Causes

What causes infection with listeriosis?

Listeria is widespread in the environment—it is found in soil, vegetation, water, sewage, some types of livestock feed and in the feces of humans and animals. Animals and humans can carry the bacteria without knowing it.

Plants and vegetables can become contaminated with Listeria from soil, water and manure-based fertilizers. Farm animals that appear healthy may also carry Listeria and contaminate foods such as meats and dairy products.

Unlike most bacteria, Listeria can survive and sometimes grow on foods being stored in the refrigerator. Moreover, foods that are contaminated with this bacteria look, smell and taste normal. Listeria can be killed by pasteurization and proper cooking procedures.

You can become infected with the bacteria by eating or drinking contaminated food or beverages. You can also become sick with listeriosis through cross-contamination during food preparation in the kitchen or in the plant where the food was processed.

About five per cent of healthy adults are carriers of Listeria and have no symptoms.

Symptoms

What are the symptoms of listeriosis?

Like other foodborne illnesses, the symptoms of listeriosis mainly involve the gut. They include

• vomiting,
• nausea,
• cramps,
• muscle aches,
• diarrhea,
• severe headache,
• constipation, and
• persistent fever.

In serious cases, when the infection spreads to the nervous system, symptoms include

• headache,
• stiff neck,
• confusion, and
• loss of balance.

In some instances, these symptoms may be followed by meningoencephalitis (an infection of the brain and its surrounding tissues) and/or septicemia (blood poisoning), either of which can result in death.

Listeriosis can be mild or severe. The mild form of foodborne listeriosis usually begins about 3 days after eating heavily contaminated food. For the more serious form of the disease, the incubation period is generally much longer—up to 70 days after exposure.

How long do the symptoms last?

The length of illness varies depending on its severity.

Risks

Are there certain foods that increase the risk of getting listeriosis?

Some foods are more likely to carry Listeria than others. Those that present a higher risk include raw or contaminated milk, soft cheeses and ready-to-eat meats such as hot dogs, pâté and deli meats. Individuals at high risk, such as pregnant women, the elderly and those with weakened immune systems, should avoid these foods to reduce the risk of becoming infected with listeriosis.

Health Canada offers a Select the Safer Alternative chart.

Are certain people more likely than others to get sick from listeriosis?

Some people face a higher risk of becoming sick with listeriosis than others, or of becoming seriously ill from the disease. Those who are at the highest risk of serious illness include:

Pregnant women and their unborn/newborn children. Pregnant women, especially those in the third trimester, are particularly at risk for Listeria infection. If a pregnant woman develops listeriosis during the first three months of her pregnancy, she may miscarry. Up to two weeks before a miscarriage, pregnant women may experience a mild flu-like illness with chills, fatigue, headache as well as muscular and joint pain. Listeriosis later on in the pregnancy can result in a stillbirth, premature birth or the birth of an acutely-ill child.

Older adults. The risk increases with age.

People with weakened immune systems, such as those undergoing chemotherapy, transplant patients, those with HIV, diabetics and alcoholics. The highest risk group includes those whose immune systems are highly compromised, such as bone marrow transplant patients, patients with cancers of the blood or lymphnodes and those with AIDS. People with AIDS are at least 300 times more likely to get listeriosis than those with a normal immune system.

Listeria is more likely to cause death than other bacteria that cause food poisoning. In fact, 20 to 30 per cent of foodborne listeriosis infections in high-risk individuals may be fatal. However, it should be noted that listeriosis is a relatively rare disease in Canada.

Are people infectious?

It’s not likely that the disease is spread person-to-person. Typically, people become infected with listeriosis by eating contaminated food. Pregnant women can pass the infection on to their unborn baby.

Is listeriosis dangerous?

In some instances, symptoms of listeriosis may be followed by meningoencephalitis (an infection of the brain and its surrounding tissues) and/or septicemia (blood poisoning), either of which can result in death.

The mild form of foodborne listeriosis usually begins about 3 days after eating heavily contaminated food. For the more serious form of the disease, the incubation period is generally much longer—up to 70 days after exposure.

Treatment

What is the treatment for listeriosis?

The disease can be effectively treated with antibiotics, but early diagnosis can be critical to the success of the treatment, especially for those at high risk. There is no vaccine to prevent listeriosis.

Those suffering from illness should drink plenty of fluids to prevent dehydration—a possible serious health effect of a foodborne illness. The most helpful fluids for protecting against dehydration are oral rehydration fluids. These products are sold as pre-mixed fluids and are commonly found in drug stores. Other drinks that do not contain caffeine or alcohol can also help with mild dehydration; however, these drinks may not replace the nutrients and minerals lost during illness.

Young children, the elderly and people with other illnesses are at greatest risk for dehydration. Symptoms of dehydration in adults and children include a decrease in urination, a dry mouth and throat and dizziness upon standing. A dehydrated child may cry with few or no tears and be unusually sleepy or fussy. Severe dehydration can be serious and the ill person may require re-hydration in a hospital. If you think you or someone under your care is dehydrated, contact your healthcare provider.

Prevention

How can listeriosis be prevented?

Proper hygiene and safe food handling and preparation practices are key to preventing the spread of all foodborne illnesses, including listeriosis. You can minimize your chances of contracting these diseases by following these steps:

• Read and follow all package labels and instructions on food preparation and storage.
• After handling foods in the kitchen, especially raw foods such as meat and fish, thoroughly clean and sanitize all surfaces used for food preparation with a kitchen sanitizer (following the directions on the container) or use a bleach solution (add 5 ml household bleach to 750 ml of water), and rinse with water.
• To avoid cross-contamination, clean all knives, cutting boards and utensils used with raw food before using them again.
• Thoroughly clean fruits and vegetables before you eat them.
• Refrigerate or freeze perishable food, prepared food and leftovers within two hours.
• Defrost food in the refrigerator, in cold water or in the microwave, but never at room temperature.
• Keep leftovers for a maximum of four days, but preferably for only two to three days. Reheat leftovers to an internal temperature of 74ºC (165ºF) before eating them.
• Check the temperature in your refrigerator using a thermometer to make sure it is at 4ºC (40ºF) or below. The higher the temperature in your refrigerator, the greater the risk that Listeria may grow in foods. The risk of getting sick increases as the number of bacteria in food rises.
• Wash and disinfect your refrigerator frequently. The more often it is cleaned, the less chance there will be for Listeria to be transferred from contaminated food and surfaces to non-contaminated foods.

Some foods are more likely to carry Listeria than others. Those that present a higher risk include raw or contaminated milk, soft cheeses and ready-to-eat meats such as hot dogs, pâté and deli meats. Individuals at high risk, such as pregnant women, the elderly and those with weakened immune systems, should avoid these foods to reduce the risk of becoming infected with listeriosis.

Health Canada offers a Select the Safer Alternative chart.

Surveillance

Does the Public Health Agency of Canada keep track of listeriosis across the country?

Yes, the Public Health Agency of Canada works with the provinces and territories to track the number of cases of listeriosis across the country.

When people get sick and go to the doctor, those doctors in many cases take samples from the patients and send them to a provincial, territorial or federal lab for testing.

Those labs test the samples to identify the genetic fingerprint of the bacteria, virus or parasite responsible for the illness.

The lab then posts the results for bacterial illnesses on PulseNet Canada, a national network that allows microbiologists to track and share genetic fingerprints for comparison across the country.

All labs then compare their results with those posted on PulseNet to find matches and identify outbreaks.

PulseNet Canada is coordinated by the Public Health Agency of Canada’s National Microbiology Laboratory in Winnipeg.

Are outbreaks of listeriosis common in Canada?

In recent years, about 132 cases of listeriosis on average were reported annually in Canada.

A case refers to illness in one person whereas an outbreak refers to two or more people linked by a common exposure within a specific time frame.

A national outbreak occurs when illness is identified in two or more provinces or territories.