Where does Listeria Bacteria come from and How is Listeriosis prevented?

In the late summer and fall of 1998 Listeria monocytogenes, a gram-positive bacterium that lives everywhere in the world except Antarctica, caused about 40 illnesses in ten different U.S. states.

The CDC in cooperation with state health departments in Connecticut, New York, Ohio, and Tennessee determined that all these cases came from a single strain of the bacterium in a production run of contaminated hot dogs produced by the Bil Mar Food company and sold under several different brand names.

Six of the victims were newborn babies who had gotten the disease from their mothers. Three elderly people and one fetus died. Another 10-state outbreak in 2000 killed 4 people and led to 3 miscarriages, this time caused by turkey meat contaminated with L. monocytogenes.

Salmonellosis is much more common than listeriosis, but listeriosis is more commonly fatal. There are an estimated 1.4 million cases of salmonellosis per year in the United States, and 500 people die from it. There are only about 2,500 cases of listeriosis per year, but of those about 500 die. Almost all listeria infections come from food. The disease can be transmitted by an infected pregnant mother to her fetus, but there is no other direct person-to-person transmission.

As with many food-borne diseases, the victims are most often under 1 year old or over 55 or have immune systems compromised by disease or medical treatment. Healthy adults and children may get infected, but it almost never results in serious illness when they do; they can be infected and have no symptoms at all. If that contaminated turkey and hot dog meat had been heated up properly, the people who ate it probably would not have gotten sick because L. monocytogenes cannot survive cooking or pasteurization.

There are six different species of the genus Listeria, but only monocytogenes causes disease in humans. It’s all over the place, not just in food but also in soil, vegetation, marine sediments, and water, and animals can carry it, too, in their intestines, just as humans can, without any symptoms of disease. It has been isolated not only in meat and cheese, but also in raw fish, cooked crabs, raw and cooked shrimp, and smoked fish. Although it will rarely survive cooking above about 140°F for longer than five minutes, it’s still pretty durable. It can grow in temperatures ranging from 31°F (i.e., colder than your refrigerator) to 113°F, and it will survive a 10 percent salt solution. Even with its ubiquity and durability, it never bothers most of us. But if you are in one of the high-risk groups, you should take precautions to avoid getting sick from it.

Pregnant women are more likely than others to get listeriosis, but no more likely to suffer serious illness than anyone else. Usually it’s very mild, and those who contract it get better. But the illness can cause miscarriages, and babies can be born infected, having caught the disease through the placenta.

In newborns, the disease can be fatal. Therefore, in addition to taking the usual precautions to prevent any food-borne illness, pregnant women and others at high risk should avoid soft cheeses such as feta, Camembert, and Brie, outbreaks of listeriosis have been traced to such foods. Hard cheeses, cream cheese, cottage cheese, and yogurt needn’t be avoided.

Although the risk is not high, pregnant women and others might want to avoid deli meats as well, or at least reheat them thoroughly before eating them. If you get a fever or a stiff neck while you’re pregnant, you should see a doctor because the only way to tell if it’s listeriosis is to test your blood or spinal fluid. If you take antibiotics early enough, infection of the baby can be prevented, although this doesn’t always work. Antibiotics are generally not used in listeriosis cases except when the victim is in one of the high-risk groups, an infant, a pregnant woman, or a person whose immune system is not functioning properly. Others recover without medical intervention.

Prevention is the same with Listeria as with other food-borne germs: cleanliness is next to healthiness. Washing raw vegetables, cooking meat thoroughly, cleaning off utensils and countertops between uses, and washing your hands frequently and especially after leaving the bathroom will prevent many bad things from happening.