The recent listeriosis outbreak, which has posed a major health threat in the United States (US) and Canada, has been linked to the consumption of contaminated food products containing the bacterium Listeria monocytogenes. Multiple fatalities and numerous cases of hospitalisation resulting from this infection has triggered extensive product recalls, including vegetables and deli meats.
The bacteria, Listeria monocytogenes, is commonly found in nature, especially in soil, groundwater, rotting vegetation, sewage, and animal and human faeces. Consuming food contaminated with L. monocytogenes could result in listeriosis, a severe infection that poses a significant risk to vulnerable individuals.
Listeriosis has two main types:
(i) Non-invasive listeriosis (febrile listerial gastroenteritis), is a mild form primarily affecting healthy individuals. Symptoms include diarrhoea, fever, headache, and muscle pain, with a short incubation period of a few days. Outbreaks typically occur after consuming foods contaminated with high levels of L. monocytogenes.
(ii) Invasive listeriosis is more severe and targets high-risk groups, such as pregnant women, cancer patients, individuals with HIV, organ transplant recipients, the elderly, and infants. This form presents severe symptoms and has a high mortality rate of 20–30 per cent. Symptoms include fever, muscle pain, septicaemia, and meningitis. The incubation period usually ranges from one to two weeks but can vary from a few days to up to 90 days.
On July 19, 2024, the Centres for Disease Control and Prevention (CDC) of the US had reported an outbreak of Listeria infection across its 12 states. The CDC defines it as a foodborne disease outbreak and the number of people infected may be higher than reported, as symptoms could take time to appear and some people may recover without medical care. It is reported that in the US outbreak, 16 out of 18 Listeria infected people had eaten deli meat, and the US Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS) has been investigating the suppliers.
Meanwhile, in Canada, a separate outbreak of listeriosis has been linked to contaminated plant-based drinks, including Silk and other great value brands. Around 15 Silk products were recalled in July 2024. The extent of the outbreak is still unknown. However, it has raised concerns about a similar 2008 Outbreak that killed 22 people. Silk has issued a precautionary recall of all its refrigerated beverages while the investigation continues.
Foodborne diseases WHO defines foodborne diseases as illnesses caused by consuming food contaminated with harmful pathogens, including bacteria, viruses, parasites, and chemical substances like heavy metals. Over 200 different diseases fall under this category, which represents a significant and growing public health issue worldwide. The impact of foodborne diseases extends beyond health, straining healthcare systems, reducing productivity, and negatively affecting tourism and trade.
Food can become contaminated at any point in the production and consumption chain, from agricultural practices to food processing, storage, and preparation. Environmental contamination, such as pollution in water, soil, or air, as well as unsafe food handling practices, contribute to the risk of foodborne illnesses.
Foodborne diseases manifest a wide range of symptoms, primarily gastrointestinal issues like diarrhoea, vomiting, and abdominal cramps. However, they can also lead to more severe health complications, including neurological, gynaecological, and immunological disorders. Diarrheal diseases are particularly problematic, disproportionately affecting low- and middle-income countries and children under five years of age. Globally, nearly one in ten people fall ill from contaminated food each year, leading to over 4,20,000 deaths, with significant mortality among young children.
Researchers warn that the more a food is handled and processed, the higher the risk of bacterial contamination. Ultra-processed foods are particularly vulnerable to harbouring bacteria, compared to less processed foods. Listeria outbreaks often stem from inadequate sanitation practices in food processing plants. Even with extensive processing, poor sanitation and insufficient monitoring could lead to contamination and the spread of Listeria.
Symptoms and Detection of Listeriosis
In most cases, individuals who consume food contaminated with Listeria do not exhibit symptoms or fall ill immediately. However, the bacteria could remain dormant in their system for up to two months before symptoms appear, making it challenging to trace the infection back to the specific food source, as the connection between consumption and illness may not be immediately apparent.
Different symptoms caused by the infection include fever, chills, muscle aches, nausea, and diarrhoea. Some people may experience headaches, stiff necks, confusion, loss of balance, and convulsions. The incubation period is usually short (one to two weeks) but can vary between a few days and up to 90 days.
People with weak immune systems, pregnant women, their babies, and older adults, aged above 65 years, are more vulnerable to listeriosis infections. Listeria could cause serious complications, including hospitalisation and death, in these groups. Pregnant women are 10 times more likely to get infected, which could lead to pregnancy loss, premature birth, or life-threatening infections in newborns. Newborns with listeriosis infections may not want to feed, be fussy, have a fever, vomit, or have trouble in breathing.
According to Dr Michael Rieder, a professor in the department of paediatrics, medicine and pharmacology at Western University, while established food and drink plants take product safety seriously, their monitoring for contaminants like Listeria could be insufficient. Certain foods like unpasteurised milk, raw sprouts, deli meats, soft cheeses, and smoked seafood are more likely to have listeriosis. The current detection methods have not been that efficient. Further, small manufacturers often rely on sending samples to labs, leading to delays in receiving results. Testing frequencies vary, with some plants testing monthly or weekly, primarily for verification purposes. There has to be more in-facility testing to obtain real-time results. To address this issue, Dr Rieder has developed a Listeria test kit, which has been approved by the US FDA and Health Canada, and has currently been working on a swab test to ensure manufacturer environments are bacteria-free.
Treatment for Listeriosis
Treatment for listeriosis depends on how severe it is. If the infection is only in the intestines, symptoms like diarrhoea and vomiting usually would appear within a day or two of eating contaminated food. This type of infection could be treated with antibiotics, including ampicillin, similar to a regular stomach infection. However, if the infection spreads beyond the intestines, it could lead to more severe symptoms, which may appear up to two weeks after consuming the contaminated food, and requires more intensive treatment.
Precautions against Listeriosis
The CDC has recommended that people, especially those in high-risk groups, avoid eating unheated deli meats, cheeses, and salads. Instead, it has suggested the use of packaged items. Deli counters could be contaminated with Listeria, and refrigeration does not kill the bacteria. To be on the safer side, it has advised to heat deli meats to an internal temperature of 165 °F before eating. This would help kill any Listeria that may be present.
In order to prevent the spread of Listeria, it has been advised to throw away any foods that may have touched the contaminated product. Additionally, it has advised to clean and sanitise the refrigerator, cutting boards, countertops, and utensils; mix bleach with hot water and use it to wipe down all surfaces; and dry everything with a clean towel. Further, it is important to wash one’s hands with soap and water for at least 20 seconds before and after cleaning the infected area. Additionally, all patients displaying symptoms, especially those in the high-risk groups, have been advised to seek medical attention immediately.
Conclusion
This outbreak underscores the importance of food safety practices. Public health officials are actively investigating the source of the contamination and advising consumers to be vigilant, especially those in vulnerable groups. The situation highlights the need for continued monitoring and preventive measures to mitigate the risks associated with foodborne illnesses. As the investigation unfolds, it is essential for consumers to stay informed about recalls and health advisories related to refined products.
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