10 Common Questions About Listeria and Their Answers


  • What is Listeria?

Listeria (pronounced liss-STEER-ē-uh) is a gram-positive rod-shaped bacterium that can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions. Of the six species of Listeria, only L. monocytogenes (pronounced maw-NO-site-aw-JUH-neez) causes disease in humans. These bacteria multiply best at 86-98.6 degrees F (30-37 degrees C), but also multiply better than all other bacteria at refrigerator temperatures, something that allows temperature to be used as a means of differentiating Listeria from other contaminating bacteria.

  • Where is Listeria found?

Listeria is a common presence in nature, found widely in such places as water, soil, infected animals, human and animal feces, raw and treated sewage, leafy vegetables, effluent from poultry and meat processing facilities, decaying corn and soybeans, improperly fermented silage, and raw (unpasteurized) milk. Foods commonly identified as sources of Listeria infection include improperly pasteurized fluid milk, cheeses (particularly soft-ripened varieties, such as traditional Mexican cheeses, Camembert and ricotta), ice cream, raw vegetables, fermented raw-meat sausages, raw and cooked poultry, and cooked, ready-to-eat (RTE) sliced meats—often referred to as “deli meats”. One study found that, over a five-year period of testing, in multiple processing facilities, Listeria monocytogenes was isolated from 14% of 1,080 samples of smoked finfish and smoked shellfish. Ready-to-eats foods have been found to be a notable and consistent source of Listeria.

  • What is the incidence of Listeria in the United States?

in the United States. [10] Such data revealed an incidence-rate of 0.27 cases per 100,000 persons, a decrease of 42% compared with 1996—1998. But, according to CDC’s Technical Information website, it is estimated that there are 1,600 cases of Listeria infection annually in the United States, based on data through 2008.

  • How is Listeria transmitted?

Except for the transmission of mother to fetus, human-to-human transmission of Listeria is not known to occur. Infection is caused almost exclusively by the ingestion of the bacteria, most often through the consumption of contaminated food.

  • What is the incubation period of Listeria?

The amount of time from infection to the onset of symptoms—typically referred to as the incubation period—can vary to a significant degree. Symptoms of Listeria infection can develop at any time from 2 to 70 days after eating contaminated food.

  • Who is most susceptible to Listeria?

Pregnant women naturally have a depressed cell-mediated immune system. In addition, the immune systems of fetuses and newborns are very immature and are extremely susceptible to these types of infections. Other adults, especially transplant recipients and lymphoma patients, are given necessary therapies with the specific intent of depressing T-cells, and these individuals become especially susceptible to Listeria as well.

  • What are the symptoms of Listeria?

When a person is infected and develops symptoms of Listeria infection, the resulting illness is called listeriosis. Only a small percentage of persons who ingest Listeria fall ill or develop symptoms. For those who do develop symptoms because of their infection, the resulting illness is either mild or quite severe—sometimes referred to as a “bimodal distribution of severity.” On the mild end of the spectrum, listeriosis usually consists of the sudden onset of fever, chills, severe headache, vomiting, and other influenza-type symptoms. Along these same lines, the CDC notes that infected individuals may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. When present, the diarrhea usually lasts 1-4 days (with 42 hours being average), with 12 bowel movements per day at its worst.

  • How is Listeria diagnosed?

Methods typically used to identify diarrhea-causing bacteria in stool cultures interfere or limit the growth of Listeria, making it less likely to be identified and isolated for further testing. On the other hand, routine methods are effective for isolating Listeria from spinal fluid, blood, and joint fluid. Magnetic-resonance imaging (MRI) is used to confirm or rule out brain or brain stem involvement.

  • What are the complications of Listeria?

For those persons who suffer a Listeria infection that does not resolve on its own, the complications (or sequelae) can be many. The most common is septicemia (bacterial pathogens in the blood, also known as bacteremia), with meningitis being the second most common. [4, 18] Other complications can include inflammation of the brain or brain stem (encephalitis), brain abscess, inflammation of the heart-membrane (endocarditis), and localized infection, either internally or of the skin.

Death is the most severe consequence of listeriosis, and it is tragically common. For example, based on 2009 FoodNet surveillance data, 89.2% of Listeria patients ended up in the hospital, the highest hospitalization rate for pathogenic bacterial infection. In persons 50 years of age and older, there was a 17.5% fatality rate—also the highest relative to other pathogens.

  • What is the treatment for Listeria?

Invasive infections with Listeria can be treated with antibiotics. When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus or newborn. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis.