July 24, 2025

expert reaction to Listeriosis-caused death in Ireland

Scientists comment on a death caused by Listeriosis in Ireland. 

Dr Ricardo Calderón González, Research Fellow, School of Medicine, Dentistry, and Biomedical Sciences, Queen’s University Belfast, said:

“Unfortunately, and although is not a very well-known infection, from time to time we see news about new outbreaks of listeriosis like this. Listeriosis is an infection caused by a bacteria called Listeria monocytogenes. It’s a ubiquitous pathogen that can be found in soil, water, animals.

“In humans, the infection is acquired by consuming contaminated food, mainly from animal origin. Usually, health individuals will be asymptomatic or experience mild, flu-like symptoms (mild fever, gastroenteritis or diarrhoea), but the infection is especially dangerous for two kinds of individuals. First, those that are immunocompromised (cancer patients under treatment, people with autoimmune diseases, those who have recently received a transplant and are under immunosuppressive therapy, newborns, or elderly people). In these cases, the infection can cause a other infections that can be dangerous if non-treated, such as sepsis, endocarditis or central nervous system infections (meningitis, encephalitis).

“Secondly, it’s really dangerous in pregnant women. The main issue is that, as I said before, in healthy individuals the infection doesn’t cause symptoms or just causes mild ones (that can be easily mistaken with the common nuisances due to pregnancy), but bacteria will head towards the placenta and then the foetus. In first stages this would cause miscarriage, and at late term it can be the cause of malformations or meningitis in the newborn.

“The incidence of the disease in Ireland and UK is low, around 0.3 cases per 100.000 individuals, which is lower than the EU average. But there are two important things to take in consideration: these numbers don’t include the number of miscarriages caused by Listeria, and despite the low incidence, mortality rate can be quite high in those that develop the disease (around 20% of those hospitalized). Main foods affected are those that require little to no cooking (as the bacteria is killed by heat), such as raw products of animal origin (cold cuts, smoked salmon), soft cheese with low or not pasteurization, and ready-to-eat meals.

“I don’t know the particular circumstances that caused this outbreak, as that would require further investigation, but it was probably caused by the presence of some affected product in the food-processing plant, and this cross-contaminated the whole chain. So, this gives us the first measure to be taken to avoid future outbreaks, which is proper cleaning of those facilities where raw products of animal origin are processed, as well as the hygiene and use of PPE by those working on them. This should go together with continuous surveillance of surfaces in contact with the food, as well as the products themselves to detect and recall affected meals, as was the case.

“I know all this can sound worrying, but there is some good news. First, the spread of infection is limited, not only because otherwise healthy individuals often present asymptomatic, but also because is not transmissible between people. Second, there are some easy and effective actions people can take to avoid not only this, but other food contaminations:

• As usual, use proper hand and surface hygiene when cooking. Also, a clean fridge helps (a spot inside it can be a reservoir for Listeria and other bacteria).
• The lower the temperature is, the slower the bacteria can grow, so try to keep the fridge temperature under 5 degrees Celsius.
• Also, the less time you give the bacteria to grow, the less it will and is thus less likely it is to cause infection. Any ready-to-eat meal should be consumed as soon as possible. It is important to respect the dates that can be found in the package.
• Those individuals at risk, like immunocompromised or pregnant women, should avoid risky foods (in general, anything from animal origin that is not properly cooked or has been pasteurized).
• Finally, cook things properly, especially if using a microwave. Although Listeria can be killed by heat, some good practices have to be on mind to avoid “cold spots” in the food where the bacteria can persist, such as tuning/moving it while heating, or using a lid/cover while it is inside the microwave (as the trapped steam will help to distribute heat all across the dish).

“It is possible that other deaths will appear due to this outbreak, although now that the alert is in place and the affected products are being retired, the chances are lower. Anyone that has consumed them and has any flu-like symptoms, especially if they’re in a high-risk group, should contact their GP immediately and explain the situation. If the infection is caught early, it is easily treatable with antibiotics as required.”

Prof Catherine Rees, Professor of Microbiology, University of Nottingham, said:

“Listeriosis is a bacterial infection that is contracted by eating contaminated food. Mild symptoms are flu-like and so go undiagnosed.  Unfortunately – unlike many other common food poisoning bacteria – listeriosis can develop into serious, life-threatening infections, most often affecting more vulnerable individuals (the elderly, pregnant women and others with a poor immune system).  

“The bacterium that causes this – Listeria monocytogenes – is sensitive to heat, and is usually associated with ready-to-eat foods, such as soft cheeses, pates or smoked salmon. So this outbreak is unusual because the list of recalled products includes dishes that should be re-heated or cooked. The food industry is very aware of the risks associated with Listeria infections, and it seems routine monitoring may have led to the identification of the source of this problem. 

“If the source of the outbreak has been identified, that will help control this outbreak, but symptoms can take some time to develop so more cases may be reported as time goes on. To help reduce the risk of further infections the public should be reminded to always heat pre-prepared food products thoroughly and to wash their hands before serving food or eating, as contaminated hands are an often overlooked source of transferring bacteria onto foods within the home.”

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

What is listeriosis, how common is it, how does it occur, how is it treated?

“Listeriosis invariably is caused by the bacterium Listeria monocytogenes and is a rare infection that can cause meningitis and other complications. It is usually transmitted through the contamination of food products and is unusual in that it can survive at low temperatures including in refrigeration. Pregnant women and over 65s are particularly susceptible to infection.

 What do we know so far about the situation so far and what is yet to be learned?

“The current situation appears to be an outbreak in the island of Ireland linked to potential contamination of a food source.”

What might have caused this particular outbreak and how will it be controlled?

“Most outbreaks of Listeriosis are often linked to persistent strains at food manufacturing sites. The strains are often difficult to detect and can withstand refrigeration and persist in factory sites for several months. Full inspection of the food processing chain and culturing of the bacterium will be required to identify the contaminating source.

How deadly is listeria and should we expect to see more deaths due to listeriosis?

“Rarely causes death, mainly in the immunocompromised. Most individuals will have mild flu-like symptoms, nausea and diarrhoea.

How worried should we be about this and future outbreaks?

“Outbreaks of Listeriosis are rare in Europe. The pathogen is very rarely transmitted between humans (except mothers to foetuses). So there should not be real alarm.

What can we do to prevent infection/future outbreaks?

“Close monitoring and testing at all points of the food supply chain”

Declared interests

Dr Ricardo Calderón González: None

Prof Brendan Wren: None

Prof Catherine Rees: None