Listeriosis is a foodborne illness. It most commonly affects newborns, people with weakened immune systems, seniors, and pregnant women. Pregnant women are at particular risk of having listeriosis, as they are 20 times more likely to acquire the disease than other healthy adults.

Cases of listeriosis are usually infrequent, although there have been several known outbreaks in the past. Listeriosis is a rare disease but it is very serious since it is more likely to be fatal in severe cases, compared to other bacteria that cause food poisoning.

Listeriosis is caused by infection with the bacterium Listeria monocytogenes. The bacterium is found in various places in the environment – in animal feed, animal and human stool, plants, and soil. Listeria infection occurs by eating food contaminated by the bacteria. Contamination can occur at any point in the farming, distribution, and food preparation process.

Listeria is different from other bacteria that cause food poisoning because it can survive and continue to grow even when in the refrigerator. Foods contaminated with listeria look, smell, and taste normal. Listeria can be killed by proper cooking methods.

Types of food commonly contaminated by listeria include dairy products, fish, meat, and vegetables.

Not everyone who is infected with listeria will develop listeriosis. Infants, people older than 50 years old (and especially over 65), or people with an impaired or weakened immune system are most likely to be affected. Symptoms of the disease may appear suddenly, as soon as 1 day after or up to 90 days after eating food contaminated with listeria.

Generally, milder forms of listeriosis will cause symptoms much sooner than more serious forms of the disease.

Symptoms of listeriosis include:

• constipation
• cramps
• diarrhea
• headache
• mild flu-like illness (e.g., chills, fatigue, muscle and joint pain)
• nausea
• persistent fever
• vomiting

Rarely, very severe forms such as meningoencephalitis (an infection of the brain and the surrounding tissues) or bacteremia (where the bacteria are present in the blood) may follow.

Listeriosis can be particularly harmful during pregnancy. A mother who is infected during the first 3 months of pregnancy may miscarry. Listeria can also be passed from a pregnant woman to her unborn baby. Acute illness, premature birth, and stillbirth are possible if the mother is infected later in pregnancy. Newborns may also have low birth weight, meningitis, or overwhelming sepsis.

Listeriosis is diagnosed by laboratory testing of blood, cerebrospinal fluid, or stool samples. Because the symptoms of listeriosis closely resemble those caused by the flu and by other foodborne illnesses, many people are unaware that they have it. This, in addition to the potential severity of the disease, makes the prevention of listeriosis of even greater importance.

Once diagnosed, listeriosis can be treated using antibiotics. There is no vaccine for listeriosis. However, listeriosis can be prevented by following proper food handling practices.

Protect yourself from listeriosis by following these food safety tips:

• Avoid eating soft cheeses (e.g., Brie, Camembert) or refrigerated meat spreads or pâtés.
• Avoid eating unpasteurized dairy products.
• Clean utensils and working area used to prepare food with a diluted bleach solution. To prepare the solution, mix 1 teaspoon (5 mL) of bleach with 3 cups (750 mL) of water.
• Defrost food in cold water, the microwave, or the refrigerator, but never at room temperature.
• Keep raw meat separate from other foods in your shopping cart while at the store, in the refrigerator, and during preparation. This will help prevent cross contamination.
• Regularly clean and disinfect your refrigerator. Listeria can survive and continue to grow even when in the refrigerator. Cleaning the refrigerator more frequently will reduce the risk of transferring Listeria from contaminated food and surfaces to other foods.
• Reheat leftovers and ready-to-eat foods like hot dogs until steaming hot.
• Thoroughly cook food before serving to kill listeria, although this does not guarantee that it is safe to eat if proper food handling processes after cooking were not followed (e.g., the cooked food is cross-contaminated by infected, raw food).
• Wash all raw fruits and vegetables before eating.
• Wash your hands frequently, especially before and after handling food.