Pregnant women and their developing babies are particularly susceptible to listeria, which can cause miscarriage or stillbirth, or even encourage premature labour. For this reason, many expectant mothers avoid foods like soft cheeses, ice-cream, and raw eggs, despite the rate of listeria infection being relatively low.
Now a new study has found that listeria may pose a greater risk of miscarriage in the early stages of pregnancy than was originally thought. Researchers at the University of Wisconsin-Madison School of Veterinary Medicine studied how pathogens affect fetal development and change the outcome of pregnancy, and determined that listeria infection is a “severe risk factor in early pregnancy”.
“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,” say the authors of the study.
Indeed, the researchers assert that listeria infection in pregnancy may go unnoticed, with the few recognizable symptoms largely indistinguishable from the discomfort most newly pregnant women feel. While the mother-to-be may not feel particularly unwell, the infection has a profound impact on the fetus.
The study team used 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 speed and progression of listeria's spread was then monitored.
It was seen that none of the monkeys showed obvious signs of infection before their pregnancies came to abrupt ends. But in tissue samples taken after each monkey experienced intrauterine fetal death, it was found that listeria had invaded the placenta, as well as the endometrium, the lining of the uterus.
The researchers believe the inflammation caused by the maternal immune response to the fast-moving listeria also affects the placenta, keeping it from protecting the fetus.
“We 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,” explained the researchers.
These results suggest listeria (and perhaps other pathogens) may be the culprit in some miscarriages that usually go without diagnosed cause, but they warn that the rapid spread of the bacteria will make it difficult to control. In addition, due to the largely asymptomatic nature of the infection, antibiotics may not be used in time.
The researchers will continue to work with listeria in order to better define how the bacterium targets the reproductive tract, its incubation time and the problems it causes leading up to miscarriage. Their goal is to provide basic knowledge about the progression of infection and the maternal immune response to intracellular pathogens in pregnancy, which may help other researchers battling similar dangers such as Zika virus.
The original study can be found at: http://mbio.asm.org/content/8/1/e01938-16