SOURCES AND SYMPTOMS
While the organism can be found in a variety of raw foods like uncooked meats, vegetables, unpasteurized milk and foods made from raw milk, processed ready-to-eat foods like cold cuts and cut-up vegetables can become tainted after processing. The spread of contamination to other foods in the home can occur when commercial foods with L monocytogenes contaminate food and kitchen surfaces directly or through packaged juices.
The organism is tough. It resists heat (though not high heat), salt, nitrite and acidity. It survives freezing and can even multiply slowly in the refrigerator. Thus, it is extremely important for people at high risk to consider all foods that are not thoroughly cooked just before consumption a potential source of L monocytogenes.
People not at high risk may not be sick at all or may develop just mild flu-like symptoms such as muscle aches, fever, nausea and diarrhea. Those symptoms occur, perhaps more severely, in people at high risk. If the infection spreads to the nervous system, the symptoms can include headache, stiff neck, confusion, loss of balance or convulsions.
Pregnant women are 20 times as likely as other healthy young women to contract listeriosis, probably because in pregnancy the immune system is dampened to prevent rejection of the fetus. A pregnant woman is likely to experience just mild symptoms. But the bacteria can penetrate the placenta and infect the fetus, resulting in possible miscarriage, premature birth or stillbirth. Newborns infected in pregnancy are at risk of dying, as well.
According to the Agriculture Department's Food Safety and Inspection Service, listeriosis cases in the US have been linked to frankfurters, deli and lunch meats, refrigerated pate, salami, brie, Mexican-style soft cheeses, shrimp, butter, raw vegetables and unpasteurized milk. Overseas, outbreaks have also been linked to shellfish, raw fish, smoked seafood, pork tongue, cream, rice salad, coleslaw and rillettes.
One problem with diagnosing listeriosis and detecting its source is that there can be a long delay, possibly 90 days, from consumption of a contaminated food and symptoms. The average time from exposure to illness is 30 days.
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