1. Organism

Listeria monocytogenes is a Gram-positive, rod-shaped, facultative bacterium, motile by means of flagella, that is among the leading causes of death from foodborne illness. It has 13 serotypes, including 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4ab, 4b, 4c, 4d, 4e, and 7. Among them, serotypes 1/2a, 1/2b, and 4b have been associated with the vast majority of foodborne infections.

L. monocytogenes is hardy; it is salt-tolerant and not only can survive in temperatures below 1°C, but also grow in these conditions, unlike many other pathogens. It is also notable for its persistence in food-manufacturing environments. The bacterium is ubiquitous in the environment and can be found in moist environments, soil, and decaying vegetation.

Of the five other species in the genus Listeria – L. grayi, L. innocua. L. ivanovii, L. seeligeri and L. welshimeri – only L. ivanovii is considered pathogenic, and mainly in ruminants, rather than in humans.

2. Disease

• Mortality: Although not a leading
cause of foodborne illness, L.
monocytogenes is among the leading
causes of death from foodborne
illness. The severe form of the infection has a case-fatality rate ranging from 15% to 30%, overall. When listerial meningitis occurs, the overall case-fatality rate may be as high as 70%; from septicemia, 50%, overall; and in perinatal/neonatal infections, more than 80%.

• Infective dose: The infective dose of L. monocytogenes is undetermined, but is believed to vary with the strain and susceptibility of the host, and the food matrix involved also may affect the dose-response relationship. In cases associated with raw or inadequately pasteurized milk, for example, it is likely that fewer than 1,000 cells may cause disease in susceptible individuals. As noted, however, the infective dose may vary widely and depends on a variety of factors.

• Onset: Gastroenteritis caused by L. monocytogenes has a relatively short incubation period, from a few hours to 2 or 3 days. The severe, invasive form of the illness can have a very long incubation period, estimated to vary from 3 days to 3 months.

• Illness / complications: L. monocytogenes infection causes two forms of disease in humans: 
1) non-invasive gastrointestinal illness, which generally resolves in otherwise healthy people. 
2) the much more serious, invasive form of the illness, which may cause septicemia and meningitis. 
Manifestations of L. monocytogenes infection tend to be host-dependent. In people with intact immune systems, it may cause acute febrile gastroenteritis, the less severe form of the disease. In vulnerable populations, however, the more severe form of the disease may result in sepsis and spread to the nervous system, potentially causing meningitis. In elderly and immunocompromised people who develop the severe form, it usually manifests in this manner. 
Pregnant women, who are disproportionately infected with L. monocytogenes, may experience mild, flu-like symptoms; however, their offspring do not fare as well – they may abort or be stillborn, and those born alive may have bacteremia and meningitis. One- third of confirmed cases of maternal-fetal L. monocytogenes infections lead to abortion or stillbirth.

• Symptoms: Otherwise healthy people might have mild symptoms or no symptoms if infected with L. monocytogenes, while others may develop fever, muscle aches, nausea and vomiting, and, sometimes, diarrhea. When the more severe form of the infection develops and spreads to the nervous system, symptoms may include headache, stiff neck, confusion, loss of balance, and convulsions.

• Duration: The duration of symptoms generally depends on the health status of the infected person. The symptoms can last from days to several weeks.

• Route of entry: Oral.

• Pathway: The pathogenesis of L. monocytogenes is unique, because the organism is able to spread directly from cell to cell in the host, rather than having to “travel” interstitially to reach other cells. Once the bacterium enters the host’s monocytes, macrophages, or polymorphonuclear leukocytes, it can reproduce, and it is bloodborne. Groups of proteins on the L. monocytogenes cell surface enable it to survive in phagocytic cells and enhance its cell-to-cell spread. 
3. Frequency 
Based on a survey collected through 1997 by the Centers for Disease Control and Prevention (CDC), listeriosis was responsible for approximately 2,500 illnesses and 500 deaths in the United States annually. By 2008, the number of L. monocytogenes infections had declined 36 percent, compared to the period from 1996 to 1998. There was a moderate increase in the incidence of L. monocytogenes from 2008 to 2009; however, it was still lower than the incidence measured 10 years before that.

4. Sources

Many foods have been associated with L. monocytogenes. Examples include raw milk, inadequately pasteurized milk, chocolate milk, cheeses (particularly soft cheeses), ice cream, raw vegetables, raw poultry and meats (all types), fermented raw-meat sausages, hot dogs and deli meats, and raw and smoked fish and other seafood. L. monocytogenes can grow in refrigerated temperatures, which makes this organism a particular problem for the food industry.

Potential contamination sources include food workers, incoming air, raw materials, and food- processing environments. Among those, post-processing contamination at food-contact surfaces poses the greatest threat to product contamination.

5. Target Populations

The main target populations for listeriosis are:

• pregnant women/fetuses/neonates - perinatal and neonatal infections;

• persons immunocompromised by, for example, corticosteroids, anticancer drugs, graft suppression therapy, AIDS;

• cancer patients, particularly leukemic;

• (less frequently reported) diabetic, cirrhotic, asthmatic, and ulcerative colitis patients;

• the elderly;

• healthy people – some reports suggest that healthy people are at risk, and that antacids or cimetidine may predispose them to the infection. Some studies suggested that healthy, uncompromised people could develop the disease, particularly if the food eaten was heavily contaminated with L. monocytogenes.

6. Diagnosis

Identification of culture isolated from tissue, blood, cerebrospinal fluid, or another normally sterile site (e.g., placenta or fetus) is needed for diagnosis of L. monocytogenes infection. Stool cultures are not informative, since some healthy humans may be intestinal carriers of L. monocytogenes.

7. Food Analysis

Methods of analyzing foods for purposes of identifying L. monocytogenes are complex and time- consuming. The present FDA method, revised in January 2003, uses a single enrichment broth, buffered Listeria enrichment broth, and requires 24 to 48 hours of enrichment, followed by a variety of agars and, finally, biochemical confirmation. Total time to identification is from 5 to 7 days. Many other enrichment broths, such as UVM broth and Fraser broth, are also included in various protocols. Agars that have been extensively evaluated include Oxford agar, PALCAM, LPM plus esculin and ferric iron and MOX.

New molecular biology techniques have been used to develop various rapid-screening kits for L. monocytogenes. These kits generally rely on ELISA, PCR, and probe-based identification.

8. Examples of Outbreaks

L. monocytogenes has caused significant outbreaks worldwide over the past decades. Some examples are listed below.

Los Angeles, 1985. A large-scale listeriosis outbreak occurred in Los Angeles County, California, due to the consumption of contaminated Mexican-style soft cheese. Human listeriosis cases reported numbered 142. Among them, 93 cases occurred in pregnant women or their offspring, and the remaining cases occurred in non-pregnant adults. The outbreak led to 48 deaths, including 20 fetuses, 10 neonates, and 18 non-pregnant adults. An investigation of the cheese plant suggested that the cheese was commonly contaminated by unpasteurized milk. The outbreak strain was serotype 4b.

U.S., 1989, 2000. A serotype 1/2a strain was isolated from a single case of human listeriosis in 1989, which was caused by the consumption of processed meat. Eleven years later, the same strain isolated from sliced turkey produced by the same processing plant was implicated in a listeriosis outbreak. This provides a powerful illustration of L. monocytogenes’s tenacity and prolonged survival in food-processing environments.

U.S., 1998 to 1999. A large scale multistate outbreak of listeriosis caused at least 50 cases in 11 states. Six adults died, and two pregnant women had spontaneous abortions. Contaminated hot dogs were linked to this outbreak. All L. monocytogenes isolates from these cases were serotype 4b.

U.S., 2002. A multistate outbreak of listeriosis in the Northeastern U.S. resulted in 46 cases, including 7 deaths and 3 stillbirths or miscarriages in eight states. The outbreak was linked to eating sliceable turkey deli meat. L. monocytogenes was isolated from 1 food product and 25 environmental samples from a poultry-processing plant. The isolate from the food product had a PFGE pattern different from the outbreak strain; however, two environmental isolates from floor drains had an identical PFGE pattern to that of outbreak patient isolates, suggesting that the plant might have been the source of the outbreak. The outbreak strain was serotype 4b.

Canada, 2008. A widespread outbreak of listeriosis occurred in Canada and was linked to deli meat produced by a Maple Leaf Foods plant in Toronto, Ontario. The outbreak caused 57 confirmed cases in seven provinces, and 22 people died. The outbreak strain was serotype 1/2a.

(sic) U.S. 2011. A total of 146 persons infected with any of the four outbreak-associated strains of Listeria monocytogenes were reported to CDC from 28 states linked to cantaloupe consumption. Thirty deaths were reported. In addition, one woman pregnant at the time of illness had a miscarriage.

9. Resources

• Loci index for genome Listeria.

• GenBank Taxonomy database.

• CDC facts about listeriosis.

• The U.S. Department of Agriculture provides information about Listeria monocytogenes.

The study was conducted UBC food microbiologist Kevin Allen and published in the journal Food Microbiology. 

According to the Vancouver Sun:

The type of foods tested included lox, smoked tuna, candied salmon and fish jerky. The listeria was found in 20 per cent of the products tested, of those, five had the more virulent variety of listeria monocytogenes.   “Additional handling of ready-to-eat foods in stores, such as slicing, weighing, and packaging, may increase the potential for cross-contamination,” Allen said in a press release. “While listeria bacteria can be killed by high heat, most people eat these fish products without further cooking.” 

The investigation is ongoing and the source of the outbreak is unknown. Alicia Cronquist, an epidemiologist at the state health department, said, “Until we have more information about the specific source of this outbreak, it is important for people to follow the standard CDC guidance about Listeria. People who are at high risk for Listeria infection can decrease their risk by avoiding soft cheeses such as queso fresco and brie unless they are made with pasteurized milk, hot dogs and deli meats unless reheated to an internal temperature of 165F, refrigerated paté or meat spreads, or refrigerated smoked seafood.”

People who are at high risk for Listeria infection include people with weakened immune systems from transplants or certain chronic diseases, immunosuppressive therapies or medications; pregnant women; and people age 60 and older.

Consumption of food contaminated with Listeria monocytogenes can cause listeriosis, an uncommon but potentially fatal infection. Healthy people rarely contract listeriosis. Symptoms can include fever and muscle aches, and also can include diarrhea, headache, stiff neck, confusion and convulsions. Listeriosis also can cause miscarriages and stillbirths.

Antibiotics given promptly can cure the illness and prevent infection of a fetus. Even with prompt treatment, some Listeria infections result in death. This is particularly likely in older adults and in people with other serious medical problems. Specific recommendations for people at high risk:

Cheeses

• 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. Make sure the label says, "Made with pasteurized milk."

Meats

• Do not eat hot dogs, luncheon meats, cold cuts, other deli meats (e.g., 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 lunch meat packages on other foods, utensils and food preparation surfaces. Wash hands after handling hot dogs, luncheon meats and deli meats.

• Do not eat refrigerated paté or meat spreads from a deli or meat counter or from the refrigerated section of a store. Foods that do not need refrigeration, such as canned or shelf-stable paté and meat spreads, are safe to eat. Refrigerate after opening.

Seafood

• 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 typically are found in the refrigerator section or deli counters of grocery stores and delicatessens. Canned and shelf-stable tuna, salmon and other fish products are safe to eat. "

General recommendations from CDC

• Thoroughly cook raw food from animal sources, such as beef, pork or poultry, to a safe internal temperature. For a list of recommended temperatures for meat and poultry, visit http://www.fsis.usda.gov/PDF/IsItDoneYet_Magnet.pdf.

• Rinse raw vegetables thoroughly under running tap water before eating.

• Keep uncooked meats and poultry separate from vegetables and from cooked foods and ready-to-eat foods.

• Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.

• Wash hands, knives, countertops and cutting boards after handling and preparing uncooked foods.

For more information, visit the CDC website:

http://www.cdc.gov/nczved/divisions/dfbmd/diseases/listeriosis/ and www.about-listeria.com.

The CDC adds that any opened package of sliced meat should be thrown out within five days.

Listeria is much less common than salmonella or E. coli. However, it can be extremely deadly.

As Spring advances and the days get warmer, there are more group gatherings and a higher consumption of processed meats.

The problem with listeria is that it does not hold back in cold temperatures - put the cold meat in the fridge and it will continue to reproduce. The CDC says the concern over listeriosis in the USA is not new; it seems people are just not paying attention.

Approximately 2,500 cases of listeriosis are estimated to occur in the U.S. each year. About 200 in every 1000 cases result in death. Certain groups of individuals are at greater risk for listeriosis, including pregnant women (and their unborn children) and immunocompromised persons. Among infants, listeriosis occurs when the infection is transmitted from the mother, either through the placenta or during the birthing process. These host factors, along with the amount of bacteria ingested and the virulence of the strain, determine the risk of disease. Human cases of listeriosis 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.

Listeria is often isolated in cattle, sheep, and fowl, and is also found in dairy products, fruits, and vegetables.

Symptoms of Listeria Infection

It is thought that ingestion of as few as 1,000 cells of Listeria bacteria can result in illness. After ingestion of food contaminated with Listeria, incubation periods (from time of exposure to onset of illness) are in the range of one to eight weeks, averaging about 31 days. Five days to three weeks after ingestion, Listeria has access to all body areas and may involve the central nervous system, heart, eyes, or other locations.

A person with listeriosis usually has fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, obtundation (decreased consciousness) or convulsions can occur. With brain involvement, listeriosis may mimic a stroke. Infected pregnant women will ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth. Pregnant women are about 20 times more likely than other healthy adults to get listeriosis; about one-third of listeriosis cases happen during pregnancy. The incidence of listeriosis in the newborn is 8.6 cases per 100,000 live births. The perinatal and neonatal mortality rate (stillbirths and early infant deaths) from listeriosis is 80%.

Diagnosis and treatment of Listeria Infection

If you have symptoms of listeriosis, a health care provider can have a blood or spinal fluid test done to detect the infection. During pregnancy, a blood test is the most reliable way to find out if your symptoms are due to listeriosis. If you are in a high-risk group, have eaten the contaminated product, and within 2 months become ill with fever or signs of serious illness, you should contact your health care provider and inform him or her about this exposure.

There are several antibiotics with which Listeria may be treated. When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis.

Preventing Listeria Infection

General recommendations include: thoroughly cook raw food from animal sources; keep uncooked meats separate from vegetables and from cooked 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; wash raw vegetables thoroughly before eating; and consume perishable and ready-to-eat foods as soon as possible.

Recommendations for persons at high risk, such as pregnant women and persons with weakened immune systems, in addition to the recommendations listed above, include: do not eat hot dogs, luncheon or deli meats, unless they are reheated until steaming hot, and wash hands after handling those products; do not eat soft cheeses (such as feta, Brie, Camembert, blue-veined, or Mexican-style cheese), unless they have labels that clearly state they are made from pasteurized milk; and do not eat meat spreads or smoked seafood from the refrigerated or deli section of the store (canned or shelf-stable products may be eaten).

References

Bennion JR, Sorvillo F, Wise ME, Krishna S, Mascola L. (2008). Decreasing listeriosis mortality in the United States, 1990-2005. Clin Infect Dis. 47(7):867-74.

Bortolussi R. (2008). Listeriosis: a primer. CMAJ. 179(8):795-7.

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

CDC. (2008). Preliminary FoodNet Data on the Incidence of Infection with Pathogens Transmitted Commonly Through Food—- 10 States, 2007. MMWR Morb Mortal Wkly Rep. 57(14);366-370.

CDC. (2009). Surveillance for foodborne disease outbreaks - United States, 2006. MMWR Morb Mortal Wkly Rep. 12;58(22):609-15.

CDC website. (2009). Listeriosis Information. Retrieved on November 1, 2009 from Centers for Disease Control and Prevention website, http://www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis.htm.

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.

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. http://www.cfsan.fda.gov/~mow/chap6.html.

Gilbert DN, Moellering RC, Sande MA. (2001). The Sanford Guide to Antimicrobial Therapy 2001. Hyde Park, VT: Antimicrobial, Inc.

Heinitz ML, Johnson JM. (1998). The incidence of Listeria spp., Salmonella spp., and Clostridium botulinum in smoked fish and shellfish. J Food Prot. 61:318-23.

Jurado RL, Farley MM, Pereira E, Harvey RC, Schuchat A, Wenger JD, Stephens DS. (1993). Increased risk of meningitis and bacteremia due to Listeria monocytogenes in patients with human immunodeficiency virus infection. Clin Infect Dis. 17(2):224-7.

Loncarevic S, Tham W, Danielsson-Tham ML. (1996). Prevalence of Listeria monocytogenes and other Listeria spp. in smoked and ‘gravad’ fish. Acta Vet Scand. 37:13-18.

Mayo Clinic. (2009). Listeria infection (listeriosis). Retrieved November 1, 2009 from Mayo Clinic website: http://www.mayoclinic.com/health/listeria-infection/DS00963.

Pinner RW, Schuchat A, Swaminathan B, Hayes PS, Deaver KA, Weaver RE, Plikaytis BD, Reeves M, Broome CV, Wenger JD. (1992). Role of foods in sporadic listeriosis. II. Microbiologic and epidemiologic investigation. JAMA 267(15):2046-50.

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.

Silver HM. (1998). Listeriosis during pregnancy. Obstet Gynecol Surv. 53:737-740.

Tappero JW, Schuchat A, Deaver KA, Mascola L, Wenger JD. (1995). Reduction in the incidence of human listeriosis in the United States. Effectiveness of prevention efforts. JAMA. 273(14):1118-22.

Voetsch AC, Angulo FJ, Jones TF, Moore MR, Nadon C, McCarthy P, Shiferaw B, Megginson MB, Hurd S, Anderson BJ, Cronquist A, Vugia DJ, Medus C, Segler S, Graves LM, Hoekstra RM, Griffin PM; Centers for Disease Control and Prevention Emerging Infections Program Foodborne Diseases Active Surveillance Network Working Group (2007). Reduction in the incidence of invasive listeriosis in foodborne diseases active surveillance network sites, 1996-2003. Clin Infect Dis. 15;44(4):513-20.

Weinberg, WG. (1996). No Germs Allowed: How to Avoid Infectious Diseases At Home and On the Road. Piscataway, NJ: Rutgers University Press.

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

• 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.

• 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).

• Bryan, FL.  (1999).  Procedures to Investigate Foodborne Illness Fifth Edition (pp. 119).  Des Moines, IA:  International Association for Food Protection.
• CDC.  (2005).  Listeriosis Technical Information.  Retrieved on August 13, 2007 from Centers for Disease Control and Prevention Web site, http://www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis_t.htm.
• CDC.  2005a.  Listeriosis General Information Retirieved on August 13, 2007 from Centers for Disease Control and Prevention Web site, http://www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis_g.htm.
• 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.
• 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 Web site, http://www.cfsan.fda.gov/~mow/chap6.html.
• Gilbert DN, Moellering RC, Sande MA.  (2001).  The Sanford Guide to Antimicrobial Therapy 2001.  Hyde Park, VT:  Antimicrobial, Inc.
• Heinitz ML, Johnson JM.  (1998).  The incidence of Listeria spp., Salmonella spp., and Clostridium botulinum in smoked fish and shellfish.  J Food Prot. 61:318-23.
• Jurado RL, Farley MM, Pereira E, Harvey RC, Schuchat A, Wenger JD, Stephens DS.  (1993).  Increased risk of meningitis and bacteremia due to Listeria monocytogenes in patients with human immunodeficiency virus infection.  Clin Infect Dis.  17(2):224-7.
• Loncarevic S, Tham W, Danielsson-Tham ML.  (1996).  Prevalence of Listeria monocytogenes and other Listeria spp. in smoked and ‘gravad’ fish.  Acta Vet Scand. 37:13-18.
• Mayo Clinic.  (2007, March 22).  Listeria infection (listeriosis).  Retrieved August 12, 2007 from Mayo Clinic Web site:  http://www.mayoclinic.com/health/listeria-infection/DS00963.
• Pinner RW, Schuchat A, Swaminathan B, Hayes PS, Deaver KA, Weaver RE, Plikaytis BD, Reeves M, Broome CV, Wenger JD.  (1992).  Role of foods in sporadic listeriosis. II. Microbiologic and epidemiologic investigation.  JAMA 267(15):2046-50.
• 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.
• Silver HM. (1998).  Listeriosis during pregnancy.  Obster Gynecol Surv. 53:737-740.
• Tappero JW, Schuchat A, Deaver KA, Mascola L, Wenger JD. (1995).  Reduction in the incidence of human listeriosis in the United States. Effectiveness of prevention efforts.  JAMA. 273(14):1118-22
• Weinberg, WG.  (1996).  No Germs Allowed: How to Avoid Infectious Diseases At Home and On the Road.  Piscataway, NJ:  Rutgers University Press.

Approximately 2,500 cases of listeriosis are estimated to occur in the U.S. each year. About 200 in every 1000 cases result in death. Certain groups of individuals are at greater risk for listeriosis, including pregnant women (and their unborn children) and immunocompromised persons. Among infants, listeriosis occurs when the infection is transmitted from the mother, either through the placenta or during the birthing process. These host factors, along with the amount of bacteria ingested and the virulence of the strain, determine the risk of disease. Human cases of listeriosis 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.

Listeria is often isolated in cattle, sheep, and fowl, and is also found in dairy products, fruits, and vegetables.

Symptoms of Listeria infection

It is thought that ingestion of as few as 1,000 cells of Listeria bacteria can result in illness. After ingestion of food contaminated with Listeria, incubation periods (from time of exposure to onset of illness) are in the range of one to eight weeks, averaging about 31 days. Five days to three weeks after ingestion, Listeria has access to all body areas and may involve the central nervous system, heart, eyes, or other locations.

A person with listeriosis usually has fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, obtundation (decreased consciousness) or convulsions can occur. With brain involvement, listeriosis may mimic a stroke. Infected pregnant women will ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth. Pregnant women are about 20 times more likely than other healthy adults to get listeriosis; about one-third of listeriosis cases happen during pregnancy. The incidence of listeriosis in the newborn is 8.6 cases per 100,000 live births. The perinatal and neonatal mortality rate (stillbirths and early infant deaths) from listeriosis is 80%.

Diagnosis and treatment of Listeria infection

If you have symptoms of listeriosis, a health care provider can have a blood or spinal fluid test done to detect the infection. During pregnancy, a blood test is the most reliable way to find out if your symptoms are due to listeriosis. If you are in a high-risk group, have eaten the contaminated product, and within 2 months become ill with fever or signs of serious illness, you should contact your health care provider and inform him or her about this exposure.

There are several antibiotics with which Listeria may be treated. When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis.

Preventing Listeria infection

General recommendations include: thoroughly cook raw food from animal sources; keep uncooked meats separate from vegetables and from cooked 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; wash raw vegetables thoroughly before eating; and consume perishable and ready-to-eat foods as soon as possible.

Recommendations for persons at high risk , such as pregnant women and persons with weakened immune systems, in addition to the recommendations listed above, include: do not eat hot dogs, luncheon or deli meats, unless they are reheated until steaming hot, and wash hands after handling those products; do not eat soft cheeses (such as feta, Brie, Camembert, blue-veined, or Mexican-style cheese), unless they have labels that clearly state they are made from pasteurized milk; and do not eat meat spreads or smoked seafood from the refrigerated or deli section of the store (canned or shelf-stable products may be eaten).