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Listeria monocytogenes is a Gram-positive bacterium, motile by means of flagella. Some studies suggest that 1-10% of humans may be intestinal carriers of L. monocytogenes. It has been found in at least 37 mammalian species, both domestic and feral, as well as at least 17 species of birds and some species of fish.  L. monocytogenes can be isolated from soil, silage, and other environmental sources. The pathogen is also able to resist, in part, the deleterious effects of freezing, drying and heat.

Listeriosis is an infection caused by eating food contaminated with the pathogen. The disease affects primarily persons of advanced age, pregnant women, newborns, and adults with weakened immune systems.  Symptoms typically include fever, muscle aches and sometimes gastrointestinal symptoms such as nausea or diarrhea. In rare cases, when the infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur. Although infected pregnant women will typically experience only mild flu-like syptoms, infections during pregnancy can in rare cases lead to miscarriage or stillbirth. When an infection does occur during pregnancy, antibiotics can often prevent infection of the fetus or newborn.

Listeria monocytogenes is typically found in soil and water. Vegetables can become contaminated from the soil or from fertilizer. Animals can carry the bacterium without appearing ill, which can lead also to the contamination of meats and dairy products. The pathogen has also been associated processed foods that become contaminated after processing, such as soft cheeses and cold cuts at the deli counter. Unpasteurized (raw) milk, or foods made from unpasteurized milk, can also become contaminated with Listeria. Although the pathogen is killed by pasteurization and cooking, certain ready-to-eat foods such as hot dogs and deli meats can become contaminated prior to packaging.

Preventative Measures include:

• Thoroughly cook raw food from animal sources, such as beef, pork, or poultry;

• Wash raw vegetables thoroughly before eating;

• Keep uncooked meats separate from vegetables and from cooked foods and ready-to-eat foods;

• Avoid unpasteurized (raw) milk or foods made from unpasteurized milk;

• Wash hands, knives, and cutting boards after handling uncooked foods; and

• Consume perishable and ready-to-eat foods as soon as possible.

Additional recommendations for high risk individuals, such as pregnant women and persons with weakened immune systems, include:

• Avoid hot dogs, luncheon meats and deli meats, unless they are fully cooked;

• Avoid getting fluid from hot dog packages on other foods, utensils, and food preparation surfaces, and wash hands after handling hot dogs, luncheon meats, and deli meats;

• Avoid soft cheeses such as feta, Brie, and Camembert, blue-veined cheeses, or Mexican-style cheeses such as queso blanco, queso fresco, and Panela, unless they have labels that clearly state they are made from pastuerized milk;

• Avoid refrigerated pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads may be eaten; and

• Avoid smoked seafood, unless it is contained in a cooked dish, such as a casserole. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna or mackerel, is most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky." The fish is found in the refrigerator section or sold at deli counters of grocery stores and delicatessens. Canned or shelf-stable smoked seafood may be eaten.

References:

www.cdc.gov/nczved/dfbmd/disease_listing/listeriosis_gi.html

www.cfsan.fda.gov/~mow/chap6.html

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