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Reporting Obligations

Individuals who have or may have listeriosis shall be reported to the local Health Unit as soon as possible. Sporadic cases of the diarrheal form of Listeriosis are not reportable.

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Aetiologic Agent

Listeriosis is an opportunistic infection caused by the agent Listeria monocytogenes (L. monocytogenes), a facultative anaerobic, nonspore-forming, motile, Gram-positive bacillus that produces a narrow zone of hemolysis on a blood agar medium.

The bacteria are found in soil, water, animals and humans. Asymptomatic fecal carriage is common in humans. Bacteria can thrive and multiply at refrigeration temperatures.

Clinical Presentation

A person with listeriosis usually has fever, diarrhea, and sometimes, nausea and vomiting. In invasive disease, the bacteria may infect the brain and the membrane lining the brain causing meningitis or septicemia. The onset of meningitis may be sudden, with fever, intense headache, nausea, and vomiting. Complications include endocarditis (the bacteria infects the membrane lining of the cavities of the heart), and internal and external abscesses.

Infected pregnant women may have mild non-specific symptoms – fever, headache, myalgia, and gastrointestinal symptoms. An infected pregnant woman may unknowingly pass on the illness to her unborn child in utero.Placental invasion can lead to fetal infection resulting in premature delivery, spontaneous abortion, stillbirth or neonatal infection (presenting septicemia or meningitis).

Twenty to thirty percent of neonatal infections are fatal. If onset of illness occurs within the first four days of life, the case-fatality rate is 50%.

Note: individuals may present with mild enteric symptoms, but could progress to more severe forms of disease.

Modes of Transmission

The main route of transmission is foodborne, through ingestion of contaminated food such as ready-to-eat meats (e.g., deli meats), unpasteurized milk and soft cheeses, and raw vegetables and cantaloupe melons. Listeria biofilms in food production systems can be transferred to food products. Vegetables can become contaminated from the soil or from manure used as fertilizer.

In utero or perinatal transmission can occur.

Incubation Period

The incubation period for self-limiting, febrile gastroenteritis following ingestion of a large inoculum is 24 hours, illness typically lasts 2 to 3 days. For invasive disease incubation period is typically 2 to 3 weeks; however, cases have occurred up to 70 days following a single exposure to a contaminated product. The incubation period for pregnancy-associated cases is longer than for nonpregnancy-associated cases.

Period of Communicability

Infected persons can shed the bacteria in stool for several months; mothers of infected newborns may shed the infectious agent in vaginal discharges or urine for 7–10 days after delivery.

Risk Factors/Susceptibility

Those at highest risk are fetuses and neonates, the elderly, immunocompromised persons and pregnant women. Healthy children and young adults are generally resistant; adults less so after age 40. Disease is frequently superimposed on other conditions such as cancer, organ transplantation, diabetes and AIDS.

Diagnosis and Laboratory Testing

Listeriosis cannot be diagnosed clinically, given the many causative agents that may present with similar non-specific symptoms.

Lab isolates are sent by the Public Health Ontario Lab to the Public Health Agency of Canada for typing.

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Treatment and Case Management

Treatment is under the direction of the individual’s health care provider. Instruct patient to keep suspect foods for laboratory analysis. Testing can be arranged through local health unit.

Patient Information

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Additional Resources

Public Health Agency of Canada. “Listeria.”

Public Health Ontario. “Listeriosis”.


Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2022.