08.07.2025

bioRxiv preprint | posted July 7, 2025

A new study demonstrates that infection with bovine H5N1 in pregnant mice results in distinct tissue tropism and enables vertical transmission of the virus, both in utero and postpartum. Specifically, infection during the second trimester (corresponding to embryonic day 10 in mice) led to detectable viral presence in the uterus, placenta, and fetus, indicating in utero transmission. When infection occurred later in pregnancy (third trimester equivalent), the offspring were born smaller and exhibited neurodevelopmental delays. In these cases, the virus was detected in milk and neonatal lung tissues, suggesting postnatal transmission via lactation. These findings provide critical insights into the pathogenesis of H5N1 during pregnancy and highlight its potential to adversely affect fetal development and neonatal health.