Researchers Are Watching H5N1 Avian Influenza and What It Could Mean for Human Health

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Often described as a surprising virus, influenza has the extraordinary ability to constantly evolve, evade immune defenses and trigger unpredictable events that can lead to pandemics. Avian influenza A, H5N1, first identified in Asia more than three decades ago, is among the most concerning influenza viruses today. Over the years, it has evolved to infect mammalian species and has spread throughout the world.

University of California San Diego School of Medicine researchers led by Carrie L. Byington, MD, professor of pediatrics, recently published a research review in the journal Pediatrics highlighting the importance of further research centering on H5N1 and the possibility of human-to-human transmission.

The School of Medicine spoke with Byington about the research and potential implications.

Why should we be concerned about H5N1?

I am a pediatric infectious diseases physician, and my background is in pandemic preparedness. One of the viruses I study is influenza. Influenza viruses cause disease every year and can lead to pandemics. Avian influenza H5N1, an influenza strain that infects birds, has been on our radar as a potential threat to humans for about 30 years.

However, around 2020, this virus started to evolve very rapidly and began to infect mammals. In 2024, the virus was found in cattle. This was really surprising for those of us who study influenza because cows were not thought to be susceptible to influenza. The most shocking thing to me was discovering that the virus was concentrated in the cow’s milk. Furthermore, dairy workers were becoming infected with avian influenza because of contact with cow’s milk through their work.

As a pediatrician, my primary concern is protecting the health of infants and children. The H5N1 outbreak in cattle raised many important questions for children. Can infants and children be infected by drinking the cow’s milk? And more importantly, what happens if a human mother gets infected with H5N1? Can the virus spread to breast milk? Those concerns led to the research review.

What is the most important takeaway from the review?

There are several, but one of the most important was looking at how influenza viruses bind to cells. Viruses use specific receptors to attach to and enter cells. So, some receptors on bird cells specifically bind avian flu and that’s why birds get sick. Some receptors in the human respiratory tract bind to the human flu and that’s why we get sick. We originally thought cows were protected because they didn't have the receptors to bind to influenza. But this strain of H5N1 can bind to receptors that cows have in their udders. We wanted to find out if humans had the same receptors in mammary tissue. Working with the Department of Pathology we completed a study and found receptors for avian influenza on cells in human breast tissue. Other groups of researchers in the U.S. and in China have recently achieved similar results, so we are confident in saying that human breast tissue has receptors that can bind avian influenza.

What should we do with this knowledge?

We know that H5N1 is now found in many species of birds and mammals. Infections in wild animals have been documented in elephant seals, dolphins, ferrets, mink, mice, lions and tigers. Importantly, infections are now found in a host of domestic animals such as livestock, like goats and cows and domestic birds like chickens, ducks and geese, and even in companion animals like cats. So, H5N1 has spread widely, and because it is found in livestock and peridomestic animals such as rodents and companion animals, humans are more likely to encounter the virus and become infected.

We are at a point of needing more than casual observation. We must be prepared and understand what the virus might do if it were to become widespread in humans. We know, theoretically, that H5N1 can bind to breast tissue, but if it does, can H5N1 infect and reproduce in human breast milk? Most importantly, can human infants be infected with H5N1 through consuming unpasteurized milk, either dairy milk or human milk? We just don't know these answers yet.

Importantly, we do know that pasteurization rapidly reduces viral titers in dairy milk and that H5N1 does not appear to be transmitted through consuming pasteurized milk. We believe pasteurizing human milk, as is currently performed in the University of California Health Milk Bank for donor milk, would also eliminate any viable H5N1 virus.

What is the next step that we should be taking?

To answer some of the important questions, we should be performing research now, both basic laboratory science and work with human populations. The SARS-CoV-2 pandemic offers some guidance. Fairly early in the pandemic there were reports that women with COVID-19 had SARS-CoV-2 detectable in their breast milk. This finding raised many concerns. Fortunately, studies done at the UC San Diego Human Milk Institute showed that although the SARS CoV-2 virus does get into breast milk, the virus is not “alive” or able to replicate, and therefore unable to infect the infant through breastfeeding. We need to be thinking about similar studies now for avian flu. We are really fortunate at UC San Diego to have the resources to assist in performing the necessary research. Protecting the human milk supply during pandemics is essential for the health of infants and one of the major goals of the scientists involved in this work.

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