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expert reaction to three previous listeria cases linked to same outbreak

Reactions to more reports of listeria cases following hospital food contamination.


Dr Sandra Wilks, Lecturer in Medical Microbiology, University of Southampton, said:

“While listeriosis causes only mild symptoms in many people, it can lead to severe and fatal infections in certain groups such as immunocompromised people as we have sadly seen in this current outbreak.  The long incubation time between consumption of contaminated food and development of symptoms makes it difficult to determine the scale of the outbreak.  It seems that these cases are linked to a specific batch of pre-packed sandwiches, the production of which has now been stopped.  Although we don’t know how the listeria got into the sandwiches, we know that Listeria can survive on surfaces, and even grow, at chilled temperatures; factors that make it essential that appropriate food hygiene, handling and storage processes are employed.”


Prof Jose Vazquez-Boland, Chair of Infectious Diseases, Infection Medicine – Microbial Pathogenesis, University of Edinburgh, said:

“The previously known cases that were last week linked to the pre-packed sandwich outbreak highlights the difficulties in establishing a cause-effect relationship between listeriosis cases and the source of infection.

“This is due to a number of factors: (i) the usually long incubation period between the consumption of the contaminated food and the manifestation of clinical symptoms, (ii) the low ‘attack rates’ i.e. only few exposed people develop the disease, and (iii) very often the lack of a clear spatial-temporal clustering of the cases because the foods involved are industrially processed and distributed across a relatively large geographical area.  Identifying the link between listeriosis cases and a specific food product requires a detailed epidemiological investigation and microbiological confirmation of the identity of the human and food bacterial isolates by whole-genome sequencing.

“This is a very serious matter.  While production at the meat production plant and the sandwich supplier has been stopped, and while in this case it has affected vulnerable hospital patients, this incident demonstrates that despite strict microbiological controls being most likely in place and regularly implemented, Listeria still continues passing unnoticed time and again, causing a sizeable number of severe, often fatal infections every year.

“Pre-packed sandwiches have been previously incriminated in listeriosis episodes in the UK.  In 2017 for example one such episodes in Yorkshire and The Humber also involved sandwiches supplied to hospitals.

“This current outbreak clearly constitutes a warning that microbiological controls for Listeria may perhaps need to be stepped up with pre-packed sandwiches and generally for ready-to-eat food products.

“As a general measure of precaution, chilled ready-to-eat foods (including among others cold meats, cold chicken, soft cheeses, pre-prepared vegetables and salads, and pre-packed sandwiches) should not be left for prolonged periods in the fridge after purchase, and perhaps be avoided by the risk groups for Listeria infection (fragilized elderly people or those suffering from underlying conditions, immunocompromised individuals, pregnant women).

“In the case of more vulnerable or severely ill patients cared for in hospitals and institutions, policies perhaps should be revised so that they are provided only fresh cooked meals.”


Dr Catherine Rees, Associate Professor in Microbiology, University of Nottingham, said:

Are these new cases, or are they previous cases that have now been identified as probably being part of the same outbreak?

“These are cases that had been reported to PHE previously, but they have now reanalysed the whole genome sequence data from these previous cases and shown that the Listeria isolated from these patients were highly likely to have come from the same source.

Is this a worrying update; was it expected?

“As these are not new cases, this is less worrying but it does suggests that the contamination event had been going on longer than previously thought, so there is the potential that more people have been exposed.  Since the source has been traced, there should be no more exposure but the long incubation period means that the risk of new cases coming to light cannot be ruled out just yet.

Is everyone affected so far a hospital patient; do normal healthy people need to worry?

“As all these cases were in a health care setting, affecting high risk patients, and the new cases have not appeared in the general population, this does not suggest there is a bigger problem likely to emerge.

“However the public should always take care with ready-to-eat foods, such as sandwiches, to make sure they are stored at the correct temperature to reduced the risk that any bacteria in these products can grow to dangerous levels.”


Dr Kimon Andreas Karatzas, Associate Professor in Food Microbiology, University of Reading, said:

“One of the major problems with Listeria outbreaks is the long incubation times which makes it difficult sometimes to identify the food responsible for a specific case.  It is really difficult to analyse most ready-to-eat foods somebody consumed for a few weeks.  These newly-identified cases are actually older listeriosis cases that were thought at the time to be individual but apparently they seem to be linked to the sandwiches outbreak.  Although rare, it is possible that more cases will be discovered or people that consumed these products before they were removed from the market might get sick.

“Due to this characteristic of the disease surveillance is very important.  In countries that do not have good surveillance, major outbreaks can go unnoticed for months as the deadliest outbreak in history that occurred South Africa a couple of years ago, with more than 216 dead.

“There should be no worry to the public as the products have been removed from the market.”


Comments issued on Friday 7 June 2019:


Dr Catherine Rees, Associate Professor in Microbiology, University of Nottingham, said:

What is listeria and why is it bad?

Listeria is a bacterium that can cause a very severe disease in humans, but usually it only affects people who don’t have a strong immune system.  It is caught by eating contaminated food and is best known for its association with soft cheeses – pregnant women are warned to stay away from this because their immunity is lower than normal, and so they are a high risk group.  People with other illnesses affecting their immune responses are also high risk – this is the second time in the UK that there have been cases associated with sandwiches sold in hospitals because patients are also a high risk group.

Where is it usually picked up from?

“This bacteria is killed if food is cooked, so infection is often caused by ready to eat foods – such as sandwiches – that become contaminated.  It’s found everywhere in the environment, so can come from salads and herbs, but it also frequently contaminates cooked meats and dairy products so it’s not just associated with one type of food.

What sort of evidence will have led to the authorities knowing these cases came from pre-packed sandwiches?

“This will have been worked out on the basis of trace back investigations – working out what common foods the different patients have eaten and looking for the common element.  PHE have pioneered the use of whole genome sequencing of Listeria as a method to allow them to confirm as fast as possible that strains found in patients are the same as those found in food processing plants.  Using this technology PHE will be very certain that the strain found in the factory is the same one responsible for the outbreak.

Is listeria dangerous for healthy people or only if you’re unwell or immunocompromised?

“As above – the immunocompromised (and that includes young children and any adult over 60) are the highest risk for developing listeriosis (the disease caused by Listeria).  However if healthy individuals ingest a high enough dose they can get ill too – but sometimes even then this will only take the form of a mild flu feeling rather than developing the more life-threatening form of the disease.

What are symptoms if contracted by people in an at-risk group?

“Classically the early stages of listeriosis are flu-like, but then move on to symptoms that are typical for systemic blood infections (bacteraemia) and can also include symptoms that are associated with the stage when the Listeria spread to the brain – so effects on balance and coordination, but this doesn’t develop in everyone.  In healthy adults, probably just flu-like symptoms that resolve within a few days.

How does it kill people in the at-risk group?

“The biggest problem is that Listeria produces enzymes that cause tissue damage – it can infect multiple organs, including the liver and spleen – and can grow at these sites and then spread to other parts of the body, including the brain.  In pregnant women it can cross the placenta and cause infections in the unborn baby.

Is there any history of outbreaks in the UK?

“Fortunately for us the last large outbreak was back in the 1980s when contaminated pate was identified as the source of the infection.  There have been other small outbreaks since then – most recently one involving frozen sweetcorn that happened all across Europe – but there have been other outbreaks associated with sandwiches and also contaminated butter.

Do these deaths suggest a systemic problem related to the hospitals or its suppliers?

“Listeria is a problem right across the ready-to-eat food chain – it is the one bacteria that most food producers are most diligent about.  However it is also one of the most difficult bacteria to deal with as it can also get into food production plants and contaminate the equipment used to make food.  One known problem is slicing machines – a large outbreak in Canada was caused when Listeria got into the inner workings of a slicer and contaminated the product just before it was packed.  The company had done everything it should to clean the machine, but the Listeria had penetrated parts of the machine that it was not thought possible it could get into.  So it is a known problem for food producers – but one that they are constantly vigilant about.  Other than cooking everything and not giving anyone any fresh, ready-to-eat foods, it is almost impossible to be 100% safe from this bacteria.

How would that bacteria have got into the food chain?  Aren’t there steps to prevent it?

“In sandwiches there will be two obvious sources – one of the ingredients (in this case sliced meats seem to have been identified as a source) and the slicing machines.  As above, companies producing these high risk ready-to-eat foods are always vigilant but this bacteria is unusual and versatile, and preventing contamination all of the time is difficult.  What we do know is that once listeria is in food, it can grow slowly even if it is kept in a fridge.  If the cold chain breaks down, then levels will rise faster – and the more bacteria in a product the more likely it is to cause infection.  So maintaining good food storage is also important to ensure the safety of product.

What steps do pre-packed sandwiches normally go through before they reach us?

“Pre-packed sandwiches are assembled on a line, sliced and packed.  Then they are transported chilled to the retailers – so not many points where you could actually intervene if a problem arises.  The most important thing is good hygienic production and a good cold chain – this keeps the risks as low as possible.  The fact that we don’t have many outbreaks actually says the food industry is generally doing a very good job controlling this.”


Prof Jose Vazquez-Boland, Chair of Infectious Diseases, Infection Medicine – Microbial Pathogenesis, University of Edinburgh, said:

What is listeria and why is it bad?

“Listeria are a group of bacteria that normally live in soil but one of its species, Listeria monocytogenes, has the ability to cause foodborne infection.

“The bacteria can be picked up by anyone but the disease caused by Listeria (known as listeriosis) is more common in certain high-risk groups of people.  It can affect the brain causing meningoencephalitis or can spread throughout the body i.e. “septicemia”.  In pregnant women Listeria infection can lead to miscarriage, stillbirth or a severe infection in the neonate, either systemic or again affecting the brain, that is highly fatal.

Where is it usually picked up from?

“Listeriosis is acquired through consumption of contaminated food.  Typical food products involved in listeriosis are soft cheeses, pâtés, processed meats, salads, sausages, smoked fish, and as in this case, prepacked sandwiches.

“Pre-packed sandwiches are recurrently incriminated in the UK in listeriosis episodes.  In 2017 for example one such episode in Yorkshire and The Humber involved also sandwiches supplied to hospitals.  There is clearly a warning here that microbiological controls for Listeria need to be stepped up with pre-packed sandwiches.

“In general, foods involved in Listeria infections are industrially processed foods that are kept at refrigeration temperatures (Listeria bacteria can grow at low temperatures) and are consumed without previous cooking.

What sort of evidence will have led to the authorities knowing these cases came from pre-packed sandwiches?

“The isolation of the same strain of the Listeria bacteria from the ill people and the incriminated food product.

Is listeria dangerous for healthy people or only if you’re unwell or immunocompromised?

“While Listeria infections can affect normal healthy people, it is very rare and most cases are seen in immunocompromised people and the elderly.

What are symptoms in at-risk groups?

“In nonpregnant adults the most common manifestation is fever of unknown origin.  If the person is developing a brain infection the fever may be accompanied by neurological manifestations with different degrees of severity.

“In pregnant women the main symptom is miscarriage or abortion.  The episode in this case is generally not preceded by any specific symptom, although a flu-like episode within the 15 days before miscarriage is often referred.

“All these manifestations have a relatively long incubation period of up to 60 days after consumption of the contaminated food.  This often makes it difficult to establish a cause-effect relationship and identification of the food responsible depends on an accurate epidemiological investigation.

“Occasionally, in healthy people Listeria infection may manifest as febrile gastroenteritis.  This occurs within hours after consuming the affected food.

How can it kill?

“Listeria are invasive pathogens that can reach the brain, the pregnant uterus or spread throughout the body causing a systemic infection.

Is there any history of outbreaks in the UK?

“The disease presents as sporadic cases (typically about 0.2 to 0.5 cases per million inhabitants per year) and these occur every year. Typical figures for England and Wales are around 150-200 listeriosis cases each year.  On occasions there are outbreaks in which between several and hundreds of people are affected by the same strain (meaning a common infection source).  These are recurrently detected in the UK, although fortunately involving a relatively small number of people.  One of the largest outbreaks in UK took place between 1987 and 1989, was caused by contaminated pâté and affected >300 people.  A recent (2018) outbreak in South Africa caused by processed meat affected 1,061 people.

Do these deaths suggest a more systemic problem related to the hospitals or their suppliers?

“Clearly Listeria foodborne outbreaks are caused by deficiencies in food safety measures and the microbiological control of food products along the production and distribution chain.

What would happen to a healthy person who ate a sandwich infected with listeria?

“What would normally likely happen is that that person would develop a subclinical infection i.e. an infection without apparent symptoms.  That subclinical infection may in fact serve to boost the immunity against Listeria infection.

How would the bacteria have got into the food chain?  Aren’t there steps to prevent it?

“The natural habitat of Listeria bacteria is soil rich in decaying organic matter.  They are ubiquitous in the farm habitat.  From there they contaminate the raw food materials, whether of animal origin or vegetables, and enter the food chain.  They then contaminate food plants where the food products that are being processed serve as a perfect growth substrate for Listeria.  Listeria bacteria are very tenacious, resist many food processing conditions and grow at refrigeration temperatures.  This means that once they contaminate a food plant they are difficult to eradicate, and can continue growing during cold storage.

“The food industry implements microbiological control checks to ensure that foods are not contaminated with Listeria.  However, these checks may fail or be insufficient and Listeria may pass undetected.  This is what has probably happened in this case.

What steps do pre-packed sandwiches normally go through before they reach us?

“All processed foods and the raw materials used in their preparation need to pass very strict microbiological controls to ensure they do not carry pathogenic organisms.”


Prof Paul Hunter, Professor in Medicine, UEA, said:

“Listeria is a bacterium that is widespread in the environment and can contaminate various foods.  People usually become infected by eating contaminated foods.  However, most who become infected have few if any symptoms.  However, in the very young, the elderly, pregnant women and people whose immune systems are weakened the infection can give rise to Listeriosis, a severe and potentially fatal infection.  When listeriosis occurs in pregnancy the woman may have few symptoms other than fever, fatigue and muscle pains, but infection may result in miscarriage or stillbirth in about 25% of cases.  In the elderly or immunocompromised the infection can cause fever, fatigue, muscle pains, and affected individuals may develop signs of meningitis – in this group the mortality rate can be up to 33%.

“A range of foods have been particularly implicated in the spread of Listeria in the past.  There have been several foodborne outbreaks reported in the past including ones associated with soft cheeses made with unpasteurised milk, ice cream, cold meats, pre-packaged salads and sandwiches.  In 2017 to 2018 the largest recorded outbreak of Listeria occurred in South Africa with some 1,060 confirmed cases of listeriosis and about 216 deaths.  This outbreak was associated with consumption of cold processed meats, especially polony.

“Previous outbreaks of listeriosis have been associated with the consumption of contaminated sandwiches including within the UK.

“Although the cause of contamination is not always identified, foodborne outbreaks of listeria in the past have occurred in settings where there are poor hygiene standards or when contaminated raw ingredients have used.  Prevention of infection depends on rigorous maintenance of hygiene standards in the food chain for ready-to-eat foods, the maintenance of low temperatures during distribution and the speedy distribution of ready-to-eat product within their shelf-life.  Listeria is one of the reasons pregnant women are advised to also avoid unpasteurised soft cheeses and pâté.”


Dr Clare Taylor, Senior Lecturer Medical Microbiology, Edinburgh Napier University, said:

What is listeria and why is it bad?

Listeria are bacteria that are ubiquitous in the environment.  There are 19 different species ofListeria but Listeria monocytogenes is the species most commonly linked to foodborne infection.  It is a problem in food because it can survive in a variety of different environments.  For example, L. monocytogenes can grow at a wide range of temperatures, including fridge temperatures, and it is very difficult to remove from contaminated environments because it can stick to lots of different types of surfaces.

Where is it usually picked up from?

“Contaminated food is the most common source ofListeria monocytogenes infection.  Lots of different types of food have been linked in the past – salad, deli meats, melons, sweetcorn, soft cheeses, pates, vegetables.

Is listeria dangerous for healthy people or only if you’re unwell or immunocompromised?

“In people with weakened immune systems,Listeria monocytogenes can cause serious infections that affect the central nervous system, such as meningitis.  This form of infection can be severe.  In pregnant women, it can cause miscarriage.  However, infection is normally rare (normally approx. 150 – 200 cases a year in the UK) and in most healthy people, illness could be like gastroenteritis (sickness) or resemble a flu-like illness.

Is there any history of outbreaks in the UK?

“Past outbreaks in the UK have been rare.

Do these deaths suggest a systemic problem related to the hospitals or its suppliers?

“Not necessarily.  Listeria infections are rare.

What would happen to a healthy person who ate a sandwich infected with listeria?

“It would depend on the amount of Listeria in the sandwich.  If there high numbers, then the person could get a gastroenteritis or flu-like illness.  In some cases there are no symptoms at all.  These type of illnesses are usually self-limiting and the body deals with it.

How would that bacteria have got into the food chain?  Aren’t there steps to prevent it?

Listeria is found in soil and various different environments and can be carried around to different places by insect carriers.  As such, food products can be contaminated during harvesting and processing.  It is possible that low numbers of bacteria could grow in contaminated food products at fridge temperatures which is why ready-to-eat produce have eat-by dates.  Food products are routinely tested for the presence of Listeria monocytogenes and other bacteria that cause foodborne infection.”


Prof Brendan Wren, Professor of Microbial Pathogenesis, London School of Hygiene & Tropical Medicine, said:

“Listeria is a bacterium that can be found occasionally in food products.  If consumed it can lead to gastroenteritis, and in compromised patients more severe disease such as meningitis and, in rare cases, death.  Pregnant women can pass the infection to their unborn children who are particularly susceptible to infection.  In this small outbreak there appears a cluster of cases in hospitalised patients that were already ill.  Listeria can survive at refrigeration temperatures which means it can persist in food processing equipment and in the food chain.”


Dr Kimon Andreas Karatzas, Associate Professor in Food Microbiology, University of Reading, said:

“Listeria monocytogenes is the most deadly foodborne bacteria, mainly associated with ready-to-eat foods that are eaten without any further processing by the consumer.  It does not cause a large number of cases but it has a high mortality that can reach up to 30%.

“It most often affects the elderly, infants, pregnant women and immunocompromised individuals (patients taking immunosuppressants).  This is the case with this outbreak that has affected hospital patients.  The company has voluntarily ceased production and hopefully this can put an end to the outbreak.  However, due to a relatively long incubation period it’s possible that there could potentially be some more cases.”


Declared interests

Prof Jose Vazquez-Boland: “I declare I have no conflict of interest.”

Dr Catherine Rees: “No interest to declare.”

Prof Paul Hunter: “I have no conflicts of interest.  I co-wrote/edited ‘The microbiological safety of food in healthcare settings’.”

None others received.


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