Pregnant women and their newborns, older adults, and people with weakened immune systems are at highest risk for Listeria infection and its complications.
The study - led by Ted Golos, a reproductive physiologist and professor of comparative biosciences and obstetrics and gynecology at the University of Wisconsin-Madison (UW-Madison) - is published in the journal mBio.
Eating food contaminated with Listeria is the most common cause of an infection called listeriosis, which, in the majority of cases, causes mild symptoms such as fever, vomiting, and diarrhea that go away after a few days.
However, in some rare cases, listeriosis can become invasive and lead to serious complications, such as septicemia and meningitis.
In these severe cases, the signs can include severe headache, stiff neck, and tremors.
According to the Centers for Disease Control and Prevention (CDC), an estimated 1,600 people in the United States get listeriosis each year, and about 260 die of it.
Listeriosis is most likely to strike pregnant women and their newborns, older adults, and people with weakened immune systems.
In the 1990s, outbreaks of Listeria were mostly linked to eating deli meats and hot dogs. Nowadays, they are more likely to be traced to consumption of dairy products, such as soft cheeses and ice cream, and other produce such as celery, seed and bean sprouts, and cantaloupe melon.
Listeria may often go unnoticed in pregnancy
Listeria infection may often go unnoticed in pregnancy - the few recognizable symptoms are so similar to the discomfort many women feel in pregnancy it may not occur to them that they have the infection.
Fast facts about Listeria
- Pregnant women are 10 times more likely to become infected with Listeria than other people.
- Soft cheeses made from unpasteurized - rather than pasteurized - milk are estimated to be 50-160 times more likely to cause infection.
- People at higher risk of infection are also advised not to eat raw melon, raw sprouted seeds and beans, and deli meats.
However, even when a Listeria infection only gives rise to mild symptoms in the pregnant woman, it can be a different story for the fetus.
Prof. Golos explains this is similar to what happens with the Zika virus - the effect on the fetus can be much more profound than the effect on the pregnant woman.
For their study, the team obtained a strain of Listeria known to have caused miscarriage, stillbirth, and premature delivery in at least 11 pregnant women in 2000.
The researchers fed doses of the Listeria strain to four pregnant rhesus macaques at the Wisconsin National Primate Research Center and monitored in detail the progression and spread of infection and changes in the fetuses. The doses were equivalent to what might be present in contaminated food.
None of the monkeys showed signs of infection before their pregnancies abruptly ended. However, samples of tissue from the uterus of each monkey after their fetuses had died showed that Listeria had invaded the placenta, as well as the lining of the uterus.
Fetus may be infected before anyone realizes
The placenta normally shields the fetus from maternal infections. "It should be a barrier," Prof. Golos explains, "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 team speculates that perhaps the inflammation caused by the maternal immune response to the rapid invasion of the Listeria cells also affects the placenta, impairing its ability to shield the fetus.
The findings suggest miscarriages that are often diagnosed as having no specific cause may be a result of infection by Listeria and perhaps other pathogens that have this effect. The challenge is how to spot and control such fast-acting, stealthy germs.
Lead author Bryce Wolfe, a graduate student at UW-Madison who is studying cellular and molecular pathology, did most of the work monitoring the speed and progression of the Listeria infection in the monkeys. He explains:
"There are effective antibiotics available. It is treatable. The issue is that because it's asymptomatic, the fetus may be infected by the time anyone realizes the mother was infected."
The team now plans to look more closely at how Listeria targets the reproductive tract, its incubation period, and the changes that occur in the lead-up to miscarriage.
The researchers want to find out more about the underlying biology of how the infection progresses and how the maternal immune system responds. They suggest such basic knowledge could also help researchers investigating other pathogens like the Zika virus.
"For many years, Listeria has been associated with adverse outcomes in pregnancy, but particularly at the end of pregnancy. What wasn't known with much clarity before this study is that it appears it's a severe risk factor in early pregnancy."
Prof. Ted Golos