Infectious Disease Consult

Listeria infection in pregnancy: A potentially serious foodborne illness

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In pregnant women and immunocompromised individuals, listeriosis can be devastating, and it poses major danger to the developing fetus



CASE Pregnant patient with concerning symptoms of infection

A 28-year-old primigravid woman at 26 weeks’ gestation requests evaluation because of a 3-day history of low-grade fever (38.3 °C), chills, malaise, myalgias, pain in her upper back, nausea, diarrhea, and intermittent uterine contractions. Her symptoms began 2 days after she and her husband dined at a local Mexican restaurant. She specifically recalls eating unpasteurized cheese (queso fresco). Her husband also is experiencing similar symptoms.

  • What is the most likely diagnosis?
  • What tests should be performed to confirm the diagnosis?
  • Does this infection pose a risk to the fetus?
  • How should this patient be treated?

Listeriosis, a potentially serious foodborne illness, is an unusual infection in pregnancy. It can cause a number of adverse effects in both the pregnant woman and her fetus, including fetal death in utero. In this article, we review the microbiology and epidemiology of Listeria infection, consider the important steps in diagnosis, and discuss treatment options and prevention measures.

The causative organism in listeriosis

Listeriosis is caused by Listeria monocytogenes, a gram-positive, non–spore-forming bacillus. The organism is catalase positive and oxidase negative, and it exhibits tumbling motility when grown in culture. It can grow at temperatures less than 4 °C, which facilitates foodborne transmission of the bacterium despite adequate refrigeration. Of the 13 serotypes of L monocytogenes, the 1/2a, 1/2b, and 4b are most likely to be associated with human infection. The major virulence factors of L monocytogenes are the internalin surface proteins and the pore-forming listeriolysin O (LLO) cytotoxin. These factors enable the organism to effectively invade host cells.1

The pathogen uses several mechanisms to evade gastrointestinal defenses prior to entry into the bloodstream. It avoids destruction in the stomach by using proton pump inhibitors to elevate the pH of gastric acid. In the duodenum, it survives the antibacterial properties of bile by secreting bile salt hydrolases, which catabolize bile salts. In addition, the cytotoxin listeriolysin S (LLS) disrupts the protective barrier created by the normal gut flora. Once the organism penetrates the gastrointestinal barriers, it disseminates through the blood and lymphatics and then infects other tissues, such as the brain and placenta.1,2

Pathogenesis of infection

The primary reservoir of Listeria is soil and decaying vegetable matter. The organism also has been isolated from animal feed, water, sewage, and many animal species. With rare exceptions, most infections in adults result from inadvertent ingestion of the organism in contaminated food. In certain high-risk occupations, such as veterinary medicine, farming, and laboratory work, infection of the skin or eye can result from direct contact with an infected animal.3

Of note, foodborne illness caused by Listeria has the third highest mortality rate of any foodborne infection, 16% compared with 35% for Vibrio vulnificus and 17% for Clostridium botulinum.2,3 The principal foods that have been linked to listeriosis include:

  • soft cheeses, particularly those made from unpasteurized milk
  • melon
  • hot dogs
  • lunch meat, such as bologna
  • deli meat, especially chicken
  • canned foods, such as smoked seafood, and pâté or meat spreads that are labeled “keep refrigerated”
  • unpasteurized milk
  • sprouts
  • hummus.

In healthy adults, listeriosis is usually a short-lived illness. However, in older adults, immunocompromised patients, and pregnant women, the infection can be devastating. Infection in the pregnant woman also poses major danger to the developing fetus because the organism has a special predilection for placental and fetal tissue.1,3,4

Immunity to Listeria infection depends primarily on T-cell lymphokine activation of macrophages. These latter cells are responsible for clearing the bacterium from the blood. As noted above, the principal virulence factor of L monocytogenes is listeriolysin O, a cholesterol-dependent cytolysin. This substance induces T-cell receptor unresponsiveness, thus interfering with the host immune response to the invading pathogen.1,3-5

Continue to: Clinical manifestations of listeriosis...


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