Could babies get bird flu through breast milk? Maybe, a study hints

Human breast tissue may be capable of hosting — and passing on — bird flu.

Human mammary glands contain sugars that avian influenza can latch onto to infect cells, researchers report August 8 at medRxiv.org. The finding, which has not yet been peer-reviewed, raises the possibility that nursing babies could be infected with bird flu through breast milk.

When H5N1 bird flu was detected in dairy cattle in 2024 — primarily in the mammary glands in the udder — and in cows’ milk shortly after, Carrie Byington began to wonder whether human mammary tissue could also harbor the virus. There were no studies addressing the question in scientific literature, and few studies in animals.

Cows have molecules in their mammary glands that both human and bird flus can use to infect cells. So Byington, a pediatric infectious diseases specialist at the University of California, San Diego, teamed up with pathologists at the school to examine if human mammary glands could similarly be infected.

The researchers used healthy tissue removed from four women who had previously undergone breast surgery. They found that the mammary glands in the tissue samples contain receptors — in this case, a certain class of sugars called sialic acids — that human and pig influenza viruses typically grab onto when infecting their hosts. But the tissue also had specific sialic acid receptors that avian influenza viruses like H5N1 use to infect cells.  

Finding receptors for bird flu in human breast tissue raises the question of whether lactating people who get infected with the virus might pass it to their infants through milk.

“That’s a very pressing question that we should be asking in this time before we’re seeing widespread transmission, or before a pandemic,” Byington says. Since September 2024, 79 people in the United States have tested positive for H5 influenza, mostly farm workers who had mild symptoms. One person died. The virus has not yet evolved the ability to spread easily from person-to-person.

The study is just one step toward understanding the effects of bird flu in human breast tissue, Byington says. The researchers are now investigating whether H5N1 viruses can survive in breast milk, and how the virus might get into the mammary glands and milk. The team also hopes to address whether antiviral medications and vaccines might reduce the risk.