Listeria (Listeriosis)

Picture of Listeria monocytogenes, the bacterium that causes listeriosis; SOURCE: CDC/Dr. Balasubr Swaminathan; Peggy Hayes

• Listeriosis is a disease caused by infection with the bacterium Listeria monocytogenes, and it is most often contracted after eating contaminated foods or liquids.
• Listeriosis most commonly affects newborns, the elderly, pregnant women, and those individuals with a poorly functioning immune system (immunocompromised).
• Most healthy individuals who come in contact with Listeria monocytogenes will have either no symptoms or a self-limiting mild gastrointestinal illness.
• In the United States, most cases occur during the summer months.
• The symptoms of listeriosis may include diarrhea, nausea, vomiting, fever, and muscle aches. More serious illnesses may lead to sepsis, meningitis, and death.
• Listeriosis is diagnosed by culturing and isolating Listeria monocytogenes from stool, cerebrospinal fluid, blood, amniotic fluid, or the placenta. A presumptive diagnosis of listeriosis can be made based on the patient's symptoms in the setting of exposure to contaminated food during a listeriosis outbreak.
• The treatment of listeriosis involves intravenous antibiotics and supportive care.
• The prognosis for individuals with listeriosis depends on several factors. Though most cases carry an excellent prognosis, those patients with underlying risk factors and severe disease are at risk for significant morbidity and mortality.
• Several measures can be taken to prevent infection with Listeria monocytogenes, including proper food handling and preparation, as well as the avoidance of certain high-risk foods and liquids.

Listeriosis is a disease caused by infection with a gram-positive, rod-shaped bacterium called Listeria monocytogenes.

• Listeria monocytogenes is ubiquitous, and it is usually found in soil, water, and decaying vegetation. Many farm animals and other domestic and wild animals can harbor the bacterium. Though many of these animals may be asymptomatic carriers of the bacterium, they can serve as a source to contaminate the foods produced from them, such as meat and dairy products. Listeria monocytogenes may also enter food-processing factories and contaminate food contact surfaces and non-food contact surfaces (for example, floors or drains).
• Listeria monocytogenes may contaminate various foods and dairy products, for example, raw vegetables and fruits, uncooked meats, packaged and processed meats (for example, hot dogs or deli meats), smoked seafood, soft cheeses, and unpasteurized milk/dairy products.
• Listeriosis is most often a foodborne illness that is transmitted to humans after they ingest foods or liquids contaminated with Listeria monocytogenes.
• Human-to-human transmission occurs when an infected pregnant mother transmits the infection to her fetus/newborn via the placenta or during delivery.
• In 2011, contaminated cantaloupes from a Colorado farm caused 146 cases of listeriosis with 32 fatalities, making it the deadliest food-borne illness outbreak in the United States since the early 1900s.

Listeriosis is a disease that rarely affects healthy individuals. However, several risk factors do exist for acquiring and developing listeriosis.

• Eating or drinking food products contaminated with Listeria monocytogenes is the greatest risk factor for developing listeriosis.
• Certain patient populations are at greater risk for developing listeriosis:
  • Newborns
  • Elderly
  • Pregnant women
  • Individuals with a poorly functioning immune system (for example, patients with AIDS, cancer, diabetes, chronic kidney disease, alcoholics, or those taking immunosuppressive medications)

The symptoms and signs of listeriosis can vary widely, and the clinical presentation often depends largely on the underlying state of health and age of the affected individual. Most healthy individuals who become infected with Listeria monocytogenes experience no symptoms at all, though rarely some may develop a mild self-limiting gastrointestinal illness. However, it is the high-risk patient populations who generally go on to develop the more severe form of the disease. There can be a substantial delay between the time of exposure to Listeria monocytogenes and the development of symptoms (the incubation period), varying anywhere from several days to two to three months.

• Diarrhea, nausea, vomiting, fever, and muscle aches are common symptoms associated with listeriosis. Often, these symptoms will resolve spontaneously after seven to 10 days.
• If the infection spreads to the central nervous system, individuals may experience headaches, stiff necks, confusion, loss of balance, or seizures. These symptoms may be seen with meningitis, encephalitis, or brain abscess.
• Infected pregnant women may experience symptoms of a mild flu-like illness. However, there is a risk of miscarriage, stillbirth, premature delivery, or sometimes a life-threatening neonatal infection after birth (pneumonia, sepsis, and meningitis, for example).
• Listeriosis during pregnancy usually occurs during the third trimester.
• In the United States, approximately a third of all cases of listeriosis occur in pregnant women.
• Rarely, infection with Listeria monocytogenes can lead to localized infections of the skin, heart, joint, or bone.
• Death from listeriosis generally occurs from a disseminated infection in high-risk individuals.

Diagnosing listeriosis promptly can be challenging, as it can initially present clinically similar to many other gastrointestinal infections. The patient's history can be critical, as it may provide information about exposure to certain food products known to harbor Listeria monocytogenes. Making a clinical diagnosis can be facilitated if there is a known outbreak of listeriosis.

The definitive diagnosis of infection with Listeria monocytogenes is confirmed by culturing and isolating the organism from blood, cerebrospinal fluid, amniotic fluid, or the placenta on specialized laboratory media. Isolating the specimen from stool samples is unreliable, as is serologic testing. Imaging studies, such as a CT scan or MRI of the brain, may be ordered to detect a brain abscess, for example. A spinal tap (lumbar puncture) to obtain cerebrospinal fluid may also be performed if there is suspicion of central nervous system infection.

Treatment for listeriosis includes intravenous antibiotics, as well as supportive care. The prompt initiation of antibiotics when the diagnosis is suspected or confirmed can hasten recovery and prevent the more serious potential complications sometimes encountered with listeriosis.

• Antibiotics
  • Ampicillin (Principen) is generally considered the antibiotic of choice, though there are other acceptable antibiotic choices.
  • Involve an infectious disease specialist to assist with appropriate antibiotic selection and duration of treatment.
  • The duration of treatment with antibiotics varies with the severity of the illness and the particular areas involved with disseminated infection.
• Supportive care
  • Intravenous fluids to prevent dehydration or to maintain adequate blood pressure may be necessary.
  • Intravenous medications for nausea and/or vomiting may be administered.
  • Patients with low blood pressure may require intravenous medications to increase their blood pressure (pressors).
  • Patients with severe listeriosis may require mechanical ventilation (breathing machine) for respiratory support.

Most authorities believe that individuals, even those at high risk, who ingest food products contaminated with Listeria monocytogenes do not require treatment if they do not have any signs or symptoms of infection. However, careful consideration must be taken in the pregnant patient, as listeriosis can be potentially devastating to the fetus and newborn.

Most individuals who ingest food products contaminated with Listeria monocytogenes will experience no symptoms (asymptomatic) and have an excellent prognosis.

For high-risk individuals (and the rare healthy individual) who develop listeriosis, the prognosis depends on many factors, such as the underlying state of health when infected and the severity of illness upon presentation to a health care professional. The prompt recognition and diagnosis of the illness are also important, as the timely initiation of intravenous antibiotics can also affect the prognosis and outcome. Nonetheless, even with prompt diagnosis and treatment, some cases of listeriosis are fatal. The overall mortality rate for clinical infections with Listeria monocytogenes is 20%-30%.

Several measures can be taken to prevent contact with foods and liquids potentially contaminated with Listeria monocytogenes. The Centers for Disease Control and Prevention (CDC) recommends the following:

• Rinse raw produce, such as fruits and vegetables, thoroughly under running tap water before eating, cutting, or cooking. Even if the product will be peeled, it should still be washed first.
• Separate uncooked meats and poultry from vegetables, cooked foods, and ready-to-eat foods.
• Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
• Clean up all spills in your refrigerator right away -- especially juices from hot dog and lunchmeat packages, raw meat, and raw poultry.
• Thoroughly cook raw food from animal sources, such as beef, pork, or poultry to a safe internal temperature.
• Use precooked or ready-to-eat food as soon as you can.
• Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.

Recommendations for people at higher risk, such as pregnant women, people with weakened immune systems, and older adults in addition to the recommendations listed above, include the following:

• Do not eat hot dogs, luncheon meats, cold cuts, other deli meats (for example, bologna), or fermented or dry sausages unless they are heated to an internal temperature of 165 F or until steaming hot just before serving.
• Avoid getting fluid from hot dog and lunchmeat packages on other foods, utensils, and food preparation surfaces, and wash hands after handling hot dogs, luncheon meats, and deli meats.
• Do not eat refrigerated pâté or meat spreads from a deli or meat counter or from the refrigerated section of a store. Foods that do not need refrigeration, like canned or shelf-stable pâté and meat spreads, are safe to eat. Refrigerate after opening.
• 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.
• 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.