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Evaluation and Management of Listeria Monocytogenes Exposure and Diagnosis in Pregnancy

Author: B. Kate Neuhoff, MD

Mentor: Patrick S. Ramsey, MD, MSPH
Editor: William Po, MD

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Listeriosis is primarily caused by eating food contaminated with the bacterium Listeria monocytogenes. Most affected persons present with a self-limiting flulike illness or gastroenteritis. In the most severe cases, individuals can develop fever, vomiting, and endocarditis or brain abscesses if immunocompromised.

The foods responsible for L monocytogenes outbreaks range from the most common culprit—unpasteurized cheese—to deli meat, unwashed produce, melon, and ice cream. While most store-bought cheeses in the United States are pasteurized, queso fresco is in many melted cheese dishes and remains an exposure risk. Mitigation of infection risk can be accomplished by avoiding high-risk foods, thoroughly washing fruits and vegetables, and practicing hand hygiene. The overall incidence of listeriosis is low; the Centers for Disease Control estimates about 1600 cases in the United States annually. However, infection is more common and more serious in vulnerable populations such as newborns and elderly, immunocompromised, or pregnant persons. Pregnant individuals are 10 to 20 times more likely to develop listeriosis, with an incidence of about 12 per 100,000.

Listeriosis is usually mild in pregnant persons. However, vertical transmission carries a risk of abortion or stillbirth as high as 20%, with a greater than 60% risk of neonatal listeriosis in the surviving infant. These risks are trimester dependent; although infection is more common in the third trimester, risk of miscarriage or stillbirth is significantly higher earlier in gestation. One cohort study noted a fetal loss following L monocytogenes infection of 100% in the first trimester, 70% in the second, and less than 5% in the third.

Diagnosis of listeriosis can be challenging, and recommendations for treatment are primarily based on symptoms and presumptive exposure. L monocytogenes is most easily isolated in blood cultures, and confirmation of infection is difficult in the absence of bacteremia. Stool culture for common enteric pathogens has a low sensitivity for L monocytogenes isolation and should not be used for diagnosis, although it can help rule out other causes of gastroenteritis. In a patient presenting with an exposure, testing and treatment are not recommended unless the patient is symptomatic. An “exposure” should be considered when a patient ingests either food implicated in a listeriosis outbreak or food with high risk of contamination. If the patient presents with symptoms but is afebrile, the benefit of treatment is uncertain. However, it is reasonable to proceed with blood cultures, stool studies, and oral antibiotic therapy (amoxicillin), with close monitoring for worsening symptoms. If a fever develops, the patient should be hospitalized and started on intravenous antibiotic therapy (ampicillin).

L monocytogenes infection is rare, but reasonable caution should be exercised in avoiding high-risk foods and ensuring good hygiene in food preparation and storage. Treatment or prophylaxis in an asymptomatic patient with an exposure is not recommended, but prompt treatment of a patient with a febrile gastroenteritis and high likelihood of exposure could improve both maternal and neonatal outcomes.  

Further Reading:

Committee Opinion No. 614: Management of pregnant women with presumptive exposure to Listeria monocytogenes. Obstet Gynecol. 2014 Dec;124(6):1241-1244. doi: 10.1097/01.AOG.0000457501.73326.6c. PMID: 25411758.

Centers for Disease Control and Prevention (CDC). Prevent Listeria

Craig AM, Dotters-Katz S, Kuller JA, Thompson JL. Listeriosis in Pregnancy: A Review. Obstet Gynecol Surv. 2019 Jun;74(6):362-368. doi: 10.1097/OGX.0000000000000683. PMID: 31216045.

Initial Publication: March 10, 2024


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