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. 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. Called an “opportunistic pathogen,” Listeria is noted to cause an estimated 2,600 cases per year of severe invasive illness.
Ingested by mouth, Listeria is among the most virulent foodborne pathogens, with up to 20% of clinical infections resulting in death. These bacteria primarily cause severe illness and death in persons with immature or compromised immune systems. Consequently, most healthy adults can be exposed to Listeria with little to no risk of infection and illness.
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”.
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. The most widely accepted estimate of foodborne transmission is 85-95% of all Listeriacases.
The infective dose—that is, the number of bacteria that must be ingested to cause illness—is not known. In an otherwise healthy person, an extremely large number of Listeria bacteria must be ingested to cause illness—estimated to be somewhere between 10–100 million viable bacteria (or colony forming units “CFU”) in healthy individuals, and only 0.1–10 million CFU in people at high risk of infection. Even with such a dose, a healthy individual will suffer only a fever, diarrhea, and related gastrointestinal symptoms.
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.
Adults can get listeriosis by eating food contaminated with Listeria, but babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. The mode of transmission of Listeria to the fetus is either transplacental via the maternal blood stream or ascending from a colonized genital tract. Infections during pregnancy can cause premature delivery, miscarriage, stillbirth, or serious health problems for the newborn.
When a person is infected and develops symptoms of Listeria infection, the resulting illness is called listeriosis. For those who do develop symptoms because of their infection, the resulting illness is either mild or quite severe.
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.
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. 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.
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.
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.
The CDC provides a comprehensive list of recommendations and precautions to avoid becoming infected with Listeria, which are as follows:
- Thoroughly cook raw food from animal sources, such as beef, pork, or poultry to a safe internal temperature.
- Rinse raw vegetables thoroughly under running tap water before eating.
- Keep uncooked meats and poultry separate from vegetables and from cooked foods and ready-to-eat foods.
- Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.
- Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
- Consume perishable and ready-to-eat foods as soon as possible.
Recommendations for persons at high risk, such as pregnant women and persons with weakened immune systems, in addition to the recommendations listed above, include:
- Do not eat hot dogs, luncheon meats, cold cuts, other deli meats (e.g., bologna), or fermented or dry sausages unless they are heated to an internal temperature of 165°F or until steaming hot just before serving.
- Do not eat soft cheese such as feta, queso blanco, queso fresco, brie, Camembert, blue-veined, or panela (queso panela) unless it is labeled as made with pasteurized milk. Make sure the label says, "MADE WITH PASTEURIZED MILK."
- Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole, or unless it is a canned or shelf-stable product. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna, and mackerel, is most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky." These fish are typically found in the refrigerator section or sold at seafood and deli counters of grocery stores and delicatessens. Canned and shelf stable tuna, salmon, and other fish products are safe to eat.
Marler Clark, The Food Safety Law Firm, is the nation’s leading law firm representing victims of Listeria outbreaks. The Listeria lawyers of Marler Clark have represented thousands of victims of Listeria and other foodborne illness outbreaks and have recovered over $800 million for clients. Marler Clark is the only law firm in the nation with a practice focused exclusively on foodborne illness litigation. Our Listeria lawyers have litigated Listeria cases stemming from outbreaks traced to a variety of foods, such as lettuce, polony, deli meat, cantaloupe, cheese, celery and milk.
If you or a family member became ill with a Listeria infection after consuming food and you’re interested in pursuing a legal claim, contact the Marler Clark Listeria attorneys for a free case evaluation.