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Risk of Miscarriage Increased by Listeria Infection

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  • Listeria has been known to cause complications particularly if it occurs at the end of pregnancy.
  • But new research suggests that if listeria occurs early in pregnancy, it increases the risk of miscarriage.
  • The maternal immune response to listeria causes inflammation that affects the placenta, preventing it from offering protection to the fetus.

Risk of Miscarriage Increased by Listeria Infection

Listeria infection during pregnancy increases the risk of miscarriage.

Listeria monocytogenes is a common food-borne bacterium, which is the third leading cause of death from food poisoning in the United States.


In U.S, about 1,600 Americans sick each year and around 260 die, according to the CDC.

Pregnant women and their newborns are much more likely to get a Listeria infection, which is called listeriosis. The outcomes maybe severe in older adults with undeveloped or weak immune system.

The researchers at the University of Wisconsin-Madison School of Veterinary Medicine are studying how pathogens affect fetal development and change the outcome of pregnancy.

"For many years, listeria has been associated with adverse outcomes in pregnancy, but particularly at the end of pregnancy," says Ted Golos, a UW-Madison reproductive physiologist and professor of comparative biosciences and obstetrics and gynecology.

"What wasn't known with much clarity before this study is that it appears it's a severe risk factor in early pregnancy." Golos added.

"The problem with this organism is not a huge number of cases. It's that when it is identified, it's associated with severe outcomes," says Charles Czuprynski, a UW-Madison professor of pathobiological sciences and director of the UW-Madison Food Research Institute.

When listeria infection occurs during pregnancy, it can go unnoticed due to only a few recognizable, indistinguishable symptoms. These symptoms do not differ from the discomfort most newly pregnant women feel.

Therefore, pregnant women are warned to avoid many of the foods like unpasteurized milk and soft cheese, raw sprouts, melon and deli meats not carefully handled as these can harbor listeria, because the bacterium is known to cause miscarriage and stillbirth, and spur premature labor.

"It's striking that mom doesn't get particularly ill from listeria infection, but it has a profound impact on the fetus," says Golos, whose work is funded by the National Institutes of Health. "That's familiar now, because we've been talking about the same difference in Zika virus."


Sophia Kathariou, a North Carolina State University professor of food science and microbiology, provided a strain of listeria that caused miscarriage, stillbirth and premature delivery in at least 11 pregnant women in 2000.

Four pregnant rhesus macaques at the Wisconsin National Primate Research Center were fed doses of the listeria comparable to what one might encounter in contaminated food.

The lead author of the study, Bryce Wolfe, monitored the speed and progression of listeria's spread.

"What's particularly striking about the work Bryce did is the detailed information we now have about the organism," Czuprynski says.

"The animal ingested it; she tracked it being shed in feces and showing up in the bloodstream. They did ultrasound analysis of the fetus, and could then show events in terms of where the organism was preceding fetal demise." Czuprunski added.


The monkeys did not show any sign of infection before their pregnancies came to abrupt ends.

On analyzing the tissue samples taken after each monkey experienced intrauterine fetal death, researchers found listeria had invaded the placenta and the lining of the uterus called endometrium.

The placenta is the medium through which the developing fetus receives nutrients and oxygen. It usually acts as a barrier by preventing infection.

"In that region, there's a rich population of specialized immune cells, and it is exquisitely regulated," says Wolfe. "When you introduce a pathogen into the midst of this, it's not very surprising that it's going to cause some sort of adverse outcome disrupting this balance."

Listeria triggers the maternal immune response that leads to inflammation. This affects the placenta, preventing it from protecting the fetus.

"It should be a barrier," Golos says. "But we're hypothesizing that the maternal immune system's attempt to clear the bacteria actually results in collateral damage to the placenta that then allows the bacteria to invade the fetus."


The results suggest listeria and perhaps other pathogens may be the culprit behind some miscarriages that usually occurs without a diagnosed cause.

The speed of invasion and absence of obvious symptoms is what makes it difficult to detect and treat the condition

"There are effective antibiotics available. It is treatable," Wolfe says. "The issue is that because it's asymptomatic, the fetus may be infected by the time anyone realizes the mother was infected."

Researchers plan to continue work with listeria to understand how the bacterium targets the reproductive tract, its incubation time and the problems it causes leading up to miscarriage.

The results are published in the journal mBio.


  1. People at Risk - Pregnant Women and Newborns - (https://www.cdc.gov/listeria/risk-groups/pregnant-women.html)
  2. Ted Golos et al. mBio; (2017)

Source: Medindia

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