What is listeriosis?

Before the summer of 2008, most Canadians had probably never heard of listeriosis. That's not surprising, since the disease is far less common than other forms of foodborne illness such as Salmonella, E. Coli or the Norwalk virus. There are more than 250 different foodborne illnesses that can be caused by a variety of bacteria, as well as viruses, parasites and toxins. According to the Public Health Agency of Canada, there are as many as 11 to 13 million cases of food-related illness in Canada annually. Of these, listeriosis cases are in the low hundreds.

* Number of cases in 2008 is preliminary and is subject to change and includes the 57 associated with the 2008 outbreak
Source: Public Health Agency of Canada

While listeriosis may be rare, it is a very serious form of foodborne illness. It accounts for only a fraction of all reported foodborne illnesses each year but is responsible for about one-quarter of all deaths resulting from them.

Nearly all cases of listeriosis are thought to be foodborne. The illness results from eating food contaminated with a bacterium called Listeria monocytogenes.

Listeria organisms are widespread in the world around us, including in soil, water and vegetation. Animals and humans can carry the bacterium without knowing it. Even farm animals that appear healthy can carry Listeria and contaminate food such as meats and dairy products. Plants and vegetables can also become contaminated with Listeria from the soil, water and manure-based fertilizers.

Listeria can be present in an assortment of foods we routinely eat, including prepared meats such as cold cuts and hot dogs, soft cheeses and fresh fruit and vegetables. Unpasteurized (raw) milk and foods made from unpasteurized milk may also contain Listeria.

Most of us can consume products containing Listeria monocytogenes without getting ill because our immune systems are strong enough to fight off infection. Healthy adults and children occasionally get infected with Listeria, but rarely become seriously ill.

Listeriosis is the name given to any form of disease or invasive infection caused by Listeria monocytogenes. It primarily affects older people, adults with compromised immune systems and, in about one-third of cases, pregnant women and their newborns. People in these groups are at higher risk of disease because their weakened immune systems make them more susceptible.

One unusual characteristic of Listeria infection, mild or invasive, compared to other foodborne illnesses is that the time between consumption of a contaminated food and the onset of illness is much longer. It ranges from three to 70 days, with a median of three weeks. In contrast, Salmonella infection occurs within 12-72 hours after eating a contaminated food.

The early symptoms of listeriosis can easily be initially mistaken for the flu. However, the illness soon becomes severe enough to send the person to a physician or hospital.

The most common forms of listeriosis are:

• Sepsis (bloodstream infection) - Patients have high fever and appear very ill. Infection of the heart valves can occur and other organs can also become infected
• Infection of the central nervous system (usually meningitis) - symptoms can include high fever, intense headache, neck stiffness, altered consciousness and convulsions
• Miscarriage, still birth or premature delivery of a newborn. The infected pregnant woman may have only a mild flu-like illness herself. If a baby is infected by its mother at the time of birth, it can develop blood stream infection or meningitis

Because the disease does not have specific symptoms, listeriosis is generally not diagnosed until the laboratory reports that Listeria monocytogenes has been cultured from blood, cerebrospinal fluid or amniotic fluid. The treatment for listeriosis is antibiotics, usually given intravenously. Despite treatment, up to 20-30% of cases end in death.

In Canada, the number of people who become seriously ill with listeriosis has been increasing steadily, from 85 cases in 2003 to an estimated 239 cases in 2008. Most cases have been isolated ones and not associated with outbreaks. Listeriosis outbreaks¹ are rare, although Canada has experienced a number over the years

Preventing listeriosis

Listeria organisms are so widespread that it is difficult, if not impossible, to keep them out of the foods we eat.

Listeria is more resistant than most bacteria to sanitation measures and treatments used to control foodborne pathogens, which are micro-organisms that can cause disease. It is very hard to remove the bacterium once it has attached itself to solid surfaces where it continues to flourish. Unlike most pathogens, Listeria can survive and grow - even in vacuum-packed products or in foods kept in cold storage. In fact, studies have shown that Listeria can be found in home refrigerators.

Given that it is so pervasive in our everyday lives, totally eliminating the risk of Listeria infection is not possible. Therefore, it is essential to reduce and manage the risks that contaminated food will be eaten. This is precisely what government regulations and food processing companies' food safety policies and practices are designed to do. These regulatory and manufacturing approaches to making food safe from Listeria contamination will be described elsewhere in this report.

There are also important steps that we can take as individuals to prevent illness. For listeriosis, special measures are required to protect people who are most vulnerable, such as pregnant women, the elderly and those with compromised immune systems. For instance, luncheon meats, deli meats and hot dogs should not be eaten by people in these groups unless they are steaming hot while certain foods, such as unpasteurized soft cheeses, refrigerated pâtés, meat spread or smoked seafood, should be avoided altogether.

Suggested Precautions For High Risk Individuals

Source: Health Canada

Next chapter

"By the end of the 2008 outbreak, listeriosis was confirmed in 57 people and was reported as the underlying or contributing cause of death for 22 of these individuals..."

¹ An outbreak is defined as occurring when two or more people experience similar illness after consuming food from a common source.