Facts About Listeria

Listeria

There are six different species of the bacterium Listeria. Only one of these species Listeria monocytogenes (L. monocytogenes) causes human illness – the others are usually harmless for humans. Listeria can be found almost everywhere and has been isolated from a number of sources, including soil, water and foods. It has evolved the ability to survive in a variety of different environmental conditions, including moist environments like refrigerators, and under a variety of stress conditions.

Strains

Within L. monocytogenes, a multitude of different strains (e.g. more than 300 strains identified in one study) have been documented. Strains can be defined by a variety of methods, including a process known as pulsed-field gel electrophoresis (PFGE) which gives different “genetic fingerprint” patterns.

Listeria and Food

Listeria can be found in unprocessed food such as raw dairy products, meat, poultry, fish, as well as processed foods such as deli meats, hot dogs, cheese and ice-cream. It is sometimes found in raw vegetables. It can also be spread with an infected product or surface, such as hands or kitchen counters during food preparation.

Listeriosis

This is the serious infection caused by eating food containing L. monocytogenes. Listeriosis usually occurs only in people with weakened immune systems (e.g. AIDS patients, organ transplant recipients, cancer patients), the elderly, pregnant women, and newborns. The infection can begin with fever and gastrointestinal symptoms, but can spread to the blood stream and/or the nervous system giving symptoms of meningitis. In pregnant women, L. monocytogenes can also spread to the fetus, causing premature delivery, miscarriages or infections of the newborn baby.

Incubation Period

The incubation period can be as much as 70 days after exposure for the more serious forms of Listeriosis, however symptoms usually appear within two to 30 days. For the milder forms, it can be as little as one day.

Incidence of Illness

The disease listeriosis is very rare, affecting an estimated 1-5 in 1,000,000 people per year in most developed countries. In people contracting the disease it can be very serious - an estimated 20% of people with this disease die due to listeriosis. Pregnant women are 20 times more likely to get listeriosis (as compared to healthy individuals), and account for about a third of all listeriosis cases. Persons with AIDS are about 850 times more likely to get listeriosis, as compared to healthy individuals.

Consumer Risk

Even if L. monocytogenes is absent in ready-to-eat meat and poultry products after processing, or found at very low prevalence and levels, additional contamination can occur after the packs are opened - especially when the meat is handled. Although L. monocytogenes can grow while refrigerated, growth is very slow at temperatures below 4ºC. In ready-to-eat foods stored at higher temperatures, for example above 7ºC, growth occurs more rapidly, increasing the risk that L. monocytogenes will reach levels more likely to cause human illness. Both safe food handling and maintaining proper refrigeration temperatures are critical to minimizing the risk of listeriosis.

Cooked Meat

Listeria is killed by cooking. Thoroughly cooking product to 165ºF/74ºC will kill the bacteria. Consumers at high risk for contracting listeriosis (e.g. pregnant women and the elderly) should reheat deli meats immediately before consumption.

Freezing

Listeria is not killed by freezing. Growth is arrested altogether, but normal growth will be resumed after thawing.

Deli Plants

Listeria is everywhere, so elimination is out of the question. Listeria is expected to enter any meat plant with raw materials and personnel and once inside, it can live almost anywhere – on floors, drains, cooling, ventilation, slicing and packaging equipment. The strategy for the food industry remains one of risk mitigation driven by robust surveillance and sanitation programs. Regular cleaning and sanitation is needed to prevent growth and survival of this organism, including disassembly and deep cleaning in harbourage points of equipment, and contamination of the surfaces in contact with ready-to-eat meats.

Surveillance

Control of Listeria in a manufacturing plant is monitored by a program of regular swabbing and sampling from the plant “environment”. There are standard remedial procedures for immediately re-testing any sites that test positive for Listeria species, including supplementary cleaning protocols. Environmental testing is industry best practice to detect and manage Listeria in a food processing plant. It is more informative than finished product testing, as it points to the source as a target for further sanitation.