Listeria is the common name for the pathogenic or disease-causing bacterium known as Listeria monocytogenes.  It is a foodborne illness that when ingested causes an infection known as listeriosis (Cossart & Bierne, 2001).  Approximately 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually (CDC, 2005).

Listeria is ubiquitous in the environment, and can be isolated from wild and domestic animals, birds, insects, soil, wastewater, and vegetation.  The bacterium easily comes into contact with farm animals as it has been found to be present in grazing areas, stale water, and poorly prepared animal feed.  In addition to being present in the environment, Listeria can live in the intestines of humans, animals and birds for long periods of time without causing infection.

Because Listeria is present in nearly every environment – including in some food processing facilities – numerous opportunities for contamination exist during the food production process (Cossart & Bierne, 2001).

Healthcare providers frequently overlook Listeria as a possible cause of illness due to its unusual growth capabilities.  First, laboratories sometimes have a difficult time growing Listeria.  When it is grown, Listeria can be confused with other less harmful contaminants and disregarded.  Second, while most bacteria grow poorly when temperatures fall below 40°F, Listeria survives at temperatures from below freezing to body temperature, and grows best at the 0°F to 50°F range, which includes the temperature range used for freezing and refrigeration.

Due to its unusual growth capabilities, Listeria may be transferred in common ready-to-eat foods that have been kept properly refrigerated.  Thus, Listeria presents many challenges because of its ability to grow in diverse environments.  These host factors, along with the amount of bacteria ingested and the virulence of the strain, determine the risk of disease.

Symptoms of Listeriosis

It is believed that the ingestion of fewer than 1,000 Listeria bacteria can cause human illness.  The incubation period (time between ingestion the onset of symptoms) for Listeria monocytogenes infection, known as listeriosis, ranges from three to seventy days, and averages 21 days (Bryan, 1999).

A person with listeriosis usually experiences fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea.  Five days to three weeks after ingestion of the bacterium, Listeria can access all body areas, including the central nervous system, heart, and eyes (FDA/CFSAN, 2003). If the infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, obtundation, or convulsions can occur. With brain involvement, listeriosis may mimic a stroke.

For unknown reasons, in immune-deficient hosts Listeria invades and grows best in the central nervous system, causing meningitis and/or encephalitis (brain infection). In pregnant women, the fetus is most heavily infected, leading to spontaneous abortion, stillbirths, or sepsis in infancy.

Pregnant women are the most at-risk population for contracting Listeria infection, and are about 20 times more likely than other healthy adults to become ill with listeriosis.  About one-third of Listeria cases occur during pregnancy.  Infected pregnant women ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth (Cossart & Bierne, 2001).  In most cases, the fetus or newborn is more likely than the mother to experience severe listeriosis associated with pregnancy (Silver, 1998); the perinatal and neonatal mortality rate is 80 percent (FDA/CFSAN 2003).

Symptoms of Listeria infection in newborns (Mayo Clinic, 2007):

  • Loss of appetite
  • Lethargy
  • Yellowing of the skin and whites of the eyes (jaundice)
  • Vomiting
  • Skin rash
  • Breathing difficulty

Others more at-risk for contracting listeriosis than the general population include:  people with weakened or compromised immune systems, cancer patients, transplant recipients, diabetics, and persons with AIDS. (CDC, 2005a)

Human cases of Listeria are, for the most part, sporadic and treatable. Nonetheless, Listeria remains an important threat to public health, especially among those most susceptible to this disease. With an increasing immunocompromised population, the risk multiplies. The fact that Listeria is a disease easily transmitted from mother to fetus through the placenta is worrisome to an expectant mother, especially since pregnant women themselves rarely show outward signs of such a devastating infection.

Many physicians overlook the possibility of Listeria food poisoning because they do not know how easily it can survive and grow in refrigerated foods.  For example, recent large outbreaks of Listeria poisoning have been caused by contaminated hot dogs and lunchmeat – foods that had not previously been considered dangerous. More research needs to be done, so that all of the mechanisms and intricacies of this bacterial strain can be understood. Above all, common myths about “proper” food storage need to be updated, so that contamination can be kept at a minimum.

Who is most susceptible to Listeria monocytogenes infection?

While the general public need not be especially concerned with preventing listeriosis, several segments of the population are at risk, and need to be informed of that risk so proper precautions can be taken to prevent listeriosis.

The body’s defense against Listeria monocytogenes and other intracellular pathogens is called “cell-mediated immunity” because it depends on our cells (as opposed to our antibodies), especially lymphocytes called “T-cells.” Therefore, it is not surprising that individuals whose cell-mediated immunity is suppressed are more susceptible to the devastating effects of Listeriosis.

Pregnant women naturally have a depressed cell-mediated immune system; many think that this occurs so that the mother’s immune system will not reject the fetus. In addition, the systems of fetuses and newborns are very immature; they are extremely susceptible to intracellular pathogens. Other adults, especially transplant recipients and lymphoma patients, are given necessary therapies with the specific intent of depressing immune T-cells, and these individuals become especially susceptible to Listeria monocytogenes as well.

Pregnant women are about 20 times more likely than other healthy adults to contract listeriosis. About one-third of listeriosis cases happen during pregnancy. The incidence of listeriosis in the newborn is 8.6 per 100,000 live births (Tappero et al., 1995).  There is no routine screening test for susceptibility to listeriosis during pregnancy, as there is for rubella and some other congenital infections. Newborns, rather than the pregnant women themselves, suffer the serious effects of infection in pregnancy.

In addition to pregnant women, fetuses, and newborns, persons with weakened immune systems due to treatment, particularly transplant recipients (Schuchat, et al., 1992) and persons receiving treatment for lymphoma, but also other cancer victims, are at significantly increased risk for Listeria infection.

  • Persons with AIDS suffer listeriosis 65-145 times more frequently than the general population (Jurado, et al., 1993).
  • Persons who take glucocorticosteroid medications (also called cortisone) are also at increased risk (Schuchat, et al., 1992). The most common medication prescribed in this class is prednisone. The threshold above which prednisone begins to have a significant effect on the immune system is 20 mg per day for 5 days.
  • The elderly and certain debilitated patients (such as those on dialysis or alcoholics) are at minor increased risk for listeriosis.

How is Listeria monocytogenes infection diagnosed?

Symptoms such as fever and a stiff neck could be the result of listeriosis.
If you think you might have listeriosis—especially if you are a pregnant woman—consult your doctor immediately. 

Your doctor can do either a blood or spinal fluid test to determine if you do have Listeriosis.  A blood test is typically the most reliable way to find out if your symptoms are due to listeriosis, but in some cases urine or amniotic fluid (the fluid that surrounds and protects a baby before birth) may be tested as well.

Treatment for Listeriosis

Listeriosis can be treated with several antibiotics.  The antibiotics that have the most activity are ampicillin, gentamicin, and trimethoprim/sulfamethoxizole (Bactrim®, Septa®) (Gilbert, Moellering & Sande, 2001).

When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus (Mayo Clinic, 2007). Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis.

Even with prompt treatment, some infections result in death. This is particularly likely in patients with central nervous system involvement, the elderly, and in persons with other serious medical problems.

Contact a health care provider if you believe you have consumed Listeria monocytogenes-contaminated foods and are experiencing symptoms of listeriosis.

How to prevent Listeriosis

Ready-to-eat foods provide a risk of transmitting Listeria, some more than others. Although this information is available to the public (Jurado, et al., 1993; Pinner, et al., 1992), a conscientious health care provider rendering care for an at-risk individual should point this information out.

  • Avoid soft cheeses (Schuchat, et al., 1992) such as feta, Brie, Camembert, blue-veined, and Mexican-style cheese. (Hard cheese, processed cheeses, cream cheese, cottage cheese, or yogurt need not be avoided.)
  • Cook leftover foods or ready-to-eat foods (Schuchat, et al., 1992) such as hot dogs and deli meat, until steaming hot before eating.
  • Undercooked chicken has been associated with listeriosis (Schuchat, et al., 1992). To make chicken safe from bacterial pathogens, the thickest section (the center of the breast) should reach 165°F.
  • Uncooked fish, whether smoked or not, has been identified as source of Listeria monocytogenes (Weinberg, 1996; CDC, 2005a). Smoked trout, “gravid” fish, sushi, sashimi, and cerviche and such uncooked fish should also be avoided by individuals at risk (Heinitz & Johnson, 1998, Loncarevic, Tham & Danielsson-Tham, 1996).


    * Abbott Cheese Listeria Outbreak


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