Listeria poses risks in early pregnancy

Listeria, stained green in this image, invades a monkey's uterus during the first trimester of pregnancy.

Image By: Gregory J. Wiepz

Researchers at the UW-Madison recently found that listeriosis, the infection caused by the foodborne bacteria called Listeria, damages the placenta and results in miscarriages during the early stages of pregnancy in non-human primates.

Listeria monocytogenes is a common human pathogen that infects humans after the consumption of contaminated foods like raw milk, cheese, deli meats or produce. It is particularly dangerous to people with defective immune systems and was traditionally considered a threat to the fetus during the late stages of pregnancy.

Collaborating with other scientists at UW-Madison, Ted Golos, a professor in the Department of Comparative Biosciences, guided his student Bryce Wolfe, in their study published in the journal mBio of the American Society for Microbiology.

“The project provides the first detailed examination of what happens in a pregnant primate between ingestion of Listeria and loss of viability of the fetus. This study also indicates that infection with Listeria earlier in pregnancy can be serious and bad for the fetus,” said Charles Czuprynski, chair of the Department of Pathobiological Sciences and director of the Food Research Institute at UW-Madison.

According to the paper, the experiments were conducted using macaque monkeys, primates with a similar reproductive tract and placenta to human beings.

When the pregnant monkeys were infected with Listeria, the organisms could be found in their feces for a period of time but eventually ended up in their blood streams, from which they made their way to the placenta and fetal tissues, said Czuprynski.

Czuprynski said that in examinations of the maternal blood samples and fetal tissues, they found “far greater numbers of organisms in the fetal portion of the placenta and in the fetus itself than in the maternal tissues.”

“So the organism has a propensity to get to the placenta, damage blood vessels in the placenta, then make its way to the fetus itself. And that will cause loss of viability for the fetus,” said Czuprynski. “We saw loss of fetal viability within one to two weeks of infection.”

Listeriosis was traditionally only considered a risk during the last trimester of pregnancy.

But in this study, pregnant monkeys were inoculated at a time equivalent to the end of the first trimester, indicating that Listeria could harm the fetus much earlier than previously thought, said Czuprynski.

He emphasized that Listeria only causes mild infection in the non-reproductive tissues of the mother and relatively mild symptoms in pregnant women, which makes it hard to detect the infection before the fetus is harmed.

According to the paper, “the maternal immune response that protects the mother from serious disease is unable to protect the fetus,” a concept newly illuminated by the recent Zika virus outbreaks.

In human cases, “the biggest problem is knowing the mother has listeriosis and if this is recognized in late pregnancy sometimes the fetus is born infected,” Czuprynski said.

He said that current treatment for babies with listeriosis is antibiotic therapy, but this is not always effective.

Centers for Disease Control and Prevention estimates that listeriosis is the third-leading cause of death from foodborne illness, with a 20 percent of case-fatality rate. Nearly 25 percent of pregnancy-associated cases result in fetal loss or death of the newborn.

It is still unclear what level of exposure to Listeria would harm the fetus, but the study provides a much more detailed understanding regarding Listeria’s growth in pregnant primates, and insights into the series of events that occur when the bacteria attacks the placenta and fetus, said Czuprynski.

After the paper was released online, the research team received an email from a woman who lost her pregnancy in its early stage due to listeriosis.

“There are not a large number of people to whom this has been reported to occur, but she was very happy to see that people were trying to understand how and why this occurs,” said Czuprynski.

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