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 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 be designed properly and 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, individuals with compromised immune systems, and the elderly 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.  Individuals at increased risk for listeriosis therefore include:

  • Pregnant women: They are about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy.
  • Newborns: Newborns rather than the pregnant women themselves suffer the serious effects of infection in pregnancy.
  • Persons with weakened immune systems
  • Persons with cancer, diabetes, or kidney disease
  • Persons with AIDS: They are almost 300 times more likely to get listeriosis than people with normal immune systems.
  • Persons who take glucocorticosteroid medications (such as cortisone)
  • The elderly

The risk of morbidity and mortality from Listeria infection is particularly acute for the elderly, as is evidenced by the astonishing death toll that the cantaloupe outbreak exacted.[5]  The occurrence of bacterial infection is a function of several major variables:  (1) the virulence of the bacterial pathogen, that is, its ability to cause severe disease; (2) how the pathogen is transmitted to the “host”—for example, whether it is airborne, foodborne, blood borne, etc.; and (3) host susceptibility—i.e., how well the host can defend itself against the bacterial pathogen.  Increased susceptibility, in turn, may result from two different processes:  a bigger infectious dose in a given case of disease may cause a more severe infection, and physical characteristics particular to an individual host may render him or her less able to limit the spread of infectious microorganisms from the intestinal tract to the bloodstream.

Morbidity and mortality in the elderly from infectious disease, generally, is far greater than in other populations.  For instance, death rates for infectious diarrheal disease alone are five times higher in people over 74 years of age than in the next highest group, children under four years of age, and fifteen times higher than the rates seen in younger adults.  Published studies attribute the elderly’s heightened risks, both of infection and mortality due to enteric infectious disease, to several factors:  (1) the aging of the gastrointestinal tract (reduced gastric acidity/reduced gastric motility); (2) a higher prevalence of underlying medical disorders (co-morbidity factors); and (3) malnutrition and a decline in the immune response that leaves the host less able to defend itself against infectious agents.

1.              Aging of the Gastrointestinal Tract—An Invitation to Infection

Inflammation and shrinkage of the gastric mucosa increase with age.  These changes lead to low gastric acidity.  In patients with gastric ulcer disease, the drugs used to treat the condition further block gastric acid production.  Because stomach acids play an important role in limiting the number of bacteria that enter the small intestine, low gastric acidity increases the likelihood of infection if a pathogen is ingested with food or water.

Gastrointestinal motility (peristalsis) decreases with age. Peristalsis, which is the mechanism that propels the stomach contents through the intestinal tract, is also the mechanical means for removing ingested, life-threatening pathogens.  The risk of infection by potentially invasive pathogens corresponds with the duration of contact between the pathogen and the intestinal mucosa.  Thus, a decrease in peristalsis delays the clearance of the pathogen from the intestinal tract and contributes substantially to the increased prevalence and severity of infection in the elderly.

2.         A Higher Prevalence of Underlying Medical Conditions—Co-Morbidity Factors

Underlying medical conditions or disease (co-morbid factors) also contribute to the morbidity and mortality of infection in the elderly.  Among hospitalized patients, those older than 65 develop pneumonia twice as often as younger patients due to poor nutrition, neuromuscular disease (poor cough reflex and aspiration), pharyngeal colonization, depressed level of alertness, endotracheal intubation, intensive care unit admission, nasogastric tube use, and antacid use.  Pneumonia is the leading infectious cause of death in the elderly.

Atherosclerosis, another common co-morbid disease, compromises circulation and blood flow to the peripheral tissues and the skin, particularly in elderly individuals who are hospitalized and bedridden with an infectious illness.  Unfortunately, it is the skin and the previously discussed mucous membranes that serve as the body’s first line of defense against invasion by infectious microorganisms.  Loss of the integrity of the skin may result in the development of pressure ulcers, which are warm, moist mediums for infectious microorganisms to rapidly multiply and are associated with a number of infectious complications.

When an infectious microorganism, regardless of source, gains access to the bloodstream, the patient may develop systemic sepsis, also know as bacteremia.  Bacteremia is most common in people who are already affected by, or are being treated for, some other medical problem (co-morbid disease); conversely, people in good health with strong immune systems rarely develop bacteremia.  The main sources of bacteremia in elderly patients are the urinary tract, gastrointestinal tract, respiratory tract, and the skin.  Other potential sources include surgical wounds, invasive tubes and catheters, intravenous lines—virtually any site where an invasive medical procedure has occurred.  Bacterial organisms most likely to cause bacteremia include members of the Staphylococcus, Streptococcus, and Escherichia coli genera.  Because bacteremia is far more prevalent in those with co-morbid conditions, which group is substantially populated by the elderly, the presence of co-morbid conditions is clearly a determinant of the mortality associated with infectious disease.

3.         A Weakened Immune System—the Inability to Fight off Infection

With advancing age come progressive weakness, decline, and dysfunction of the immune system.  Many of the body’s natural physiologic responses to infection are therefore blunted in the elderly; and the intensity of many clinical signs and symptoms in an elderly patient with an infectious process are heightened when compared to those in a younger person.  This age-related decline contributes significantly to the increased risk of severe illness and mortality in elderly persons with infectious disease.

The effect of a weakened immune response on the health of an elderly person often manifests most apparently during periods of intense stress (e.g., surgery, sepsis, multiple organ failure, malnutrition, dehydration).

C.        Symptoms of Listeriosis

It is believed that the ingestion of fewer than one-thousand 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 three to 70 days, and averages 21 days.[6]

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.[7]  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 immune-compromised 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.

Bill Marler is an accomplished personal injury and products liability attorney. He began litigating foodborne illness cases in 1993, when he represented Brianne Kiner, the most seriously injured survivor of the Jack in the Box E. coli O157:H7 outbreak. Bill settled Brianne’s case for $15.6 million, creating a Washington state record for an individual personal injury action.

[1]           Cossart P, Bierne H.  (2001).  The use of host cell machinery in the pathogenesis of Listeria monocytogenes.  Curr Opin Immunol (England).  13(1):96-103.

[2]           CDC.  (2005).  Listeriosis Technical Information.  Retrieved on March 19, 2009 from Centers for Disease Control and Prevention Website, http://www.cdc.gov/nczved/dfbmd/disease_listing/listeriosis_gi.html.

[3]           Cossart P, Bierne H.  (2001).  The use of host cell machinery in the pathogenesis of Listeria monocytogenes.  Curr Opin Immunol (England).  13(1):96-103.

[4]           Schuchat A, Deaver KA, Wenger JD, Plikaytis BD, Mascola L, Pinner RW, Reingold AL, Broome CV.  (1992).  Role of foods in sporadic listeriosis.  I. Case-control study of dietary risk factors.  JAMA.  267(15):2041-5.

[5]           At 38 dead, or even 33, this outbreak has the dubious distinction of being one of the deadliest in U.S. history.

[6]           Bryan, FL.  (1999).  Procedures to Investigate Foodborne Illness Fifth Edition (pp. 119).  Des Moines, IA:  International Association for Food Protection.

[7]           FDA/CFSAN.  (2003).  Foodborne Pathogenic Microorganisms and natural Toxins Handbook: The ‘Bad Bug Book.’  College park, MD:  Center for Food Safety and Applied Nutrition, Food and Drug Administration.  Retrieved on January 11, 2008 from FDA/CFSAN Website, http://www.cfsan.fda.gov/~mow/chap6.html.