Listeriosis

• Reporting criteria
• The disease and its epidemiology
• Case definition

• Laboratory testing
• Case investigation
• Disease control measures

• References
• Contact

What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency  

What to report to the Colorado Department of Public Health and Environment (CDPHE) or local health agency

• All confirmed and probable listeriosis cases should be reported to public health within four (4) days of diagnosis or a positive laboratory test.
  • All positive Listeria laboratory tests should be reported to public health, regardless of symptoms. Public health will make the determination of whether the case meets the case definition.
  • If both a mother and an infant are infected (have positive tests), each should be entered into EpiTrax as a separate case.
• Cases should be reported using EpiTrax Reportal, fax, or telephone to CDPHE or local health departments (telephone and fax numbers are listed at the end of this document).
• All potential foodborne, waterborne, and enteric disease outbreaks should be reported immediately to CDPHE or local health departments and entered into EpiTrax, even if the causative agent is not yet known.
  • In general, an “outbreak” is defined as an increase in the number of illnesses above what is normally expected among a specific population, within a given area, over a certain period of time.
  • Specifically, a foodborne disease outbreak occurs when two or more people get the same illness from the same contaminated food or drink.

Purpose of surveillance and reporting

• To identify cases for investigation and potential outbreaks
• To monitor trends in disease incidence

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Etiologic agent

Listeriosis refers to disease caused by Listeria monocytogenes, a gram-positive rod-shaped bacterium. Serotypes 1/2a, 1/2b, and 4b cause 95% of human infections with a known serotype.

Clinical description

Listeriosis is an uncommon infection that rarely causes severe illness in healthy adults and children. Newborns, persons who take oral steroid medications, immunocompromised persons, organ transplant patients, the elderly, and persons with HIV or AIDS, cancer, diabetes, kidney disease, or liver disease are at greater risk of developing listeriosis and experiencing severe outcomes. 

There are two clinical presentations of listeriosis. Non-invasive listeriosis is typically a mild, self-resolving gastrointestinal illness that is rarely clinically diagnosed or investigated by public health. Invasive listeriosis is a severe illness requiring public health investigation. This chapter focuses on invasive Listeria infections.

Listeriosis can be severe in non-pregnant, high-risk individuals (see above), presenting as fever, flu-like symptoms, headache, stiff neck, confusion, loss of balance, and seizures. Severe complications such as sepsis, meningitis, and/or encephalitis can occur, and the illness can be fatal. The overall case-fatality rate for non-pregnant adults is approximately 20-30%.

Infected pregnant individuals may experience only a mild, influenza-like illness but can result in miscarriage, stillbirth, premature delivery, or a life-threatening infection for the newborn, such as pneumonia, meningitis, or septicemia. The mother usually recovers fully; however, the case-fatality rate is 20-30% in infected newborns and approaches 50% when onset occurs in the newborn in the first four days of life. Spontaneous abortion can occur at any point in pregnancy.

Reservoirs

Listeria monocytogenes is ubiquitous in the environment and can be found in soil, water, and decaying vegetation. Domestic and wild mammals, birds, and humans (especially slaughterhouse workers and people who work with the bacteria in a laboratory) can carry the bacteria without showing any symptoms. 

Listeria bacteria can survive and multiply even in refrigerated or frozen conditions; it grows well in biofilms (such as in drains). Consequently, food and food production environments (especially ones that process raw or unpasteurized products) may serve as a persistent reservoir for the bacteria.

Modes of transmission

The main transmission route for listeriosis is by consuming contaminated food, which is the source for most human infections. Food items can become contaminated with L. monocytogenes when food is harvested (as listeria is ubiquitous in soil) or when it is processed, prepared, packed, transported, and/or stored in manufacturing or production environments where the bacteria is present. Unlike most other bacterial foodborne pathogens, Listeria can multiply in contaminated foods held at refrigeration and frozen temperatures (Listeria can multiply between 32°F and 113°F). Food contaminated with Listeria looks, smells, and tastes normal. 

Vegetables and fruits can become contaminated from the soil or from manure used as fertilizer. Animals can carry the bacteria and contaminate food of animal origin such as meats, poultry, and dairy products. The bacteria have been found in processed foods that become contaminated after processing, such as soft cheeses, Mexican-style cheeses, frozen dessert products, smoked seafood, hot dogs, pâtés, and cold cuts at the deli counter. Unpasteurized (raw) milk or foods made from unpasteurized milk may contain the bacterium. Listeria is killed by pasteurization and cooking. 

Listeriosis is not spread person to person except when a mother spreads it to her child during pregnancy or childbirth (vertical transmission). Newborns can acquire listeriosis in utero or during passage through the birth canal if their mothers are infected after eating contaminated foods during pregnancy, even if the mother did not have severe illness. Those at increased risk for infection can potentially develop infection after eating food contaminated with even a few bacteria. 

Rare cases of nosocomial transmission (i.e., hospital-acquired) have been reported in hospital nurseries and have been attributed to contaminated equipment or materials.

Incubation period

The median incubation period for listeriosis is usually 2-3 weeks. Pregnant individuals usually have a longer incubation period of 2-4 weeks. In rare cases, the incubation period can be as short as a few days and as long as two months.

Infectious period

Infected individuals can shed the organism in their stools for several months, however person-to-person transmission does not occur. Mothers of infected newborn infants can shed the organism in vaginal discharge and urine for 7-10 days after delivery, rarely longer.

Epidemiology

Listeriosis is the third leading cause of death from foodborne illness in the United States. Healthy children and adults rarely become seriously ill. Newborns, pregnant people, people with immunosuppressive medical conditions, and the elderly are at greater risk of developing serious illness.

During 2023, eight listeriosis cases were reported in Colorado, with a five-year average of 11 cases per year (2019-2023). The majority of cases occurred in individuals aged 60 and older; on average, fewer than one case per year was reported in an infant.

In 2022, 963 listeriosis cases were reported in the United States, with a five-year average of 902 cases per year (2018-2022). During this time period, 96% of listeriosis cases in the FoodNet catchment area were hospitalized and 21% of cases died. Five percent of cases were outbreak-associated, and approximately 3% were acquired internationally.

While the majority of listeriosis cases are sporadic, isolated cases, notable foodborne outbreaks have occurred. In 2011, there was a large multi-state outbreak of Listeria monocytogenes associated with whole cantaloupes grown in Colorado. The outbreak resulted in 147 confirmed cases and 33 deaths among residents of 28 states, making it one of the deadliest foodborne outbreaks in United States history.

More recently, in 2024, there was a multistate outbreak linked to meats sliced at deli counters, including Boar’s Head brand liverwurst. There were a total of 61 people infected in 19 states (none in Colorado) with 10 deaths reported. Other recent outbreaks have been linked to bagged peaches, plums, and nectarines, queso fresco and cotija cheese, and premade supplement shakes.

For more data, see Colorado reportable disease data (Colorado data), CDC WONDER (national data), and CDC FoodNet Fast (FoodNet catchment area data).

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Clinical criteria

Invasive listeriosis:

• Systemic illness caused by L. monocytogenes manifests most commonly as bacteremia or central nervous system infection. Other manifestations can include pneumonia, peritonitis, endocarditis, and focal infections of joints and bones.
• Pregnancy-associated listeriosis has generally been classified as illness occurring in a pregnant individual or in an infant age ≤ 28 days. Listeriosis may result in pregnancy loss (fetal loss before 20 weeks gestation), intrauterine fetal demise (≥ 20 weeks gestation), preterm labor, or neonatal infection, while causing minimal or no systemic symptoms in the mother. Pregnancy loss and intrauterine fetal demise are considered to be maternal outcomes.
• Neonatal listeriosis commonly manifests as bacteremia, central nervous system infection, and pneumonia, and is associated with high fatality rates. Transmission of Listeria from mother to baby transplacentally or during delivery is almost always the source of early-onset neonatal infections (diagnosed between birth and 6 days) and the most likely source of late-onset neonatal listeriosis (diagnosed between 7–28 days).

Non-invasive Listeria infections:

• Listeria infections can manifest as an isolate from a non-invasive clinical specimen suggestive of a non-invasive infection, which can include febrile gastroenteritis, urinary tract infection, and wound infection.

Laboratory criteria for diagnosis

Confirmatory laboratory evidence:

• Isolation of L. monocytogenes from a specimen collected from a normally sterile site reflective of an invasive infection (e.g., blood or cerebrospinal fluid or, less commonly: pleural, peritoneal, pericardial; or other sterile sites, including organs such as spleen, liver, and heart, but not sources such as urine, stool, or external wounds)

OR

• For maternal isolates: In the setting of pregnancy, pregnancy loss, intrauterine fetal demise, or birth, isolation of L. monocytogenes from products of conception (e.g. chorionic villi, placenta, fetal tissue, umbilical cord blood, amniotic fluid) collected at the time of delivery

OR

• For neonatal isolates: In the setting of live birth, isolation of L. monocytogenes from a non-sterile neonatal specimen (e.g., meconium, tracheal aspirate, but not products of conception) collected within 48 hours of delivery.

Presumptive laboratory evidence:

• Detection of L. monocytogenes by culture-independent diagnostic testing (CIDT) in a specimen collected from a normally sterile site (e.g., blood or cerebrospinal fluid or, less commonly: pleural, peritoneal, pericardial; orthopedic site such as bone, bone marrow, or joint; or other sterile sites, including organs such as spleen, liver, and heart, but not sources such as urine, stool, or external wounds)

OR

• For maternal isolates: In the setting of pregnancy, pregnancy loss, intrauterine fetal demise, or birth, detection of L. monocytogenes by CIDT from products of conception (e.g., chorionic villi, placenta, fetal tissue, umbilical cord blood, amniotic fluid) collected at the time of delivery

OR

• For neonatal isolates: In the setting of live birth, detection of L. monocytogenes by CIDT from a non-sterile neonatal specimen (e.g., meconium, tracheal aspirate, but not products of conception) collected within 48 hours of delivery.

Supportive laboratory evidence:

• Isolation of L. monocytogenes from a non-invasive clinical specimen (e.g., stool, urine, wound) other than those specified under maternal and neonatal specimens in the confirmatory laboratory evidence section.

Epidemiologic linkage

For probable maternal cases:

• A mother who does not meet the confirmed case criteria BUT
• Who gave birth to a neonate who meets confirmatory or presumptive laboratory evidence for diagnosis 

AND

• Neonatal specimen was collected up to 28 days of birth.

For probable neonatal cases:

• Neonate(s) who do not meet the confirmed case criteria

AND

• Whose mother meets confirmatory or presumptive laboratory evidence for diagnosis from products of conception

OR

• A clinically compatible neonate whose mother meets confirmatory or presumptive laboratory evidence for diagnosis from a normally sterile site.

Criteria to distinguish a new case from an existing case

There is currently insufficient data available to support a routine recommendation for criteria to distinguish a new case of listeriosis from prior reports or notifications. Duplicate or recurring reports of listeriosis in an individual should be evaluated on a case-by-case basis.

Case classification

Confirmed

• A person who meets confirmatory laboratory evidence

Probable

• A person who meets the presumptive laboratory evidence

OR

• A mother or neonate who meets the epidemiologic linkage but who does not have confirmatory laboratory evidence.

Suspected

• A person with supportive laboratory evidence. 

Case classification comments

Pregnancy loss and intrauterine fetal demise (i.e., stillbirth) are considered maternal outcomes and would be counted as a single case in the mother. 

Cases in neonates and mothers should be reported separately when each meets the case definition. A case in a neonate is counted if live-born.

Interpretation note: Suspect cases of listeriosis (individuals with detection of L. monocytogenes from a non-invasive clinical specimen, e.g., stool, urine, wound) are rarely investigated and not transmitted to CDC. 

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• The Colorado State Public Health Laboratory (“State Lab”) provides testing services to support public health investigations.
• The State Lab may test clinical and/or environmental samples when determined to be necessary for public health investigations. These services vary by pathogen but may include:
  • Testing for individuals who have been restricted or excluded from work or child care by public health
  • Testing for outbreak investigations, with prior approval from CDPHE
  • Other testing services as discussed with CDPHE
• The State Lab maintains a test directory, guidance for submitting samples, and other helpful resources on their webpage.
  • Additional questions can be directed to the Lab Coordinators by email (cdphe_labcoordinators@state.co.us; preferred method) or phone 303-692-3069.
• Clinical and commercial laboratories are required to submit Listeria isolates or clinical material to the State Lab for confirmation and whole genome sequencing (WGS; i.e., molecular subtyping). While these submissions do not require prior approval from CDPHE, a complete lab requisition form is required for each isolate/clinical material submission to the State Lab.
  • WGS results are used for public health surveillance only and cannot be released to clinical providers or used for clinical decision-making.
  • The State Lab does not perform routine antimicrobial susceptibility testing (AST); providers interested in AST should order it from a clinical or commercial laboratory.

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Listeria case interviews are conducted by CDPHE. All listeriosis cases are interviewed to determine:

• Potential source of infection, and implement disease control measures as appropriate
• If others are ill (i.e., could this be an outbreak?) 

Forms 

CDPHE interviews all cases using the CDC Listeria Case Form that can be found in the Communicable Disease Manual. After the patient is interviewed, CDPHE will complete the EpiTrax record and upload the data into the CDC’s Listeria Initiative Database. CDPHE will share the CMR with the LPHA in EpiTrax so they are aware of the case and exposures. CDPHE will email LPHAs if disease control follow up is needed or a case may be part of an outbreak. 

Reported incidence is higher than usual/outbreak suspected

CDPHE will notify LPHAs if the number of reported cases of listeriosis in your jurisdiction is higher than usual, or if an outbreak is suspected. LPHAs are responsible for investigating the situation to determine the source of infection and mode of transmission. CDPHE staff can assist local public health agencies to investigate outbreaks and determine disease control recommendations to prevent further cases, as well as coordinate surveillance of cases that cross county lines. 

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Listeria (Listeriosis) | CDC

Treatment

Treatment with antibiotics is recommended for severe infections. When infection occurs during pregnancy, antibiotics given promptly to the pregnant individual can often prevent infection of the fetus or newborn. Even with prompt treatment, some infections result in death.

Prophylaxis

No prophylactic treatment of close contacts is recommended.

Education

Persons at high-risk for infection can limit exposure by avoiding certain foods:

• Avoid eating hot dogs, luncheon meats, deli meats, or leftover foods unless they are reheated until steaming hot.
• Avoid consuming raw (unpasteurized) milk or raw milk products, including raw milk cheese, raw milk kefir, and raw milk yogurt. Pasteurized milk and dairy products may be eaten.
• Avoid eating soft cheeses such as feta, Brie, goat cheese and Camembert; blue-veined cheeses; or Mexican-style cheeses such as queso blanco, queso fresco, and queso panela; unless they have labels that clearly state they are made from pasteurized milk. Hard cheeses, processed cheeses, cream cheese, cottage cheese, and yogurt may be eaten.
• Avoid eating refrigerated pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads may be eaten.
• Avoid eating refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole. Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna, or mackerel is most often labeled as “nova-style”, “lox”, “kippered”, “smoked” or “jerky”. The fish is found in the refrigerated section or sold at deli counters of grocery stores and delicatessens. Canned or shelf-stable smoked seafood may be eaten.
• Avoid getting fluid from hot dog packages on other foods, utensils, and food preparation surfaces.

Everyone, including high-risk individuals, should handle food properly and practice good hygiene:

• Always wash hands thoroughly with soap and water for at least 20 seconds before eating or preparing food, after handling hot dogs and deli meats, after using the toilet, after handling animals, and after changing diapers.
• Thoroughly cook raw food from animal sources, such as beef, pork, or poultry. Cooking will kill Listeria bacteria.
• Wash raw fruit and vegetables thoroughly before eating, including melon rind.
• Keep uncooked meats separate from vegetables, fruits, cooked foods, and ready-to-eat foods.
• Consume perishable foods and ready-to-eat foods as soon as possible.
• Avoid unpasteurized (raw) milk, including unpasteurized goat’s milk, unpasteurized milk products, and foods made from unpasteurized milk.
• Wash knives and cutting boards after preparing uncooked foods.

Managing special situations

Food recalls

In recent years, outbreaks of listeriosis have resulted in large food recalls due to Listeria contamination. The risk of an individual person developing listeriosis after consuming a contaminated food is very small. If a person has eaten contaminated food and does not have any symptoms, testing and treatment are not recommended, even if that person is in a high-risk group. However, if a high-risk individual has eaten the contaminated product and within two months becomes ill with fever or signs of serious illness, they should contact a physician and inform them of their exposure.

Patients in health care facilities (hospitals and long-term care facilities)

If nosocomial transmission is suspected, CDPHE will coordinate with the Health Facilities Division.

Food handlers/health care/child care/preschool/school/community residential programs

Because listeriosis is not spread through person-to-person transmission, there are no special actions to be taken if a case is a food handler, is a health care worker, attends a child care center/preschool/school, or is a resident in a community residential program.

Environmental measures

• Implicated food items must be removed from the environment.
• A decision about testing suspect/implicated food items must be made in consultation with CDPHE.
  • The general policy of the State Lab and the Communicable Disease Branch is only to test food samples associated with outbreaks, not in single cases.
• If a commercial product is suspected, CDPHE Communicable Disease Branch will coordinate follow-up with the CDPHE Division of Environmental Health and Sustainability and relevant outside agencies. If the food item is believed to be distributed to health care facilities (hospitals, long-term care facilities, etc.), then CDPHE Communicable Disease Branch will coordinate with Health Facilities. 

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American Academy of Pediatrics. Red Book: 2024-2027 Report of the Committee on Infectious Diseases, 33rd Edition. Illinois, Academy of Pediatrics, 2024.  

CDC Website: www.cdc.gov (click on “Diseases and Conditions”)

Centers for Disease Control and Prevention. National Notifiable Diseases Surveillance System (NNDSS) Annual Summary Data for years 2016-2022, United States, CDC WONDER online database. Accessed at http://wonder.cdc.gov/nndss-annual-summary.html on May 21, 2025 9:55:58 PM

Centers for Disease Control and Prevention (CDC). FoodNet Fast: Liserta Surveillance Tool. Atlanta, Georgia: U.S. Department of Health and Human Services. Available from URL: http://wwwn.cdc.gov/foodnetfast. Accessed 05/15/2025

Heymann DL, ed. Control of Communicable Diseases Manual, 21st Edition. Washington, DC, American Public Health Association, 2022.

Human Food Program (2025, January 16). Listeria (Listeriosis). U.S. Food And Drug Administration. https://www.fda.gov/food/foodborne-pathogens/listeria-listeriosis

Listeriosis (Listeria monocytogenes) 2019 Case Definition | CDC. (n.d.). https://ndc.services.cdc.gov/case-definitions/listeriosis-2019/

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CDPHE Communicable Disease Branch

• Phone: 303-692-2700 or 800-866-2759
• Fax: 303-782-0338
• After hours: 303-370-9395

CDPHE Lab Coordinators: 

• Email (preferred): cdphe_labcoordinators@state.co.us  
• Phone: 303-692-3069

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