On the UN’s first World Food Safety Day on 7 June, UK health authorities announced three hospital patients had died after eating sandwiches contaminated with listeria. It came just weeks after French authorities reported a woman had died and another had lost her baby after eating cheese contaminated with the bug. So just how worried should we be about listeria?

What is listeria?

Listeria monocytogenes is a foodborne bacteria that causes only mild symptoms in healthy people, but can lead to a serious infection in pregnant women, the elderly and patients with weakened immune systems.

The pathogen is so prevalent in the environment that food processing plants are subject to ‘relentless exposure’, warns a 2014 FSA study. Unlike other foodborne bacteria, it continues to grow at low temperatures. But it is killed by cooking food thoroughly, so it’s mostly a concern in “chilled foods that do not require further cooking,” says BMPA director Nick Allen. That includes salads, cooked meats, smoked fish, pates and some cheeses.

Factory worker on scotch egg processing line

Listeria can be passed to food via raw materials, packaging and even workers

How does listeria contaminate food?

Listeria can find its way into factories via raw materials, packaging materials and even workers. It is killed by thermal processing, but post-cooking contamination is a ‘major hazard’, warns the FSA study. And while most big food processors have stringent controls to prevent cross-contamination, it does slip through the net. This month, Müller recalled batches of Cadbury’s desserts made at its Minsterley plant in Shropshire after its own routine testing found traces of listeria contamination.

”We are in the midst of a comprehensive review of the whole supply chain as it relates to the ingredients and the manufacture of these products and we will act in line with the findings of this review to prevent any reoccurrence,” says a Müller spokesman. “Production of these products will remain suspended until this point.”

According to FSA data, recalls involving listeria have increased threefold in the past year, from four in 2017/2018 to 13 in 2018/19. That’s still less than 7% of total alerts though. And typically, only low levels of listeria are transferred to the final product during cross-contamination in a processing plant, says the FSA study.

Store made sandwiches

A survey of sliced cooked meat from smaller retailers found the bug in 3.8% of samples

Why do temperature controls matter?

In order for an outbreak of foodborne disease to occur, there must also be inadequate refrigeration between production and consumption, which allows the bacteria to multiply enough to cause an infection.

The risk of this happening in supermarket supply chains, which have strict temperature controls, is low. A 2007 FSA survey of sliced cooked meat samples from the mults found listeria monocytogenes in 1.5% of samples, but at levels too low to pose a risk to human health.

However, a subsequent survey of sliced cooked meat from smaller retailers found the bug in 3.8% of samples, and 71.3% of those samples had been stored above the industry guideline of 5°C, which ”could put consumers at risk,” warned the which “could put consumers at risk,” warned the Advisory Committee on the Microbiological Safety of Food. That’s particularly worrying because a PHE study of human listeriosis in England found that high-risk groups, such as the elderly, were more likely to buy foods from smaller convenience stores than the general population.

Patients in hospitals could be at increased risk if hospitals fail to adhere to best practice and keep sandwiches cold enough.

Pre-packed sandwiches

Listeria is one of the most dangerous foodborne diseases

Is listeria a risk to public health?

Listeriosis remains very rare in the UK. There were 135 cases reported in 2017, according to PHE, which is a 17.7% decline compared with the average number of cases reported in the preceding six years (2010-2016).

But it’s still one of the most dangerous foodborne diseases. A quarter of all cases in pregnant women result in miscarriage or still birth, while the fatality rate among non-pregnancy cases is over 30%. So “it remains imperative that sporadic cases of illness and clusters of disease continue to be monitored and investigated by the FSA, to inform the continued risk assessment of the food chain”, says Allen.