both linked back to Poland as the source of that. I think until they identify the source, the source won’t naturally stop.”
Prof Mark Stevens, the chair of microbial pathogenesis at the University of Edinburgh, added that the outbreak was also unusual because cases were occurring at the same time, whereas salmonella cases across the UK were usually more spread out.
He said: “This is obviously a real challenge for the health protection agencies because they have to now identify that source and try to restrict its entry into the food system.
“It is of concern to have an outbreak of this size. There are thousands of salmonella cases each year, but rarely are they on this scale where hundreds of people are being infected by a common strain.”
Stevens added: “I think until we really understand the source of these infections, it’s possible that we’ll continue to see more of them.”
