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What is Listeriosis?

Listeriosis is a serious infection caused by eating food contaminated with the bacterium Listeria monocytogenes. The disease affects primarily persons of advanced age, pregnant women, newborns, and adults with weakened immune systems. However, persons without these risk factors can also rarely be affected.

What kind of germ is Listeria?

There are seven species of Listeria, which are then divided into two genomically distinct groups. The first group contains the species L. murrayi and L. grayi, which is considered nonpathogenic (not disease-causing) (Baltimore, 2007). The other group consists of the five remaining species; of those, two are nonhemolytic (L. innocula and L. welshimeri), and the remaining three are hemolytic (L. monocytogenes, L. seeligeri, and L. ivanovii) (Baltimore, 2007). L. ivanovii causes disease primarily in animals (Baltimore, 2007).

L. monocytogenes causes the vast majority of disease in both humans and animals, and is the only serotype that is pathogenic for humans (Lorber, 2010). There are at least 13 serotypes of L monocytogenes; however, almost all disease is caused by one of three serovars: 4b, 1/2a, and 1/2b (Lorber, 2010). This species is also widespread in nature, and is commonly found in soil, water, decaying vegetation, and as part of the fecal flora of many mammals (Lorber, 2010). L. monocytogenes has also been isolated from sewage and silage, where it can survive for over 295 days (Baltimore, 2007).

Listeria grows best at 86 to 98.6 degrees Fahrenheit (30 to 37 degrees Celsius); however, the Listeria also grow better than other bacteria at refrigerator temperatures (39.2 to 50 degrees Fahrenheit or 4 to 10 degrees Celsius) (Lorber, 2010). Additionally, the bacteria grows best in environments that are neutral to slightly alkaline pH, and dies in an environment with a pH below 5.5 (Lorber, 2010).

What are the symptoms of listeriosis?

A person with listeriosis has fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur (CDC, 2011).

Infected pregnant women may experience only a mild, flu-like illness; however, infections during pregnancy can lead to miscarriage or stillbirth, premature delivery, or infection of the newborn (CDC, 2011).

Who is At Risk for Listerosis?

Each year, an estimated 1,600 persons in the United States fall seriously ill with listeriosis (CDC, 2011). Of these, approximately 260 die (CDC, 2011). Between 1989 and 1993, the annual incidence of listeriosis decreased by 34%; from 1996 to 2006, it declined 36% (CDC, 2011). However, outbreaks continue to occur. For example, in 2002, an outbreak traced to consumption of turkey meat resulted in 54 illnesses, 8 deaths, and 3 fetal deaths across 9 different states (CDC, 2011). Further, an increasing rate of listeriosis has been reported in several European countries in recent years (Allerberger and Wagner, 2009).

A number of groups are at higher risk for developing listeriosis, including:

  • Pregnant women;
  • Newborns;
  • Persons with weakened immune systems such as those resulting from organ transplants and from certain diseases, therapies, and medications;
  • Persons with cancer, diabetes, alcoholism, liver or kidney disease;
  • Persons with AIDS;
  • Persons who take glucocorticosteroid medications; and
  • The elderly.

(CDC, 2011).

Healthy children and adults occasionally get listeriosis, but they rarely become seriously ill (CDC, 2011). Children younger than 1 month of age and adults over the age of 60 experience the highest rates of infection (Lorber, 2010). Some studies have demonstrated higher rates of infection in males, and, in the Northern hemisphere, a seasonal predominance occurring in late summer and fall (Baltimore, 2007).

Pregnant women are approximately 20 times more likely than other healthy adults to get listeriosis (CDC, 2011). In fact, about one in six (17%) of listeriosis cases happen during pregnancy (CDC, 2011). In the 10- to 40-year age group, pregnant women account for 60% of cases (Lorber, 2010). However, as mentioned above, pregnant women do not usually experience severe symptoms, and generally experience only a mild, flu-like illness (CDC, 2011). Instead, newborns – rather than the pregnant women themselves – suffer the most serious effects of infection in pregnancy (CDC, 2011). Approximately 50-90% of infected fetuses that are delivered immaturely do not survive (Baltimore, 2007). While the mortality rate is greater than 50% for fetuses infected in utero, it is only 30% for early onset neonatal sepsis, 15% for late-onset neonatal meningitis, and less than 10% in older children promptly given appropriate antimicrobial therapy.

Around 70% of nonperinatal infections occur in individuals who have hematologic malignancy, have received an organ transplant, are receiving corticosteroid treatment, or have AIDS (Lorber, 2010). Individuals infected with AIDS are almost 300 times more likely to get listeriosis than people with normal immune systems (CDC, 2011).

How do you get listeriosis?

Generally, an individual gets listeriosis by eating food contaminated with Listeria. Babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. Persons at risk can prevent Listeria infection by avoiding certain high-risk foods and by handling food properly.

Epidemic human listeriosis has been associated with foodborne transmission in a number of large outbreaks (Baltimore, 2007). These outbreaks are disproportionately tied to consumption of soft cheeses; improperly pasteurized milk and milk products; vegetables grown on farms where the ground is contaminated with the feces of colonized animals; and contaminated raw and ready-to-eat beef, pork, poultry and packaged meats (Baltimore, 2007).

There have also been small clusters of nosocomial person-to-person transmissions in obstetric suites and hospital nurseries (Baltimore, 2007).

Zoonotic transmission with cutaneous infections occasionally occurs in farmers and veterinarians who contract the disease when handling sick animals (Baltimore, 2007).

How does Listeria get into food?

Listeria monocytogenes is found in food, soil and water, but nearly all medical authorities agree that over 99% of the identified victims of Listeriosis acquire it from eating food that has become contaminated wit Listeria. Animals (and humans) can carry the bacterium without appearing ill and can contaminate foods such as meats and dairy products. Cross contamination can lead to the contamination of fruits and vegetables, ice cream, and other products. The bacterium has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in processed foods that become contaminated after processing, such as soft cheeses and cold cuts at the deli counter, cantaloupes, sprouts, and frozen foods (Listeria thrives at cool temperatures and is not affected by short-term freezing). Unpasteurized (raw) milk or foods made from unpasteurized milk routinely contain the bacterium. Listeria can get into food processing factories, where they are then able to live for years, and sometimes contaminate food products (CDC, 2011).

Listeria, unlike most other bacteria, can grow and multiply in the refrigerator (CDC, 2011).

Listeria is killed by pasteurization and cooking; however, in certain ready-to-eat foods such as hot dogs and deli meats, contamination may occur after factory cooking but before packaging (CDC, 2011).

What are the symptoms of listeriosis?

Generally, a person with listeriosis experiences fever and muscle aches (CDC, 2011). These symptoms are often preceded by diarrhea or other gastrointestinal symptoms (CDC, 2011). In cases where a diagnosis of listeriosis occurs, the infection is generally an “invasive” infection, which means that the Listeria has spread beyond the gastrointestinal tract and into other parts of the body (CDC, 2011). The specific symptoms of listeriosis depend on the person infected and the kind of infection.

What are the clinical syndromes associated with listeriosis?

Infection in Pregnancy

Pregnant women often experience only mild-flu like illness (CDC, 2011). For unexplained reasons, central nervous system infection is extremely rare during pregnancy in the absence of other factors that put an individual at high risk of infection (Lorber, 2010). However, this does not mean that the fetus is not affected. Infections during pregnancy can also lead to miscarriage, premature delivery, stillbirth, or life-threatening infection of the newborn infant (CDC, 2011).

When bacteremia occurs during pregnancy, it generally manifests as an acute febrile illness (Lorber, 2010). This may be accompanied by symptoms including myalgias, arthralgias, headache, and backache (Lorber, 2010).

Generally, illness occurs during the third trimester of pregnancy (Lorber, 2010). This is most likely due to the fact a major decline in cell-mediated immunity occurs at 26 to 30 weeks gestation (Lorber, 2010).

Neonatal infection

When a pregnant woman is infected with Listeria, it may cause spontaneous abortion of the fetus, and the fetus may be stillborn or die within hours of a form of listerial infection known as granulomatosis infantiseptica, which involves widespread granulomas and microabscesses (Lorber, 2010). These microabscesses are particularly prevalent in the liver and spleen (Lorber, 2010).

However, listeriosis in infants more commonly manifests in one of two forms: early-onset sepsis syndrome or late-onset meningitis (Lorber, 2010).

Early-onset sepsis syndrome is associated with premature birth and is usually acquired while the fetus is in utero (Lorber, 2010). It occurs less than 5 days after birth, but is generally seen within 2 days (Baltimore, 2007). The mortality rate associated with early-onset neonatal disease is approximately 20-30% (Baltimore, 2007).

Late-onset meningitis occurs about 2 weeks after birth (Lorber, 2010). The epidemiology associated with late-onset neonatal disease is not well understood (Baltimore, 2007). It often occurs in infants that were carried to full term, and the mothers are usually asymptomatic and do not test positive for the bacteria (Baltimore, 2007). If adequately treated, late-onset meningitis has a mortality rate of under 20% (Baltimore, 2007).

Febrile Gastroenteritis

Many individuals who go on to develop bacteremia or listerial central nervous system infection have a history of gastrointestinal symptoms (Lorber, 2010). These symptoms often include diarrhea, nausea, and vomiting, and are often accompanied by a fever (Lorber, 2010). Additional symptoms may include muscle and joint pains (Lorber, 2010). Typically, illness occurs about 24 hours after ingesting a large number of Listeria; however, gastrointestinal symptoms can appear as quickly as 6 hours or as long as 10 days after the initial exposure (Lorber, 2010). Generally, symptoms reside after 1-3 days, though they may last as long as a week (Lorber, 2010).


After the neonatal period, bacteremia with no apparent focus is the most common manifestation of listeriosis (Lorber, 2010). Symptoms generally include fever, myalgias, diarrhea, and nausea (Lorber, 2010). In healthy people, there is a comparatively good chance that blood would not be cultured and the infection would therefore go undetected (Lorber, 2010).


Listerial endocarditis, or the inflammation of the inner lining of the heart, accounts for approximately 7.5% of adult listerial infections (Lorber, 2010). The syndrome produces valve disease in both native and prosthetic valves (Lorber, 2010). Further, listerial endocarditis is associated with a high rate of mortality: 48% of those afflicted do not survive (Lorber, 2010). Children rarely are afflicted (Lorber, 2010).

Central Nervous System Infection

L. monocytogenes often affects the brain; in particular, it often involves the brain stem and meninges (Lorber, 2010). The organism is the fourth most common cause of bacterial meningitis (Lorber, 2010). Of the five bacteria responsible for the majority of bacterial meningitis cases, Listeria is associated with the highest meningitis-related mortality rate (22%) (Lorber, 2010)

Encephalitis (swelling of the brain) is a rare form of central nervous system listeriosis (Lorber, 2010). An infected individual generally is in a state of altered consciousness or cognitive dysfunction. Rhombencephalitis (brain stem encephalitis, or swelling of the brain stem) is an unusual form of listerial encephalitis. As opposed to other listerial central nervous system infections, rhombencephalitis generally occurs in healthy adults (Lorber, 2010). The illness is biphasic, meaning that it essential comes in two stages. First, there is a period of about four days that involves symptoms including fever, headache, nausea, and vomiting; this is followed by the abrupt onset of cerebellar signs, asymmetrical cranial nerve deficits, and hemisensory deficits and/or hemiparesis (Lorber, 2010). Respiratory failure develops in approximately 40% of rhombencephalitis cases (Lorber, 2010). Associated mortality is high, and many who survive experience serious complications (Lorber, 2010).

About 10% of listerial central nervous system cases involve the formation of microscopic brain abscesses (Lorber, 2010). Like in cases of rhombencephalitis, mortality is high, and many survivors experience serious sequelae (Lorber, 2010).

How is listeriosis detected?

Both pregnant and non-pregnant individuals face the same dilemma concerning detection of Listeria. The fact is that most cases of Listeriosis are not identified epidemiologically. Most (over 95%) of victims who get Listeria will develop gastroenteritis and will recover without seeking medical attention, or if they seek medical attention, will not be tested or provide a stool sample which often misses Listeria in standard stool cultures. There is also no routine screening test for listeriosis during pregnancy, as there is for rubella and some other congenital infections. Victims who have symptoms such as fever or stiff neck may consult a physician, and there are options in cases of suspected Listeriosis for a blood or spinal fluid test (to cultivate the bacteria). Many of those who do present with invasive listeriosis are already suffering other medical conditions, are elderly or very young, and in the cases of suspected meningitis, many will forgo the intrusive cerebral spinal tap in favor of administration of broad-spectrum antibiotics. When antibiotics are administered, later CSF cultures can lead to false negatives.

During pregnancy, a blood test is the most reliable way to find out if symptoms are due to listeriosis, but these often do not detect Listeria in the placenta, and post-DNC or still birth, placental culturing is rarely performed.

How can I reduce the risk for listeriosis?

General recommendations:

  • Cook raw food from animal sources – such as beef, pork, or poultry – thoroughly, to a safe internal temperature;
  • Wash raw vegetables thoroughly before eating;
  • Keep uncooked meats separate from vegetables and from cooked foods and ready-to-eat foods;
  • Avoid unpasteurized (raw) milk or foods made from unpasteurized milk;
  • Wash hands, knives, and cutting boards after handling uncooked foods; and
  • Consume perishable and ready-to-eat foods as soon as possible.

(CDC, 2011).

Are there additional precautions I can take if I’m in a group at high risk for listeriosis?

In addition to following the general recommendations found above, individuals in high risk groups can decrease their risk for listeriosis by following specific guidelines on how to handle certain foods, as well as by avoiding consumption of particular foods known to be frequent sources of Listeria bacteria.

According to the CDC, when contemplating which meats to consume, and how to handle them, at-risk individuals should:

  • Avoid eating hot dogs, cold cuts, luncheon meats, other deli meats (for example, bologna), or fermented dry sausages unless heated to an internal temperature of 165 degrees Fahrenheit or steaming hot just before serving;
  • Avoid getting fluid from lunch meat and hot dog packages on other foods, utensils, and food preparation surfaces;
  • Wash hands after handling deli meats, hot dogs, and luncheon meats; and
  • Avoid eating refrigerated pate or meat spreads from meat counters, deli counters, and refrigerated sections of the store

(CDC, 2011).

Additionally, following certain recommendations may keep food safe. These guidelines advise individuals to:

  • Be aware that Listeria can continue to grow and multiply in refrigerated foods, and keep a appliance thermometer to ensure that the temperature remains under 40 degrees Fahrenheit in your refrigerator and under 0 degrees Fahrenheit in your freezer;
  • Clean up all spills that occur in your refrigerator immediately, particularly spills involving juices from hot dogs, lunch meat packages, raw meat, and raw poultry;
  • Clean and rinse the inside walls and shelves of your refrigerator using liquid soap and hot water;
  • Use shallow containers for leftovers in order to promote rapid, even cooling, and ensure that your containers use airtight lids or enclose them using plastic wrap or aluminum foil;
  • Use leftovers within 3 to 4 days;
  • Use ready-to-eat and precooked foods as quickly as possible, making sure not to keep the product in the fridge past the “use-by” date; and
  • Following USDA refrigerator storage time guidelines regarding hot dogs (keep opened package no longer than 1 week and unopened package no longer than 2 weeks in the refrigerator) and luncheon and deli meats (keep factory-sealed, unopened package no longer than 2 weeks and store opened packages and meat sliced at a local deli no longer than 3 to 5 days in the refrigerator)

(CDC, 2011).

The CDC further advises at risk individuals to avoid soft cheeses including feta, queso blanco, brie, queso fresco, Camembert, panela (queso panela), or blue-veined chesses unless made with pasteurized milk, which should be indicated on the label by the words “MADE WITH PASTEURIZED MILK” (CDC, 2011).

Finally, the CDC recommends that members of at-risk groups, including pregnant women and immunocompromised individuals, not eat seafood unless it is contained in a cooked dish (for example, a casserole) or is a canned or shelf-stable product (CDC, 2011). Canned and shelf-stable tuna, salmon, and other products are thus generally safe to eat (CDC, 2011).

Can listeriosis be treated?

When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus or newborn (CDC, 2011).

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 the elderly and in persons with other serious medical problems (CDC, 2011).

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