Since 2022, over 168 million poultry have been infected with a strain of H5N1 avian influenza — commonly known as “bird flu.” While largely known to be present in birds and pigs, the first case of this H5N1 strain was confirmed in dairy cattle last March. Now, the virus has begun spreading to people. This multi-species
outbreak is already considered a pandemic among animals and may now pose a direct threat to
humans.

H5N1, like the common cold, is a pathogenic virus; it is able to cause disease by infecting and replicating inside cells. While H5N1 primarily affects birds, it can also be found in pigs. The most recent outbreak adds cows and humans to the mix.

According to Bill Messer, an associate professor in the Department of Molecular Microbiology and Immunology at the Oregon Health and Science University (OHSU), H5N1’s name describes the antigenic markers of the influenza viruses it possesses. Messer says, “One of the ways that influenza is able to escape the human immune system and cause repeated infections in people over time is (that) it can reassort or
rescramble the H component and the N component in its genome.”

Viruses such as H5N1 occur naturally in wild aquatic birds who can then pass it on to other wild birds and poultry through secretions or direct contact. This specific H5N1 outbreak’s ability to spread to dairy cattle as well is likely due to direct contact between cattle and wild birds and possibly from cross-contamination between cattle and poultry in factory farms where livestock are kept in close proximity. According to Messer, the process by which viruses can then infect their hosts “really depends on the virus’ ability to get into the cells lining the airways of the animal, including the human animal, that it’s infecting. Once it gets into the airways, it can get into the cells of the animal and then disseminate into the animal … The virus replicates in the animal and then is able to get out of the animal again and spread
to something else.”

The factors that contribute to the spread of H5N1, which range from the migration of birds to contact between animals and humans, make it a complex ecological virus. “Influenza is also a highly environmental or ecologic virus because its evolution relies on multiple hosts. It uses multiple hosts when they commingle to alter its genetics, and it can also use wild environmental hosts to move around the world, kind of free from our attempts to control the movement and spread of the virus,” says Messer.

Because H5N1 is an ecological virus, it is evaluated and treated under the One Health Framework, a structural approach used to monitor the close intersection of human, animal and environmental health. This January, the Centers of Disease Control and Prevention (CDC) released a four-year plan to address health issues such as zoonotic diseases, which are passed between humans and animals, including H5N1. In the CDC’s One Health Zoonotic Disease Prioritization Report, zoonotic influenza was listed as the number one priority among 55 other zoonotic diseases, including anthrax and Lyme disease. If the goals of the One Health Framework succeed, identifying and preventing pandemics in animals such as H5N1 could be made more efficient and effective.

As of May 2025, there have been 70 confirmed human cases of H5N1 since the current outbreak began. The majority of these cases, however, have surfaced in people who work with sick poultry or cattle or who have been in contact with wild birds. Additionally, there has been no recorded human-to-human transmissions of this strain of the H5N1 virus.

For human-to-human transmission to occur, Messer says the virus must first develop the ability to both infect human tissue and spread directly to other humans. “The number of mutations needed in the genet-
ics of the virus … to make the transition from bird-specific to human-specific, or both human and bird, can be many or it can be small,” he says. “It kind of depends on where the virus is starting. But we know that these viruses not (only) have the capacity to but (really) do constantly evolve. And every time a human gets infected with this virus, it is a potential opportunity for the virus to evolve in the direction of making it easier to infect humans.”

Preventing the spread of H5N1 to humans entails curbing its spread within farms. However, existing methods of mitigation are limited. One of the most common strategies farmers use to slow the spread of the disease amongst their livestock is culling, or selectively slaughtering, infected animals. As a result, over 30 million chickens were culled in the U.S. during the last three months of 2024, diminishing the supply of egg-laying chickens and contributing to nationwide egg shortages.

According to Katie Bolton, the president of Unger Farms, if a number of chickens in a
flock start showing signs of bird flu, the entire flock has to be culled. Located in Cornelius, Oregon, her farm is primarily a berry farm that also raises a small flock of egg-producing chickens. Bolton says that since Unger Farms only has one free-range flock, they’re fairly isolated from the spread of the virus. “Most of the effect of bird flu, for us, is more that we hear a lot about it in the marketplace. We know that there’s a shortage of eggs so usually there becomes more interest in our delivery service,” she says.

Another possible strategy to slow the spread of the virus is by vaccinating farm animals. While vaccination can limit the number of animals being infected, it comes with its own set of risks. Messer says, “We already
know that we can’t control the virus itself because it’s in wild bird populations. So we can vaccinate all of the domestic animals, and we still won’t be able to eliminate the virus because it doesn’t require domestic animals to continue to propagate and move around.” This is where the issue lies: “In some scenarios, if your vaccine is not very effective but it is effective enough to put pressure on the virus, your vaccine can, in theory, actually accelerate evolution of the virus to becoming something different.”

The H5N1 outbreak has also generated concern over the contamination of animal products such as dairy and eggs. Messer says that while the raw forms of these products contain the virus if they are produced by an infected animal, cooking and mandated milk pasteurization eliminates any existing virus. “When you crack an egg … if you fully cook it, that should have the same effect as pasteurization and prevent you from getting infected by that virus.”

Although humans currently have extremely low chances of contracting H5N1, there is concern that the U.S. government is not acting sufficiently to control and monitor the virus. Messer believes that the recency of the COVID-19 pandemic has negatively impacted treatment of H5N1 outbreaks. “My instinct, with the sort of uncertainty around H5N1 influenza, is that we are currently under-reacting … I think that if you took COVID-19 off the table, then this H5N1 avian influenza would actually be considered a public health emergency,” he says.

Since its initial transmission from wild birds to poultry, the virus has spread from feather to hoof, infecting some humans in the process. Strategies like culling, vaccination and the implementation of the One Health Framework could help slow the continued spread of the virus between farm animals and from those animals to humans. However, as it is predominantly carried by wild birds, the virus’ spread cannot be fully controlled. H5N1 does not currently pose a significant threat to humans as the virus has not mutated to spread between people yet. Nonetheless, it remains unknown when that mutation could occur — especially as more humans are infected.

“The risk itself is kind of hard to put a number on, but the capacity of the virus to do this is absolutely present,” Messer says. “If (the spread of H5N1) goes on long enough, it’s really likely that the virus will ac- quire the ability … to facilitate human-to-human transmission.”