Avian influenza, commonly referred to as the “bird flu,” is spreading rapidly across the United States. In a recent “Health 1+1” episode, Xiaoxu Sean Lin, a U.S. virology expert and former researcher at the Walter Reed Army Institute of Research, said that human infections with avian influenza were extremely rare prior to 2023. However, Lin warns that certain key mutations in the virus could significantly increase the risk if not closely monitored.

The Scale of the Outbreak

More than 160 million poultry have either died from avian influenza or been culled to control the spread of the infection, according to the U.S. Centers for Disease Control and Prevention. At the same time, instances of cross-species transmission have continued to increase. To date, 70 confirmed human cases of avian influenza infection have been confirmed, most of which are linked to transmission from dairy cattle.

H5N1, which primarily spreads among birds, has nonetheless crossed species barriers, with infections now confirmed in dairy cows, cats, and other mammals, Lin said. This suggests that the virus may be adapting to new hosts.

The first cases of H5N1 avian influenza in dairy cows were reported in the United States in March 2024. As of March 10, 2025, at least 17 states have reported outbreaks among cattle. Additionally, there have been multiple reports of domestic cats contracting and dying from avian influenza. At the end of last year, 20 big cats at a wildlife sanctuary n Washington State,  including an Amur/Bengal tiger, four cougars, an African caracal, two Canada lynx, a Geoffroy cat, a Bengal cat, a Eurasian lynx, four bobcats, and five African servals, died from the virus.

Lin pointed out that while H5N1 is primarily transmitted among poultry, cross-species infections have occurred, spreading to dairy cows, cats, and other mammals. This suggests that the virus may be adapting to different hosts. He emphasized that if the virus continues to spread within populations, its adaptation to humans could accelerate.

There is no evidence of widespread human-to-human transmission of H5N1. While there have been occasional reports of human cases, they have typically been linked to direct contact with infected poultry. The risk of human-to-human transmission remains low, but experts continue to monitor the situation closely, as the virus’s ability to adapt to new hosts could potentially change over time.

Mutation Risks of the Avian Influenza Virus

The highly pathogenic H5N1 avian influenza virus, specifically the Clade 2.3.4.4b lineage, is currently the predominant strain circulating in the United States. Due to the virus’s frequent mutations, its classification has become increasingly complex, Lin noted.

The H5N1 virus has acquired mutations at several critical sites, which may enhance its adaptability and ability to infect other species, Lin noted.

Key mutations identified include:

• PB2 protein: The E627K amino acid substitution has been identified, enabling the virus to replicate more efficiently in mammalian hosts, including humans, pets, and livestock.


• Hemagglutinin (HA) protein: Mutations in this protein have increased the virus’s binding affinity to host cell receptors, potentially broadening its host range.


• Neuraminidase (NA): Frequent reassortment events—processes where viruses exchange genetic material— involving the neuraminidase (NA) gene have been observed, which may lead to the emergence of novel recombinant viruses.

These key mutations could facilitate a “host jump,” allowing the virus to cross species barriers—shifting from infecting birds to mammals, including humans—and successfully establish infection. This makes disease surveillance and containment more challenging.

“Just one or two additional critical mutations could enable the virus to spread widely among humans,” Lin said.

Vaccine Effectiveness and Concerns

Last month, the U.S. Department of Agriculture announced a $1 billion initiative to combat avian influenza and address rising egg prices. The funding will be distributed as follows:

• $500 million to strengthen biosecurity measures at poultry farms, such as preventing wild birds from transmitting the virus to commercial flocks.


• $400 million to support poultry farmers, helping those affected by culling to restore their operations.


• $100 million for vaccine research and the development of treatment options.

Additionally, the USDA will explore strategies such as increasing egg imports and reducing exports.

Lin noted that China has already implemented a nationwide poultry vaccination program. While it offers partial protection for poultry, it has not prevented outbreaks in the country. Widespread vaccination can lead to viral mutations that enable the virus to evade immunity, similar to what occurred with COVID-19. Despite global vaccination efforts, the Omicron variant emerged and continued to spread, developing a stronger ability to bypass immune defenses.

Lin recommends not only advancing antiviral drug development alongside vaccines but also reassessing current containment strategies, including whether entire flocks should be culled upon detecting infections or if pharmaceutical interventions could help poultry combat the virus more effectively.

Precautionary Measures Against Avian Influenza

Lin advises the U.S. to boost bird flu monitoring and testing, expedite test approvals, and collaborate internationally to track viral changes and predict outbreaks.

Amid the ongoing spread of avian influenza, Lin recommends a few practical, common sense steps to reduce the risk of infection:

Monitor the health of poultry

Households with backyard chickens should watch for signs of illness—such as sudden, unexplained deaths of multiple birds—and report any concerns to local health authorities immediately.

Avoid contact with dead birds

If you find dead birds, including ducks, along shorelines or in other areas, do not touch or move them. Instead, notify local health authorities for proper handling and disposal.

Avoid consuming raw milk

For now, refrain from drinking raw (unpasteurized) milk, as dairy cows infected with avian influenza may produce milk containing the virus. A recent study from Stanford University found that the avian influenza virus in raw milk can remain infectious even after five days of refrigeration.

Maintain Good Hygiene

Although there are currently no confirmed cases of human-to-human transmission of avian influenza, it is important to remain vigilant. Practice good hygiene, by washing hands regularly, especially if you work with or handle animals.