A.        Sources, Characteristics, and Identification

Listeria is the common name for the pathogenic or disease-causing bacterium known as Listeria monocytogenes.  It is a foodborne bacterium that, when ingested, causes an infection known as listeriosis.[1]  Approximately 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually.[2]

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.[3]

Listeria prevention—particularly in food processing facilities—is both a necessary and obvious concern.  To this end, food processing facilities must follow sanitary procedures designed to prevent Listeria contamination; the failure to do so is careless in the face of such a pervasive threat to food safety.

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.

B.        Who is most susceptible to Listeria monocytogenes infection?

Several segments of the population, including pregnant women and those with compromised immune systems, are particularly at risk for listeriosis.  The body’s defense against Listeria and other intracellular pathogens is called “cell-mediated immunity” because it depends on the body’s cells (as opposed to 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[4] and lymphoma patients, are given necessary therapies with the specific intent of depressing immune T-cells, and these individuals become especially susceptible to Listeria as well.  The elderly and certain debilitated patients are at increased risk for listeriosis.

C.        Symptoms of Listeriosis

It is believed that the ingestion of fewer than 1,000 Listeria bacteria can cause human illness.  The incubation period—i.e., the time between ingestion of bacteria and the onset of symptoms—for Listeria infection ranges from 3 to 70 days, and averages 21 days.[5]

A person with listeriosis usually experiences fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea.  Five days to three weeks after ingestion, Listeria bacteria can invade all body areas, including the central nervous system, heart, and eyes.[6]  If the infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, reduced alertness (obtundation), and convulsions can occur.  With brain involvement, listeriosis sometimes mimics a stroke.

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 the devastating infection.  Even with prompt treatment, some infections result in death.  This is particularly likely in the elderly and in persons with other serious medical problems.