Communicable Diseases



A bacterial infection that usually causes a mild febrile illness, but that can cause meningoencephalitis and/or septicemia in newborns and adults. The healthy host acquiring infection may exhibit only an acute mild febrile illness; in pregnant women infection can cause preterm delivery and fetal infection, with infection most likely resulting from transplacental transmission, although some infection may be the result of ascending infection from vaginal colonization. Infants may be stillborn or born with septicemia, or may develop meningitis in the neonatal period even though the mother may be asymptomatic at delivery. Listeria can also cause spontaneous abortions, although the incidence is difficult to estimate since bacterial cultures are not routinely obtained from spontaneously aborted fetuses or products of conception. Spontaneous abortions occur more commonly in the second half of pregnancy; perinatal infection is acquired during the third trimester or possibly from nosocomial infection in the case of late-onset neonatal disease. Listeria has not been associated with recurrent pregnancy loss. The postpartum course of the mother is usually uneventful, but the case-fatality rate is 30% in newborns and approaches 50% when onset occurs in the first 4 days. Listeriosis is associated with a higher mortality rate than other common foodborne pathogens such as Salmonella. In pregnancy-related cases, the postpartum course of the mother is usually uneventful, but the case-fatality rate is 20–30% in infected newborns. The overall case-fatality rate among non-pregnant adults is approximately 30%, with the case-fatality rate higher among patients ≥50 years old (24%) than in other age groups (14%). In a recent epidemic, the overall case-fatality rate among non-pregnant adults was 35%: 11% in those below 40 and 63% in those over 60.

Those at highest risk are neonates, the elderly, immunocompromised individuals, pregnant women, and alcoholic, cirrhotic or diabetic adults. Non-pregnant adults frequently present with sepsis, meningitis, or meningoencephalitis. The onset of meningoencephalitis (rare in pregnant women) can be sudden—with fever, intense headache, nausea, vomiting and signs of meningeal irritation—or subacute, particularly in immunocompromised or elderly hosts. Rhomboencephalitis may rarely occur. Delirium and coma may appear early; occasionally there is collapse and shock. Endocarditis, granulomatous lesions in the liver and other organs, localized internal or external abscesses, and pustular or papular cutaneous lesions may occur on rare occasions. In pregnant women, symptoms may be mild and nonspecific: fever, headache, myalgia, or gastrointestinal symptoms.

Diagnosis is confirmed only after isolation of the infectious agent from CSF, blood, amniotic fluid, placenta, meconium, lochia, gastric washings, and other sites of infection. Listeria monocytogenes can be isolated readily from normally sterile sites on routine media, but care must be taken to distinguish this organism from other Gram-positive rods, particularly diphtheroids. Selective enrichment media improve rates of isolation from contaminated specimens. Microscopic examination of CSF or meconium permits presumptive diagnosis; serological tests are unreliable, and not recommended at the present time.

Infectious agent

Listeria monocytogenes, a Gram-positive rod-shaped bacterium; human infections are usually (95%) caused by serotypes 1/2a, 1/2b, 1/2c and 4b.


An uncommonly diagnosed infection that occurs worldwide; in the USA, the incidence of illness reported in areas under active surveillance requiring hospitalization is about 3.1 cases per 1 million. Although Listeria accounts for a small fraction of all foodborne illnesses, in Europe it is an important contributor to severe illness and accounts for approximately 4% of hospitalizations and 28% of deaths due to foodborne disease. It is often associated with consumption of non-pasteurized milk or milk products, including cheese and ready-to-eat meats. Infection often occurs sporadically; several outbreaks have been recognized in recent years. About 30% of clinical cases occur within the first 3 weeks of life; in non-pregnant adults, remaining infections occur mainly after 40 years of age. Nosocomial acquisition has been reported. Asymptomatic infections probably occur at all ages, although they are of clinical importance only during pregnancy, because of the risk of fetal loss.


The organism mainly occurs in soil, forage, water, mud, livestock food, and silage. The seasonal use of silage as fodder is frequently followed by an increased incidence of listeriosis in animals. Animal reservoirs include infected domestic and wild mammals, fowl, and people. Asymptomatic fecal carriage is common in humans without known exposure (up to 5%), and can be much higher in slaughterhouse workers and laboratory workers who work with Listeria monocytogenes cultures, and in asymptomatic household contacts of persons with invasive listeriosis. Soft cheeses may support the growth of Listeria during ripening, and have caused outbreaks. Unlike most other foodborne pathogens, Listeria can multiply in refrigerated foods that are contaminated; it is extremely hardy in comparison to most bacteria. Studies have shown that Listeria can form—and exist in—biofilm, enabling attachment to, for example, stainless steel surfaces in food production systems. Bacteria in biofilm can show increased resistance to sanitizers, disinfectants and antimicrobial agents. Listeria growing or surviving in biofilm in production facilities can be transferred to food products.

Mode of transmission

Outbreaks have been reported in association with ingestion of raw or contaminated milk, soft cheeses, vegetables, and ready-to-eat meats such as hot dogs, pâté, and deli meats. A substantial proportion of sporadic infections results from foodborne transmission. Papular lesions on hands and arms may occur from direct contact with infectious material. In neonatal infections, the organism can be transmitted from mother to fetus in utero, or during passage through the infected birth canal. There are rare reports of nursery outbreaks attributed to contaminated equipment or materials.

The question of the dose-response relationship for Listeria is debated. Newer risk assessment modeling has enabled the preparation of dose-response curves, reflecting the fact that the infection process should be viewed as a probability of infection related to the dose ingested. These models seem to suggest a 10−9 to 10−13 probability for infection with a dose of 100 organisms, and a 10−6 to 10−9 probability for infection at 1 000 000 organisms.

Incubation period

Variable, and longer than most common foodborne pathogens; cases have occurred 3–70 days after a single exposure to an implicated product. Estimated median incubation is 3 weeks.

Period of communicability

Mothers of infected newborn infants can shed the infectious agent in vaginal discharges and urine for 7–10 days after delivery, rarely longer. While fecal-oral transmission from mother to child during vaginal birth may account for some infections in newborns and nosocomial transmission has been documented in newborn nurseries, the primary modes of transmission are transplacental in neonatal cases and foodborne in others. Asymptomatic carriage of Listeria monocytogenes has been well documented; infected individuals can shed the organisms in their stools for several months. Secondary infections among household contacts have not been identified.


Fetuses and newborns are highly susceptible. Infection in children and young adults generally cause less severe disease than in the immunocompromised and the elderly. There is a strong association between decreased immunity (particularly cell-mediated) and invasive listeriosis, and disease is often superimposed on other debilitating illnesses or conditions such as malignancy, organ transplantation, diabetes, cirrhosis, renal disease, heart disease, HIV infection, and in those on corticosteroids. In immunocompetent hosts, Listeria may be more likely to manifest as febrile gastroenteritis. There is little evidence of acquired immunity, even after prolonged severe infection.

Methods of control

  1. Preventive measures:
    1. Pregnant women and immunocompromised individuals should avoid ready-to-eat meats and other foods (unless heated until steaming hot); smoked fish; and soft cheeses made with unpasteurized milk. They should cook leftovers or foods such as hot dogs until steaming hot. They should also avoid contact with potentially infectious materials, such as aborted animal fetuses on farms.

    2. Ensure safety of all foods of animal origin. Pasteurize all dairy products where possible. Irradiate soft cheeses after ripening or monitor non-pasteurized dairy products, such as soft cheeses, by culturing for Listeria.

    3. Processed foods found to be contaminated by Listeria monocytogenes (e.g. during routine bacteriological surveillance) should be recalled.

    4. Thoroughly wash raw vegetables before eating.

    5. Thoroughly cook raw food from animal sources such as beef, pork, or poultry.

    6. Wash hands, knives, and cutting boards after handling uncooked foods.

    7. Avoid the use of untreated manure on vegetable crops.

    8. Veterinarians and farmers must take proper precautions in handling aborted fetuses and sick or dead animals, especially sheep that died of encephalitis.

  2. Control of patient, contacts and the immediate environment:
    1. Report to local health authority: Obligatory case report required in many countries, Class 2; in others, report of clusters required, Class 4 (see Reporting).

    2. Isolation: Enteric precautions.

    3. Concurrent disinfection: Not applicable.

    4. Quarantine: Not applicable.

    5. Immunization of contacts: Not applicable.

    6. Investigation of contacts and source of infection: Case surveillance data—especially strain characteristics—should be analyzed frequently (weekly) for possible clustering. Patients in all suspected clusters should be interviewed promptly to identify common-source exposures, for rapid outbreak identification. Relatively low incidence and long incubation periods can make identifying Listeria outbreaks difficult; therefore, prompt and thorough investigation of all cases is important.

    7. Specific treatment: Penicillin or ampicillin alone or together with aminoglycosides. For penicillin-allergic patients, trimethoprim-sulfamethoxazole or erythromycin is preferred. Cephalosporins, including third-generation cephalosporins, are not effective in the treatment of clinical listeriosis. Tetracycline resistance has been observed. A Gram-stain smear of meconium from clinically suspected newborns should be examined for short Gram-positive rods resembling L. monocytogenes. If positive, prophylactic antibiotics should be administered as a precaution.

  3. Epidemic measures: Investigate suspected outbreaks to identify a common source of infection, and prevent further exposure to that source.

  4. Disaster implications: None.

  5. International measures: WHO Risk assessment of Listeria monocytogenes in ready-to-eat food:






[CCDM19: M. Iwamoto, C. Olson, J. Schlundt]
[CCDM18: P. Martin]

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