Researchers in Australia and Ireland have uncovered why, in pregnant mice, the flu can be more dangerous during pregnancy, identifying an immune sensor that becomes overactive and triggers harmful inflammation. 

In most people with flu, the virus remains in the upper respiratory tract — mainly the nose and throat — and is cleared before spreading further. While severe flu — due to influenza viruses spreading beyond the lungs into the heart and blood vessels — can develop in some people, it is more frequent during pregnancy, increasing the risk of severe complications for the mother, such as bronchitis and pneumonia, and the baby, including premature birth, lower birth weight andimpaired brain development. 

Researchers from RMIT University, Adelaide University and EARA member Trinity College Dublin, investigated the role of an immune sensor that helps the body recognise influenza viruses, called Toll-like receptor 7 (TLR7). In pregnant mice infected with influenza virus, the researchers found that animals genetically modified to lack TLR7 developed much milder symptoms despite having the same levels of viruses in the lungs as non-modified mice. The absence of TLR7 reduced harmful inflammation and prevented damage to blood vessels normally caused by flu infection during pregnancy. 

The offspring of mice lacking this immune sensor also had higher body weight and lower levels of inflammation in the developing brain compared with offspring from infected mice with TLR7. 

“With this new information, we can start to work out how to switch off this TLR7 sensor, which could help prevent the harmful inflammation that makes flu in pregnancy so dangerous,” said Stella Liong from RMIT University and one of the lead authors of the study published in Science Advances.