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Listeria: Are You at Risk?
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FAQ on Deadly, Little-Understood Listeria Bug Behind Cantaloupe Outbreak
By Daniel J. DeNoon
Reviewed by Laura J. Martin, MD
Sept. 29, 2011 -- Are you at risk from listeria, the deadly bacteria now in the news?
The death toll is rising in the ongoing outbreak from contaminated cantaloupe. It's now the largest outbreak of food-borne illness in more than a decade.
Suddenly the spotlight is on listeria. What is it? Where is it found? Who's at risk? What can we do about it? What are the symptoms of listeriosis, and how is it treated? Here's WebMD's FAQ.
What Is Listeria?
Listeria monocytogenes leads a double life. It's commonly found in the environment, where it feeds on decaying plant matter. It's found in soil, animal feed, groundwater, and sewage. It can also be carried in the guts of cattle and poultry.
But when listeria gets into humans, it changes form. It becomes a bacterial parasite that lives inside -- and feeds on -- human cells. The disease caused by listeria is called listeriosis.
Because it's spread from cell to cell, it avoids the antibody arm of the immune system. Fortunately, the human cell-mediated immune system usually is very good at seeking out and destroying listeria-infected cells.
People with lowered cell-mediated immunity -- the elderly, cancer patients, people taking immunity-suppressing drugs, and pregnant women -- are particularly vulnerable to listeriosis.
How Do People Get Listeriosis?
By far the most common way people get listeriosis is by eating foods contaminated with listeria.
However, newborns can be directly infected during birth. For every 100,000 U.S. births, there are 8.6 neonatal infections. Listeriosis is one of the most common causes of neonatal meningitis.
What Makes Listeria Dangerous?
"Listeria is a very uncommon cause of illness," Patricia Griffin, MD, chief of the CDC's enteric disease epidemiology branch, tells WebMD.
That said, mild gastrointestinal illness from listeria probably is quite common. Listeriosis -- when listeria escapes the gut and invades the bloodstream -- is rare. The CDC now estimates there are only about 800 listeriosis cases a year in the U.S.
Whether listeria causes illness, Griffin says, depends on a combination of three things: a person's susceptibility, how much listeria a person has been infected with, and the virulence of the particular listeria strain.
Another thing that makes listeria dangerous is that it can survive for a long time, even at refrigerator temperatures.
"Listeria can live in microfilms," Griffin says. Microfilms are sticky mats of bacteria that don't easily wash away.
Microfilms may be one reason why listeria can survive and thrive for years in food processing plants.
But what makes Listeria most dangerous is that once it has entered the bloodstream, it gets into the lymph system and into the brain. Encephalitis and meningitis are major causes of death and disability in people with listeriosis.
Why Are Pregnant Women Susceptible to Listeria?
As pregnancy progresses, a woman's cell-mediated immune responses are suppressed. This makes her body more vulnerable to invasion by listeria, particularly during the third trimester.
Even so, pregnant women only rarely develop serious listeriosis. In one study of listeriosis in pregnant women, about a third of the women had flu-like symptoms, two-thirds had a fever, and about 29% had no symptoms at all.
The real threat is to the fetus. About half of women with listeriosis deliver preterm. About 10% to 20% of cases result in miscarriage, and just over 10% of cases resulted in stillbirth.
Griffin says that of the 72 listeriosis cases in the current outbreak, two involved pregnant women. The status of their pregnancies is not yet known.
What Foods Typically Carry Listeria?
According to the FDA, foods typically linked to listeria food poisoning are:
What Are Listeriosis Symptoms?
People with listeria food poisoning often come down with a case of diarrhea, often with a fever. Over days or weeks, more serious symptoms develop: fever, stiff neck, confusion, muscle weakness, and/or vomiting.
While symptoms may appear as soon as three days after consuming contaminated food, symptoms usually appear in one to three weeks. However, some people become ill two months after eating contaminated food.
What Should I Do If I Bought a Suspect Cantaloupe?
All of the cantaloupes in the current listeria outbreak came from Jensen Farms, a Colorado-based company. Although some of these cantaloupes carry a distinctive sticker, not all contaminated fruit will be marked. Ask your grocer if the cantaloupe you bought is from Jensen Farms.
If you suspect that you have a contaminated cantaloupe, do not try to wash off the listeria. Griffin of the CDC notes that it's not clear whether a listeria-contaminated melon carries listeria on the inside as well as on the outside.
So dispose of suspect cantaloupe in a sealed bag, and make sure it will not be eaten by animals or other people.
But that's not all you should do.
One study found that once a listeria-contaminated food product was in a person's home, 11% of all food samples in their refrigerators also were contaminated. Nearly two-thirds of people with listeria infections turn out to have listeria growing in their refrigerators.
So clean your refrigerator if you think you may have purchased a contaminated cantaloupe. Wash the fridge thoroughly with soap and water. Then wipe it down with a diluted solution of chlorine bleach.
How Is Listeriosis Treated?
Listeriosis patients almost always begin treatment in the hospital. Treatment includes two weeks of antibiotics -- four weeks if a spinal tap shows infection of the spinal fluid.
SOURCES: Patricia Griffin, MD, chief, enteric diseases epidemiology branch, CDC, Atlanta.CDC web site.FDA web site.Lamont, R.F. Journal of Perinatal Medicine, 2011; vol 39: pp 227-236.Freitag, N.E. Nature Reviews Microbiology, published online September 2009.