What you need to know about Listeria monocytogenes
September 28th, 2011
12:35 PM ET

What you need to know about Listeria monocytogenes

With an outbreak of sickness linked to  bacteria-tainted cantaloupe now reported in more than a dozen states, here's a quick backgrounder on listeria monocytogenes.

What is it?
Listeria monocytogenes is a “hardy” bacterium that is resistant to extreme hot and cold. This bacterium is pathogenic – meaning that it is infectious to humans, causing the illness listeriosis.

Where does it come from?
Listeria is found in soil, water and the intestines of some animals. Unfortunately, most animals show no symptoms so the bacterium is transferred to raw foods such as unpasteurized dairy products, raw vegetables and raw meats. And unlike similar types of bacteria, listeria can grow in the low temperatures of a refrigerator.

Studies also suggest that up to 10% of humans may be carriers. The FDA says that when listeria gets into a factory environment it can live for several years.

But how did it get inside of cantaloupes?
Cantaloupes and other fruits with porous surfaces can become infected by listeria when soil and/or water seeps through their skin. Any bacterium that remains on the outside can be transferred from the shell to the inside when the fruit is sliced open by a knife or handled.

Who is most at risk?
Listeriosis primarily affects the elderly, pregnant women, newborns, and people with weakened immune systems. In the U.S., an estimated 1,600 people become seriously ill each year; approximately 16% of these illnesses result in death. Cervical infections caused by listeriosis in pregnant women may result in spontaneous abortion during the second or third trimesters or stillbirth.

What are the symptoms of listeriosis?
The early symptoms of listeriosis are similar to those of the flu with nausea, muscle aches and a high fever. Gastrointestinal symptoms such as diarrhea may also appear. The length of time between infection and the appearance of symptoms is unknown.

How can I protect myself?
Recommendations for keeping food safe from listeria are similar to those used to protect against other foodborne illnesses. Remember to cook meat to the USDA’s recommended temperatures and to wash all raw vegetables and fruit. Keep your fridge below 40 degrees Fahrenheit and eat leftovers within three to four days, the CDC says.

I’ve eaten a cantaloupe! What should I do?
Your risk of developing listeriosis is extremely low, even if the cantaloupe you ate was contaminated. If you do not have any symptoms you’re most likely in the clear. However, if you’re in one of the high risk groups and showed symptoms up to two months after eating a possibly contaminated product, contact your doctor.

Has this happened before?
About 800 cases of Listeria infection are diagnosed each year in the United States, along with three or four outbreaks of Listeria-associated foodborne illness. A multi-state outbreak of listeria from contaminated hot dogs and deli meats occurred in 1998, killing 21. Between 1998 and June 2009, 48 deaths from listeria-caused illnesses were reported by the CDC’s Foodborne Outbreak Online Database.

Got a question about the outbreak? Ask it below and we’ll do our best to find an answer.

To learn more visit the FDA's Bad Bug Book or the CDC's section on listeriosis.

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