Listeria recall: What you need to know about the foodborne bacteria

Listeria can contaminate a wide variety of foods. Here's why it can be a health hazard and how to protect yourself.

Listeria monocytogenes, which is widely referred to as Listeria, can be found in soil and vegetation, sewage and water, as well as the feces of both animals and humans, food safety organizations say.

People who eat foods contaminated with Listeria may carry the bacteria and not develop listeriosis illness.

Listeria made headlines most recently in May 2016, as a recall of frozen produce in Canada, the U.S. and Mexico expanded. 

The Canadian Food Inspection Agency lists the affected products. 

How common is it?

There are about 132 cases of listeriosis reported each year in Canada on average, said Keith Warriner, a professor in the department of food sciences at the University of Guelph. 

"At the moment what we have is better diagnostics than solutions and when you get that imbalance, the only net result is more recalls," Warriner said.

Who is at risk?

Listeria is not considered very dangerous for healthy people, said Lori Burrows, a professor of biochemistry and biomedical sciences at McMaster University in Hamilton, Ont.

During listeriosis outbreaks, health officials often focus on highly vulnerable groups, such as the elderly, infants, patients with a compromised immune system and pregnant women.

Invasive listeriosis is rare but can be deadly. The 2008 listeriosis outbreak traced to ready-to-eat meat, resulted in the deaths of 23 Canadians, the Canadian Food Inspection Agency says.

At least 90 per cent of people who get listeriosis are in highly vulnerable groups, according to the U.S. Centers for Disease Control and Prevention. 

While pregnant women are especially susceptible to listeriosis, their symptoms are typically mild, the American College of Obstetricians and Gynecologists says. The infection can cross the placenta, which can result in complications such as miscarriage, preterm labour and stillbirth and infection in the newborn.

Health Canada has said L. monocytogenes is more likely to cause death than other bacteria that cause foodborne illness. About 20 to 30 per cent of foodborne listeriosis infections in high-risk individuals may be fatal, the department says. 

 How can I tell if my food is contaminated?

Foods that are tainted with Listeria won't look different, or have a distinct taste or smell.

When levels of the bacteria build up on equipment in food processing plants then outbreaks can occur. Listeria can be killed through cooking or pasteurization. 

Researchers have shown that the bacteria can even survive in fridges and freezers.

What are the symptoms?

People may experience a range of symptoms, including: 

• Diarrhea.
• Vomiting.
• Nausea.
• Stomach cramps.
• Severe headache.
• Muscle aches.
• Constipation.
• Persistent fever.
• Stiff neck.
• Loss of balance.
• Convulsions.

Symptoms can come on quickly, or weeks after someone comes into contact with a tainted product.

How does listeria differ from other foodborne bacteria?

Listeria likes to grow in communities called a biofilm on surfaces, said Burrows, the biochemistry and biomedical sciences professor.

"Even though the plants disinfect and they're inspected regularly for contaminants like listeria, if it gets into cracks in the machinery or things like that, it's harder to get rid of it," Burrows said.

Once the contamination is traced in a plant, it is aggressively disinfected — such as by taking equipment apart to get at any cracks where bacteria may have been missed by regular disinfecting. Then regulators will inspect it again and clear production if it meets standards.

How much does it take to make you ill?

Low numbers of listeria, less than about three thousand per serving, are unlikely to cause most humans to become ill unless someone is really severely immune compromised or pregnant, said Prof. Rick Holley, a food safety expert at the University of Manitoba in Winnipeg.

How is listeriosis diagnosed and treated?

For patients with symptoms, health-care providers confirm the diagnosis by isolating the bacteria from blood or spinal fluid that are normally sterile, the CDC says.

How is it treated?

Listeriosis is treated with antibiotics, and people are also encouraged to drink lots of fluids to make sure they don't get dehydrated.

How can I lower my risk?

• Read and follow all package labels on food preparation and storage.
• Wash hands before and after handling foods.
• Use a mild bleach solution to clean and sanitize surfaces and kitchen utensils after preparing raw foods to avoid cross-contamination.
• Clean fruits and vegetables.
• Don't leave food to defrost at room temperature and instead defrost food in the refrigerator, the microwave or in a cold water bath.
• Don't eat hotdogs or frozen vegetables directly out of the package as the fluid within the package may be contaminated. Cook hotdogs until they are steaming.
• Cook meat, poultry and fish thoroughly.
• Avoid raw, unpasteurized milk.
• People in high-risk groups should avoid soft cheeses including brie, camembert, feta and queso blanco fresco, refrigerated pâtés, deli meats and smoked fish.

How are outbreaks investigated?

Health officials match the strain causing outbreaks in multiple locations using a type of barcode of its DNA and checking if it's the same as what is found in patients. The whole genome of the bacterium can also be sequenced to compare its distinctive profile, Burrows said.