A new study in monkeys shows that the flu forces the placenta into a strong defensive mode. This effort may harm the placenta itself. The work appeared in Science Advances. The researchers saw the same reaction in mothers who felt almost fine and in mothers who became very ill.
Dr Kristina Adams Waldorf led the research as the senior author. She is a professor of obstetrics and gynaecology at the University of Washington School of Medicine. She explained that the placenta uses its energy to fight the virus. That leaves less energy for keeping its cells healthy and growing.
Doctors have known for years that the flu during pregnancy can cause serious illness in the mother. It can also lead to early birth and stillbirth. The new results suggest the placenta’s extra workload may put the baby at risk. Adams Waldorf said the placenta may be pulled away from its main job, which is caring for the foetus. She believes a mother’s flu may threaten foetal health more than experts once thought.
She said the findings show why pregnant women should get a flu shot every year. The shot protects both the mother and the baby. She noted that an early flu season may be coming. Protection takes a few weeks to build, so she urged pregnant women to get the shot now. Only about half of pregnant women in the United States get vaccinated, even though the shot lowers the risk of severe illness and stillbirth. The American College of Obstetricians and Gynaecologists recommends it.
How the study worked
The team infected 11 pregnant pigtail macaques with the 2009 pandemic H1N1 flu strain. The infections took place in the third trimester. They compared these animals with 16 pregnant macaques that stayed healthy. Macaques are a good model for human pregnancy. They carry their young for a long time, and their placentas look much like ours.
Five days later, researchers checked the mothers, placentas and foetuses. Under the microscope, all 11 infected mothers showed pneumonia in their lungs. Four of them had shown no outward signs of sickness.
What they found
In the infected placentas, a type I interferon response turned on. This is the body’s early warning system against viruses. Inflammation also rose. At the same time, genes that support the placenta’s normal work became less active. These included genes for cell division, metabolism and protein recycling. This pattern is common in stressed cells.
The placenta’s reaction did not match the mother’s symptoms. It appears to act as a guard for the foetus. When the mother is infected, it goes into full defence no matter how sick she feels.
The virus almost never reaches the foetus. None was found in any foetal organ that was tested. Only tiny traces of viral genetic material showed up in two placentas. Still, two of the 11 infected pregnancies ended in stillbirth. None of the 16 healthy pregnancies did.
Because of the small number of cases, the team could not determine the cause of the stillbirths. They warned against blaming the placental response alone. However, studies in people already connect flu in pregnancy to stillbirth. The authors believe a placenta that neglects its upkeep deserves closer study.
The team also did exploratory work on the foetuses. They saw changes in immune cells in the lungs, lymph nodes and blood. These changes matched how sick the mothers had been.
Orlando Cervantes, now at the Tulane National Biomedical Research Center, was the lead author. He did the work at the UW School of Medicine. The National Institutes of Health funded the work.
