Epidemiology

Listeriosis is rare, with a laboratory-confirmed incidence of 0.24 cases per 100,000 people.⁴ However, an estimated one in seven cases are associated with pregnancy (ie, either the pregnant patient or their neonate), underscoring the need for obstetrical providers to be aware of the symptoms of infection and the associated pregnancy-related risks.⁵ The higher incidence during pregnancy may be due to immune alterations that increase the risk of developing listeriosis. Additionally, there may be increased testing and diagnosis in pregnant patients because of the known risks of listeriosis during pregnancy. Among individuals of female sex ages 15 to 44 years, listeriosis risk is higher in Hispanic and non-Hispanic Asian individuals than non-Hispanic White individuals.⁵ Although the case-fatality rate of invasive listeriosis is 20% across all populations, the case-fatality rate during pregnancy is low for pregnant patients but approximately 25% for the fetus or neonate.⁴

Transmission

Listeriosis is a foodborne illness, meaning it is transmitted through ingestion of bacteria-contaminated food.^1,3 Because L. monocytogenes is present in the environment, including soil, water, and decaying vegetables, it has the potential to contaminate a wide variety of foods.⁶ Historically, infections caused by L. monocytogenes were commonly associated with consumption of raw or unpasteurized soft cheeses, queso fresco-type cheeses, and cold deli meats (both packaged and deli sliced), which is why it is recommended to avoid these foods during pregnancy.⁷ However, the range of food sources in outbreaks has expanded to include produce, such as sprouts, melons, leafy greens, and enoki mushrooms; caramel apples; ice cream products; precooked meats; prepared pasta dishes; and other packaged prepared meals (Box 1).⁷ For a list of current or prior L. monocytogenes outbreak investigations, visit the Centers for Disease Control and Prevention’s (CDC’s) web page on Listeria outbreaks (https://www.cdc.gov/listeria/outbreaks/index.html).⁸

In addition to the risk of maternal and fetal in utero infection, which typically occurs due to maternal bacteremia, neonatal infections can also present after birth. Transplacental transmission during pregnancy is the most common cause of neonatal infections.¹

Although the presence of L. monocytogenes has been reported in breast milk, there is insufficient evidence regarding the risk of neonatal transmission with breastfeeding.⁹ Maternal listeriosis is not considered a contraindication to breastfeeding.^10,11

Box 1. Examples of foods associated with L. monocytogenes outbreaks.

Prevention

Education to reduce the risk of listeriosis is particularly important during pregnancy, given the nearly 18-fold increased risk of developing listeriosis during pregnancy compared with the nonpregnant population.³ Preventive measures include regular handwashing; washing all fruits and vegetables before consumption; and heating foods that are at risk for L. monocytogenes colonization, such as deli meats, hot dogs, and fermented or dry sausages, to 165°F or until steaming hot before eating. Avoidance of foods known to have higher rates of L. monocytogenes contamination, including unpasteurized milk and cheeses; cold deli meats, hot dogs, and fermented or dry sausages; pasteurized or unpasteurized queso fresco cheeses; refrigerated pâté or meat spreads; and refrigerated smoked seafood, is also recommended during pregnancy to further reduce the risk of infection. (See Box 2.) Additional prevention methods can be found at the CDC’s web page on preventing Listeria infection (www.cdc.gov/listeria/prevention).⁷

Box 2. Methods to reduce the risk of listeriosis during pregnancy.

Pregnancy-Specific Risks of Listeriosis

Those who are pregnant and their neonates are at increased risk for severe symptoms and complications of listeriosis following exposure to contaminated foods._1,3,4 Cell-mediated immunity plays a critical role in fighting intracellular pathogens such as L. monocytogenes. During pregnancy, adaptations in cell-mediated immunity allow tolerance of the antigenically foreign fetus but reduce the ability to mount effective responses against intracellular infections. As a result, pregnancy is associated with an increased susceptibility to listeriosis and a higher risk of more severe disease compared with the nonpregnant state.^1,3

High-dose L. monocytogenes inoculation in pregnant macaque primates caused an extensive neutrophilic inflammatory response in the placenta with disruption of the maternal-fetal barrier. Vasculitis, thrombosis, and necrosis of the decidual spiral arteries and the presence of bacteria in the intervillous maternal circulation, villous capillaries, and umbilical cord were observed.¹³ In addition, L. monocytogenes is an actively invasive pathogen endowed with an efficient cell-to-cell spreading mechanism that bypasses the dependency on cell and tissue damage for successful dissemination, while allowing escape from immune control.¹⁴ Following placental invasion, the bacteria replicate rapidly, typically within trophoblast cells and fetal placental macrophages without reentering the bloodstream, thereby evading the host immune system. Placental infection with abscess formation may occur and lead to fetal septicemia, increasing the risks of miscarriage, preterm labor, and stillbirth.^1,3,15

Maternal Risks

A recent retrospective cohort study evaluating obstetric and maternal morbidity associated with listeriosis during pregnancy in 134 participants reported that pregnant patients with listeriosis were 21 times more likely to experience severe maternal morbidity compared with pregnant patients without listeriosis. The study noted an increased risk of acute respiratory distress syndrome, maternal shock, need for mechanical ventilation, and cesarean delivery. No maternal deaths were reported in this study.¹⁶

Fetal Risks

One of the most serious risks of maternal listeriosis is the risk to the fetus or newborn, with approximately 25% of cases during pregnancy resulting in stillbirth or neonatal death.⁴ In a review of 200 cases of listeriosis during pregnancy, 20% resulted in pregnancy loss or stillbirth. Among surviving neonates, 68% had listeriosis.¹⁷ This is consistent with other studies which show that more than 50% of neonates born to individuals with listeriosis develop symptomatic neonatal infection, including sepsis, pneumonia, or meningitis.¹⁵ Additionally, maternal listeriosis carries a high risk of preterm delivery and its associated risks to the neonate. In one review that identified 248 cases of listeriosis associated with pregnancy, approximately 50% of pregnant patients affected by suspected listeriosis delivered preterm, and 75% of those preterm newborns had meconium staining.¹⁸

Clinical Presentation

The presentation of listeriosis depends on whether the infection is noninvasive (isolated to the gastrointestinal tract) or invasive (disease beyond the gastrointestinal tract). The clinical presentation of listeriosis during pregnancy is directly related to the severity of maternal disease and can be similar to the symptoms seen in other infections, highlighting the critical importance of having a high index of suspicion for listeriosis.^4,6

In otherwise healthy immunocompetent individuals, most cases of listeriosis in nonpregnant patients are noninvasive and self-limited, with either a mild febrile gastrointestinal illness or no symptoms at all. If an individual develops symptoms, it typically presents within 24 hours to a few days of L. monocytogenes exposure, with a brief episode of fever, nausea, and vomiting for which many individuals do not seek medical attention. Those who are pregnant may also be asymptomatic or have only mild symptoms.^2,4

Invasive listeriosis often presents initially with a fever and flu-like symptoms.⁴ It can also present with headache, stiff neck, confusion, ataxia, and seizures. These symptoms can be manifestations of bacteremia, endocarditis, meningitis (including meningoencephalitis), brain abscess, septic arthritis, and osteomyelitis.^4,6 Pregnant patients with invasive listeriosis are more likely to present with symptoms in the second half of pregnancy. The incubation period during pregnancy is longer than outside of pregnancy, with a median incubation period of 19 to 28 days.¹ Many pregnant patients present with nonspecific flu-like symptoms, such as fever, chills, myalgia, abdominal pain, or back pain. They may also present with other signs of infection, including chorioamnionitis (even with intact membranes) or sepsis.^3,4 As a result of placental infection and maternal bacteremia, preterm labor or fetal heart rate tracing abnormalities may also be seen. Because stillbirth is common with listeriosis, some patients may have stillbirth and signs of infection as their first presenting symptoms.^1,15

Regarding fetal findings, there are no data regarding pathognomonic ultrasound findings for maternal or fetal listeriosis, and no specific fetal anomalies have been associated with perinatal infection. A single case report described findings of mild ascites and echogenic bowel in a fetus, ultimately leading to a diagnosis of L. monocytogenes infection, and suggested that ultrasound findings with listeriosis may be similar to those seen with early fetal infection caused by a variety of infectious agents.³

In neonates, invasive listeriosis can be divided into two categories: early onset (presenting within the first week after delivery) and late onset (presenting from 1 week of delivery onward). Early-onset disease often follows preterm birth and presents with pneumonia and septicemia. Amniotic fluid at the time of delivery may appear meconium-stained or green-brown. Granulomatosis infantiseptica, a rare finding seen in some neonates with early-onset listeriosis and stillbirths, presents as an erythematous rash with histologic presence of granulomas and microabscesses. Late-onset disease most often presents with meningitis-like signs or symptoms.¹⁵

Diagnosis

Testing should be considered in pregnant patients who have signs or symptoms that are commonly seen with listeriosis, especially when a possible L. monocytogenes exposure is reported. Testing in asymptomatic individuals, even with potential exposure to a high-risk or outbreak-associated product, is not currently recommended.¹⁹ The diagnosis of listeriosis is made through cultures of body specimens, most commonly via blood cultures. Cerebrospinal fluid cultures should be obtained in any patient with central nervous system (CNS) symptomatology or concerns for meningitis, as individuals may test negative on blood cultures alone.^1,19,20 In febrile pregnant patients with CNS symptoms undergoing a diagnostic lumbar puncture for suspected meningitis, fluid analysis and culture for L. monocytogenes is warranted in addition to any other infectious agents being considered. Amniocentesis with amniotic fluid culture should be considered, as it may aid in diagnosis, and there should be a low threshold for amniocentesis given the severe risks to the pregnancy. Cultures of placenta or other products of conception after delivery may also be helpful.^1,19 Regardless of the specimen, the microbiology or pathology laboratory should be notified regarding the concern for listeriosis, as L. monocytogenes may be misidentified as a contaminant (eg, confused with diphtheroids or streptococci) on Gram stain and dismissed by the laboratory otherwise.¹⁹ Such organisms do not always stain gram-positive, which may complicate results.²¹ Stool testing for L. monocytogenes has extremely low sensitivity, even in a patient with diarrhea, and is not recommended as part of the diagnostic workup.²¹ However, stool testing to rule out other, more common infectious causes of diarrheal infections can be considered. Vaginal testing for L. monocytogenes has no useful role in the diagnosis of an asymptomatic pregnant patient with possible exposure. In symptomatic pregnant patients with suspected exposure to L. monocytogenes, the recommended diagnostic test is blood culture, along with a complete blood cell count to assess for leukocytosis.^19,21 Specific antibody testing for L. monocytogenes is not routinely available and is not recommended in the setting of potential exposure.²¹

Management

The management of L. monocytogenes exposures depends on the exposure risk, history, clinical presentation, and disease severity. The American College of Obstetricians & Gynecologists (ACOG) has published guidance based on expert review on the management of listeriosis in pregnancy, and the Society for Maternal-Fetal Medicine (SMFM) agrees with this guidance.¹⁹ Management is guided by two main considerations: whether the patient is symptomatic and whether the patient is febrile (see Table 1).¹⁹

Table 1. Diagnosis and management of listeriosis based on exposure and symptoms.

Asymptomatic Patients

A common clinical scenario arises when a patient calls or presents with concerns regarding recent consumption of a food product that was recalled due to possible Listeria contamination. If the patient is asymptomatic, testing and treatment are not indicated. Postexposure antibiotic “prophylaxis” in the absence of symptoms, even for pregnant patients, is not recommended. It is important, however, to counsel patients regarding the prolonged incubation period (up to 70 days) for L. monocytogenes and to encourage patients to continue close monitoring for at least 2 months after the consumption of the recalled food. If symptoms arise, the patient should notify their obstetric care provider immediately.¹⁹

Symptomatic but afebrile patients

There are no data to guide the management of a patient who presents following possible Listeria exposure with mild symptoms. SMFM supports the guidance from ACOG that either expectant management or testing with blood cultures can be considered. Expectant management may be considered for patients who have mild gastrointestinal and flu-like symptoms (eg, myalgia, nausea, vomiting, or diarrhea) without fever. It is also reasonable to pursue blood culture testing, but it is important to notify the microbiology laboratory regarding the concern for L. monocytogenes. Some experts treat patients preemptively, though there are no clear data to determine the benefits or risks of treatment. If the blood culture returns positive results, then admission for treatment with intravenous antibiotics is indicated.¹⁹

There is no consistent or evidence-based guidance regarding antenatal testing or assessment of overall fetal well-being in the setting of possible L. monocytogenes exposure.¹⁹ The decision to perform a detailed fetal ultrasound examination as well as antenatal testing should be individualized based on the patient’s symptomatology and medical history.

Symptomatic and febrile patients

Pregnant patients with a possible exposure to Listeria who are symptomatic and febrile require expedient evaluation, including possible hospitalization for antibiotic therapy. Blood cultures should be obtained before any antibiotics are started, followed by empiric intravenous ampicillin administration; if concerns are significant based on symptoms and laboratory data (blood cell count and differential), antibiotics should be started before blood culture results are returned.¹⁹ If there are abnormal neurologic findings or meningeal signs on physical examination, cerebrospinal fluid testing with cultures should be considered, and double-agent antibiotic treatment should be administered (see “Treatment considerations”).²⁰ Consultation with an infectious disease specialist should be considered and the pediatric team should be notified in cases of imminent delivery.

Treatment considerations

In the absence of a true penicillin allergy, the first-line treatment for invasive listeriosis is high-dose intravenous ampicillin (2 g every 4 to 6 hours) for at least 14 days (see Table 2).^19,21 Oral antibiotics will not achieve adequate therapeutic levels for maternal or fetal treatment and are not an acceptable alternative. Gentamicin or a comparable aminoglycoside (depending on drug availabilities or institutional formularies) is occasionally used in combination with ampicillin to create a synergistic effect and to lower mortality rates, particularly in invasive cases with meningitis.^14,21

If the patient reports a history of a significant penicillin allergy (eg, anaphylaxis, urticaria), penicillin desensitization can be considered before proceeding with ampicillin treatment.²² Individuals with a history of any of the following signs or symptoms may not have a true penicillin allergy and are likely to tolerate ampicillin: nonspecific symptoms unlikely to be an allergic reaction (eg, gastrointestinal distress, headaches, yeast vaginitis), nonurticarial maculopapular (morbilliform) rash without systemic symptoms, pruritus without rash, family history of penicillin allergy but no personal history, or patient reports history but has no recollection of symptoms or treatment. However, for patients with more serious symptoms related to past penicillin treatment, including anaphylaxis, urticaria, or pruritus with rash, ampicillin should be avoided.²³ Given the typical urgency of starting maternal treatment in these cases – and the delay typically associated with coordinating desensitization on an inpatient emergency basis – alternative antibiotic therapy should instead be initiated expeditiously.¹⁶ Because cephalosporins are not effective in treating listeriosis, the use of cefazolin or another cephalosporin as an alternative is not recommended.²⁴ The first-line alternative agent in pregnancy is trimethoprim-sulfamethoxazole, given intravenously. Vancomycin is the second-line alternative, because vancomycin treatment failures have been reported, especially in severe cases with CNS involvement. Neonatal meningitis is a frequent manifestation of fetal infection; therefore, if ampicillin cannot be used to treat maternal infection, gentamicin should be added to any second-line agent used for maternal-fetal therapy.^24,25 Many pregnant patients commonly receive iron supplementation, which may enhance the pathogenicity of L. monocytogenes. Iron supplements, especially intravenous iron, should be withheld until antibiotic treatment completion.^26,27

Cases of successful treatment of invasive listeriosis in the first trimester with subsequent live birth have been reported. A literature review by Chan et al described eight cases of maternal listeriosis in the first trimester that were successfully treated.²⁸ Of these, six neonates were delivered at term, and two were delivered at 35 weeks of gestation. All neonates were asymptomatic and tested negative for L. monocytogenes at delivery, and placental evaluation was normal.²⁸ Given the limited number of reported cases in the first trimester, there are insufficient data to guide fetal well-being assessment, antenatal surveillance, or delivery timing after successful treatment in the first trimester. It is reasonable to manage these cases similarly to other perinatal infections with detailed ultrasound examination, fetal growth assessment, and antenatal testing guided by fetal growth and the overall clinical picture.²⁹ Successful treatment of listeriosis in the second or third trimester without delivery remains uncommon, as many patients present with advanced disease, preterm labor, stillbirth, or evidence of intraamniotic infection, which generally precludes expectant management. When maternal listeriosis is diagnosed in the second or third trimester, antenatal surveillance, fetal well-being assessment, and timing of delivery should be individualized.

Table 2. Antibiotic treatment regimens for invasive listeriosis during pregnancy.

Conclusion

Listeriosis is rare but carries significant risks to pregnant patients and their fetuses, including fetal or neonatal death. By providing patient education and counseling on prevention methods, providers may help reduce infections due to L. monocytogenes. Additionally, early identification, diagnosis, and treatment, may reduce the severity of illness and improve outcomes for pregnant patients and their fetuses.

References

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25. Fernandez Guerrero ML, Torres R, Mancebo B, et al. Antimicrobial treatment of invasive non-perinatal human listeriosis and the impact of the underlying disease on prognosis. Clin Microbiol Infect. Jul 2012;18(7):690-695. https://doi.org/:10.1111/j.1469-0691.2011.03616.x

26. Shah AA, Donovan K, Seeley C, et al. Risk of infection associated with administration of intravenous iron: a systematic review and meta-analysis. JAMA Netw Open. Nov 1 2021;4(11):e2133935. https://doi.org/:10.1001/jamanetworkopen.2021.33935

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29. Khalil A, Sotiriadis A, Chaoui R, et al. ISUOG Practice Guidelines: role of ultrasound in congenital infection. Ultrasound Obstet Gynecol. Jul 2020;56(1):128-151. https://doi.org/:10.1002/uog.21991

Suggested Citation: Society for Maternal-Fetal Medicine (SMFM). Diagnosis and Management of Listeriosis during Pregnancy. Washington, DC: SMFM; 2026. Available at: https://www.smfm.org/listeria . Retrieved [enter date]. [DOI Number Pending]

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