Updated October 28, 2024
NCCID Disease Debriefs provide Canadian public health practitioners and clinicians with up-to-date reviews of essential information on prominent infectious diseases for Canadian public health practice. While not a formal literature review, information is gathered from key sources including the Public Health Agency of Canada (PHAC), the USA Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and peer reviewed literature.
This disease debrief was prepared by Mayaline Youssef. Questions, comments, and suggestions regarding this disease brief are most welcome and can be sent to nccid@manitoba.ca.
What are Disease Debriefs? To find out more about how information is collected, see our page dedicated to the Disease Debriefs.
Questions Addressed in this debrief:
- What are important characteristics of Listeriosis?
- What is happening with current outbreaks of Listeriosis?
- What is the current risk for Canadians from Listeriosis?
- What measures should be taken for a suspected Listeriosis case or contact?
What are important characteristics of Listeriosis?
Cause
Listeriosis is a serious infection caused by Listeria monocytogenes, a bacterium commonly found in the environment. The bacterium can contaminate various foods and water sources: although cases are relatively rare, outbreaks can be of serious public health concern. Rates range from 0.1 to 10 cases per million people annually, depending on the countries and regions of the world.
Listeria monocytogenes can be present in soil, sewage, and untreated water, and can contaminate fruits, vegetables, fish, meat, seafood, and dairy products. Some foods are more susceptible to contamination, particularly those with a long shelf life that are stored at refrigeration temperatures. High-risk items include unpasteurized dairy products, and ready-to-eat meat products such as pâté, deli meats, frankfurters, smoked salmon, and hot dog wieners. Prepared salads and raw vegetables, such as coleslaw and sprouts, may also contain the bacterium.
Certain groups face a heightened risk of contracting listeriosis. Pregnant women are 20 times more likely to become infected, while individuals with compromised immune systems, including those with HIV, are up to 300 times more vulnerable than healthy adults. Listeriosis symptoms can be particularly severe for these groups, as well as newborns, as they may develop septicemia or meningitis.
While outbreaks of listeriosis are relatively infrequent, the disease’s high fatality rate, long incubation period, and the challenge of identifying the source of contamination make it a critical public health concern. Stringent food safety practices, from proper food handling and preparation, to monitoring and controlling contamination in food production and processing environments, are necessary to prevent listeriosis.
PHAC: Causes of Listeriosis (Listeria)
Signs and Symptoms
There are two main types of listeriosis: non-invasive and invasive.
Non-invasive listeriosis (febrile listerial gastroenteritis) is a mild form of infection that primarily affects otherwise healthy individuals. It is rarely diagnosed because laboratories do not routinely test for L. monocytogenes. Symptoms, such as diarrhea, fever, headache, and muscle pain, begin within 24 hours of consuming contaminated food and usually last 1–3 days. Though generally mild, it can sometimes progress to invasive illness. Outbreaks are often linked to foods containing high levels of L. monocytogenes, with a short incubation period of a few days.
Invasive listeriosis is more severe and occurs when the bacteria spread beyond the intestines. It primarily affects high-risk groups such as pregnant women, cancer patients, individuals with weakened immune systems, the elderly, and infants. The incubation period can range from a few days to 90 days, with symptoms including fever, muscle pain, septicemia, and meningitis. For pregnant individuals, the symptoms may be mild or absent, but the infection can lead to miscarriage, stillbirth, premature delivery, or newborn infections. Non-pregnant individuals often experience more severe symptoms like fever, stiff neck, confusion, and seizures, with a mortality rate of 20–30%. Nearly 1 in 6 non-pregnant individuals with invasive listeriosis die from the infection.
PHAC: Outbreak of Listeria infections linked to recalled plant-based refrigerated beverages
Severity and Complications
Severe cases of listeriosis can lead to brain infections like meningitis or encephalitis, as well as blood poisoning (septicemia). For pregnant women, the infection poses specific risks including miscarriage (loss of the baby before 20 weeks of pregnancy), stillbirth (loss of the baby after 20 weeks), and premature birth. Babies born prematurely due to listeriosis may face life-threatening illnesses shortly after birth.
Complications arising from severe listeriosis can be particularly devastating for vulnerable populations like the very young, elderly, and those with compromised immune systems. These groups face an increased risk of death from the infection. Common symptoms associated with the more severe forms of listeriosis include headache, stiff neck, confusion, loss of balance, and convulsions. The more severe forms of the disease may take anywhere from three days to three months to fully develop. Hospitalization is often necessary for individuals who develop severe complications such as meningitis or septicemia, and death rates in this group are high.
CDC: People at Increased Risk for Listeria Infection
PHAC: Risks of listeriosis (Listeria)
US Food and Drug: Listeria (Listeriosis)
Epidemiology
Listeria has a global distribution. About 1,600 people are infected with Listeria each year in the United States, and approximately 260 people die as a result of the infection: one of the highest mortality rates among foodborne illnesses. In Canada, about 134 cases of invasive listeriosis are reported annually. In Canada, a recent outbreak led to 20 laboratory-confirmed cases of L. monocytogenes in Alberta (1 case), Ontario (13 cases), Quebec (5 cases), and Nova Scotia (1 case). These cases occurred between August 2023 and mid-July 2024, with an illness reporting period of 9 to 29 days. Fifteen people were hospitalized, and three individuals died. The outbreak was linked to plant-based beverages, with 70% of cases occurring in individuals over the age of 50, and 65% of cases affecting females. In October 2024, the Ontario Ministry of Health reported that four people contracted L. monocytogenes after consuming beef tongue.
PHAC: Listeriosis Surveillance Data
FDA: Listeria Contamination in Food Products
Transmission
L. monocytogenes is transmitted through the consumption of contaminated food or water. Pregnant women can transmit the infection to their unborn babies. It can also spread through direct contact with infected animals, particularly in agricultural settings, such as farmers or veterinarians assisting with animal births, or meat processing. Human-to-human transmission is rare.
PHAC: For health professionals: Listeriosis (Listeria)
Diagnosis
The initial diagnosis of listeriosis is based on clinical symptoms and the detection of L. monocytogenes bacteria through smear tests of blood, cerebrospinal fluid (CSF), or samples from newborn meconium, fetal tissue (in abortion cases), feces, vomit, or contaminated food. Reliable diagnostic methods, such as polymerase chain reaction (PCR), are available for detecting L. monocytogenes in humans. During pregnancy, blood and placenta cultures are the most accurate methods to confirm a diagnosis of listeriosis.
In the laboratory, listeriosis can be diagnosed through:
- Cultivation of the organism
- Detection of the infectious agent or its products in tissues or body fluids
Several rapid diagnostic kits based on enzyme-linked immunosorbent assay (ELISA) and PCR technology are commercially available for detecting L. monocytogenes. However, none of these methods have been validated as standard diagnostic tools.
PHAC: For health professionals: Listeriosis (Listeria)
Prevention and Control
Monitoring food recalls and outbreaks, cooking food properly, and following storage guidelines can reduce the risk of listeriosis. For individuals at higher risk, including pregnant women, older adults, and those with weakened immune systems, food safety precautions should be taken to prevent infection. Some overarching guidelines include:
- Avoid deli meats unless they are dried, salted, or heated until steaming hot (165°F).
- Avoid unpasteurized dairy products and soft cheeses such as Brie, Camembert, and blue-veined cheeses.
- Choose canned, shelf-stable, or fully cooked smoked seafood rather than refrigerated smoked seafood.
- Avoid foods like raw or lightly cooked sprouts, and cut melons that have been left out for more than two hours.
Listeria is highly resistant to common food preservation methods such as salt, smoke, and acidic conditions, and can survive and multiply at refrigeration temperatures. Key control strategies include:
- Implementing good hygienic practices (GHP) and good manufacturing practices (GMP) throughout the food production chain to minimize contamination risks.
- Developing food safety management systems based on Hazard Analysis Critical Control Points (HACCP) to identify and manage potential hazards related to L. monocytogenes.
- Conducting regular microbiological testing to validate the effectiveness of control measures.
- Monitoring the environment in food production facilities to identify and eliminate niches where L. monocytogenes can thrive.
Adhering to guidelines like the “General Principles of Food Hygiene to the Control of Listeria monocytogenes in Ready-to-Eat Foods” and utilizing advanced technologies like Whole Genome Sequencing (WGS) can help trace and prevent outbreaks.
CDC: How to Prevent Listeria Infection
WHO: Control Methods and Prevention
PHAC: Prevention of listeriosis (Listeria)
Vaccination
There is no vaccine available to prevent listeriosis.
PHAC: Risks of listeriosis (Listeria)
Treatment of Listeriosis
Listeriosis can be treated effectively if diagnosed early. The choice of treatment depends on the severity and type of illness.
- Rehydration: People experiencing diarrhea should stay hydrated by drinking plenty of fluids to prevent dehydration.
- Antibiotics:
- Invasive Listeriosis: Severe cases, such as those involving meningitis, require antibiotic treatment. If the infection occurs during pregnancy, prompt antibiotic administration can help prevent transmission to the fetus or newborn. Treatment is with ampicillin or amoxicillin combined with gentamicin or erythromycin.
- Intestinal Listeriosis: Most cases of intestinal illness resolve on their own without the need for antibiotics. However, antibiotics may be necessary for patients who are severely ill or at high risk of complications.
PHAC: Treatment of listeriosis (Listeria)
What is happening with the current outbreak of Listeriosis?
The recent outbreak in Canada linked to L. monocytogenes has been connected to plant-based refrigerated beverages from Silk and Great Value brands. A total of 20 confirmed cases have been reported across Alberta (1), Ontario (13), Quebec (5), and Nova Scotia (1). Fifteen people have been hospitalized, and three deaths have occurred. The affected individuals had consumed recalled plant-based beverages prior to falling ill.
In October 2024, the Ontario Ministry of Health reported that four people contracted listeriosis after consuming beef tongue.
CDC: Illnesses and Deaths in Listeria Outbreak
What is the current risk for Canadians from Listeriosis?
According to the Public Health Agency of Canada (PHAC), while the overall risk of listeriosis remains low for most Canadians, certain individuals are at higher risk for severe outcomes. These include pregnant women, older adults, individuals with weakened immune systems, and those with underlying medical conditions.
PHAC advises that Canadians should take special care to avoid high-risk foods, such as unpasteurized dairy products, ready-to-eat deli meats, and certain refrigerated foods, which are prone to contamination by L. monocytogenes. Listeria is particularly dangerous because it can survive in refrigerated environments and on surfaces, potentially contaminating food products even after cleaning.
PHAC: Listeriosis (Listeria) For Health Professionals
What measures should be taken for a suspected Listeriosis case or contact?
Case and Contact Management
Listeriosis is a notifiable disease in Canada, and healthcare providers are required to report confirmed or suspected cases to public health authorities for tracking and investigation. Exposed individuals, particularly those in high-risk categories, should monitor for symptoms like fever, muscle aches, and gastrointestinal issues for up to 70 days. Public health officials may initiate food recalls and facility investigations to prevent further spread of the infection.