The Growing Problem of Listeria and Listeriosis: What are the risks and how can you stay protected?

Listeria also can form biofilms that help them attach to surfaces like floor, drains and equipment – making them more difficult to eradicate with cleaning, and protecting them from drought, heat, standard cleaning and disinfection chemicals.

While Listeria outbreaks are less common than those caused by other foodborne pathogens such as Salmonella and E.coli, recent years have seen a rise in food product recalls due to cases of Listeria contamination. Over the last five years, recall notifications have been issued worldwide ^1,2 for wide-ranging products including pre-prepared salad, brie and camembert cheese, chilled fish and ready to eat chicken, all due to contamination from Listeria.

As more outbreaks of Listeria contamination are observed it follows that cases of listeriosis in individuals are also on the rise – 2022 saw 2,738 people across the European Union be affected by the disease, an increase on the previous year of almost 16%³. The United States are also gravely affected, accounting for 1,600 cases in 2024. 

Listeria bacteria are found in many places, including soil, water, rotting vegetation and animals. There are ten different species of Listeria, but the specific type which causes listeriosis is Listeria monocytogenes which have been found to enter the food chain via several routes. If manufacturing or production environments are contaminated (potentially by raw materials, water, soil or air) these bacteria can be transmitted to our food at any point from when it is initially harvested and prepared, through to when it is transported and stored, where bacteria can go on to multiply to dangerous levels. These bacteria can also be spread in our homes by pets if they eat food contaminated with L. monocytogenes. Once Listeria has entered a facility it can be very challenging to fully remove, which is why it is so important to take all possible steps to prevent contamination in the first place.

Certain food types are more prone to contamination with Listeria than others. For example, soft cheeses and those made with raw milk are more likely to become contaminated than hard cheeses, as they are made in a way that allows listeria bacteria to grow more easily. Other foods that are more likely to contain L. monocytogenes include smoked fish, meats and raw vegetables.

Listeriosis has a range of signs and symptoms, often depending on how severe the illness is and what form it takes. In cases of less severe, non-invasive listeriosis (where the infection remains in the intestine), typical symptoms include fever, muscle aches, nausea, vomiting and diarrhea. These symptoms normally appear within a few hours or days, however in some cases, symptoms of listeriosis may take up to two months to appear after the contaminated food was first eaten.

In healthy individuals, listeriosis typically causes mild symptoms which last for only a few days, however in vulnerable populations such as newborns, adults over the age of 65, pregnant women, individuals with weakened immune systems and those undergoing certain medical treatments (e.g. chemotherapy for cancer), the impacts of listeriosis can be more severe and dangerous.

These vulnerable groups are at increased risk of developing the invasive form of listeriosis, which has a high mortality rate of 20-30% ⁴. In cases of invasive listeriosis, symptoms are more severe and can lead to serious and life-threatening complications such as meningitis and sepsis. Additionally, listeriosis infections experienced during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.

Importantly, listeriosis can be treated with antibiotics if it is detected and diagnosed early. Determining the initial source of infection can be challenging however, depending on how long it takes for symptoms to appear. 

Given the potential dangers that listeriosis presents to vulnerable populations, it is vital that any individual or company involved in the preparation, storage or distribution of food take the required steps to minimize the risk of Listeria contamination. As previously discussed, the control of Listeria bacteria presents specific challenges in light of its characteristics – the bacteria are highly resistant to common methods for preserving food, such as salting, smoking or using acid. Furthermore, Listeria bacteria are capable of surviving and growing under refrigeration.

For this reason, it is important that food suppliers take a fully integrated approach to listeria control at every stage of the food chain, reflecting Good Hygienic Practices and Good Manufacturing Practices, and ensuring food safety management systems are compliant with the principles of Hazard Analysis Critical Control Points (HACCP). To further minimize the risk of Listeria contamination, these procedures must be rigorously tested to validate and verify that they are working correctly. Where food manufacturers are producing products considered at higher risk of listeria contamination, environmental monitoring is also needed. 

If Listeria contamination is detected, conducting a root cause analysis across the production chain is essential to pinpoint and eliminate the source. By tracing contamination back to its origin – whether from raw materials, equipment, environmental conditions or hygiene practices – manufacturers can take corrective action to prevent future instances. The process of identifying the cause of outbreaks has been strengthened by the introduction of improved detection tools, such as those that employ whole genome sequencing – and may provide another reason as to why we are seeing more incidences of Listeria contamination and product recalls. Genetic testing can also help to establish if the same strain of bacteria is making people sick in multiple places, enabling health authorities to track and map outbreaks.

In a home setting, risk of Listeria contamination can be minimized by practicing good hygiene and food safety, such as handwashing regularly, including before and after preparing food, sanitizing surfaces and separating raw and cooked foods in the refrigerator. It is also important to store food at a safe temperature, ensuing the refrigerator is working properly and set to 5C or below, and to respect use by dates and storage instructions on ready-to-eat foods. Cooking food thoroughly before eating is another important step to kill off any bacteria present.    

To further reduce personal risk, those in the previously identified vulnerable groups are advised to avoid consuming dairy products made of unpasteurized milk along with deli meats, pre-prepared meat products and cold-smoked seafood.