Listeriosis is caused by one species in a group of bacteria called Listeria monocytogenes found in cow's milk, animal and human feces, soil, and leafy vegetables. In the past 10 years there have been several outbreaks that seem to have been linked to eating deli meats and soft cheeses such as feta, blue-veined cheeses, and some types of Mexican cheese. one recent study found that 2 percent of hot dogs tested contained the bacterium L. monocytogenes. Children with impaired immune systems also can catch the disease from undercooked chicken.
The bacteria is remarkably tough, resisting heat, salt, nitrite, and acidity much better than many other organisms. It can survive on cold surfaces and can multiply slowly at temperatures as low as 34°F. Freezing the food will stop the bacteria from multiplying, and commercial pasteurization will eliminate the organism in dairy products. Listeria does not change the taste or smell of food.
When Listeria is found in processed products, the contamination probably occurred after processing, rather than due to poor heating or pasteurizing.
Babies can be born with listeriosis if their mothers ate contaminated food during pregnancy. Pregnant women are 20 times more likely than other healthy adults to get the disease; about a third of all cases occur during pregnancy. However, it is new-borns rather than their mothers who suffer the most serious effects of infection during the pregnancy.
If the fetus is affected early in the pregnancy, the baby will probably be born prematurely. Infants in this situation are usually quite ill, with low birth weight, breathing problems, blue skin, and low body temperature. If the baby survives, there may be a bloodstream infection or meningitis. Half of these babies infected with Listeria die, even if treated.
Fetuses affected later in the pregnancy may be carried to term and be born at normal weight. If infected during delivery, the infants may develop meningitis; if so, 40 percent die. Some surviving babies who contract meningitis may have permanent brain damage or mental retardation.
Most normal, healthy children who become infected with the bacteria suffer few symptoms, or may experience a flu-like illness with fever, muscle aches, and nausea or diarrhea. If infection spreads to the nervous system, however, it can cause a type of meningitis.
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