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Grebe wing

International Bird Rescue photo by Ariana Gastelum

Emily Werdal, a wildlife rehabilitation technician at the conservation group International Bird Rescue's Los Angeles center, examines the wing of a Western grebe that was caught in an oil spill. Center staff wear personal protective equipment, such as custom-fitted masks, gloves and gowns, to protect them from avian influenza.

When a particularly virulent form of bird flu was first detected in the United States in January 2022, it was hoped the ensuing outbreak would burn out fast. After all, the last major national outbreak of highly pathogenic avian influenza (HPAI) had fizzled in a matter of months. But by January 2024, two years later, concerns were growing among veterinarians that this new outbreak might be extraordinarily persistent. Now, they are confronting an uncomfortable possibility that the virus is here to stay.

Since it was initially detected in wild birds in Europe in 2020, this aggressive form of HPAI — known technically in epidemiology circles as H5N1 clade 2.3.4.4b — has reached every continent on Earth, apart from Australia. Its prevalence meets the criteria of a panzootic, the nonhuman animal version of a pandemic.

Now known to be present in the U.S. for more than four years, the possibility is solidifying that the outbreak isn't so much an epidemic, defined as a widespread but temporary occurrence of a disease, but endemic, meaning that it has established a consistent presence. The same goes for other places where HPAI isn't already well established, such as in countries throughout Europe and South America.

Endemic status could have big financial implications for commercial poultry operators, which, in the U.S. alone, already have lost more than 200 million birds since 2022 due to the virus. And it would further test the mental wherewithal of farmworkers and the veterinarians who work alongside them, often by assisting in the culling of millions of birds at any one time to prevent the virus's spread.

Among wild birds, persistent HPAI could drive to extinction some vulnerable and endangered species, such as whooping cranes, western snowy plovers and Humboldt penguins. And if HPAI is left to circulate in the environment for years, the risk will increase that the virus — which already has infected many other types of animals, among them cats, dogs, cattle, seals and humans — could mutate into something more dangerous to human health.

Dr. Gail Hansen, an expert in public health and veterinary epidemiology in Washington, D.C., has been watching the virus mutate with growing alarm.

"In the past, we'd always thought that aquatic wild birds would carry the virus but wouldn't get ill from it. But this one is different. It's killing wet birds like geese and ducks," she said. Then came 2024, when the virus was found to infect dairy cows, a stupefying development.

"Everyone was like, 'Cattle don't get influenza, period, let alone HPAI,' " Hansen said. "So all the things we thought we knew. Whew. We need to show a little more humility, I think, by telling people what we do and don't know — because there's a lot we don't know."

A lingering presence

HPAI has moved through U.S. commercial poultry operations in waves since early 2022, with monthly case numbers sometimes dropping to zero or near zero before rebounding, according to a VIN News Service analysis of data posted by the U.S. Department of Agriculture.

March 2022 is one of the worst on record: More than 20 million birds were culled that month. The disease remained present in 2023 and during most of 2024, although the number of birds that were killed waned. Then, in the winter of 2024-25, the virus came roaring back. In January 2025, more than 23 million birds were culled, a new record. So far in 2026, more than 20 million birds have been culled in the first not-quite-three months of the year.

Whether the virus has become endemic in the U.S. depends on whom you ask, the type of birds you're referring to and how you define "endemic." Hansen, a poultry consultant whose resume includes a stint as a Kansas state public health veterinarian and epidemiologist, said there appears to be a growing consensus that HPAI is now endemic in the U.S.

"I've been to enough meetings with USDA people who've basically said, 'It's endemic in our poultry,' " she said. "I have no qualms about saying, 'Yep, it's endemic here.' I mean, it's been going on since 2022, and endemic just means that there's this base level of infections that we're expecting — and that's what we're seeing."

Those with a different view include Dr. Suzanne McComb, who is executive vice president of the American Association of Avian Pathologists (AAAP), a trade group for poultry veterinarians.

She accepts the disease is prevalent in wild birds, but when referring strictly to commercial poultry, she doesn't think HPAI has reached endemic status.

"The definition of 'endemic' is basically that a disease is commonly in a geographical area — so, consistently and often," she said. "And we are not at that point in the United States for commercial poultry. It's still not normal. It's still not common to find. It's still sporadic and on a case-by-case basis."

A relatively low number of cases this year so far in commercial turkeys, which typically constitute a higher proportion of infected birds, is a positive sign, McComb maintains. "That does make you feel like maybe 2026 won't be as severe, but we don't know that yet," she said.

The fact that the virus is predominant in wild birds isn't lost on Dr. Annika McKillop, a poultry veterinarian in Maryland who previously has worked for both that state's Department of Agriculture and the USDA. She reasons that if HPAI persists in wild birds, they will present a constant menace to commercial flocks.

"It's definitely endemic in wild bird populations," McKillop said. "The virus is also infecting wild mammals, but current evidence shows the sources of infection are coming from wild birds and their droppings. So there is always going to be a potential for it to be around because of that." (Like McComb, McKillop does not consider HPAI endemic in U.S. commercial poultry.)

Positive cases still are being identified in a host of bird species taken in by conservation group International Bird Rescue, which runs wild bird rescue centers in Los Angeles and San Francisco.

The ongoing HPAI threat is complicating its usual efforts to treat birds injured by oil spills or fishing gear. "It's been disheartening," said Dr. Rebecca Duerr, a wildlife veterinarian and the group's research director. "It's made everything more difficult."

As a case in point, Duerr and colleagues recently spent hours quarantining more than 30 western grebes — a species of diving waterbird — after another bird in the Los Angeles center tested positive for HPAI. "Imagine quarantining pools in the pouring rain. It's super fun," Duerr said wryly.

Every day, center staff are having to don copious personal protective equipment, including fitted face masks, hair covers, gowns and gloves, to protect themselves from infection.

Duerr reckons HPAI has been endemic in the U.S. for two years now — with the caveat that the definition of "endemic" is more nuanced for migratory birds. "To a point, it's not always local," she said. "But it's here when the birds are here; when the environmental conditions are right for them to migrate in, based on the season and environmental temperatures and such."

HPAI has long been considered endemic in countries with less-advanced veterinary infrastructure, such as Egypt, China, India, Indonesia, Nigeria and Vietnam, according to the USDA. As for the U.S., the USDA did not respond to a request for comment.

Bird flu

VIN News interviewed the chief veterinarian of the Food and Agriculture Organization of the United Nations (FAO), Dr. Thanawat Tiensin. When asked if HPAI is now endemic in more countries, he replied: "Let's say it like this: Outbreaks have been introduced repeatedly in past years. You can see repeated outbreaks in poultry production: in chicken farms, in turkey farms, in duck farms."

Biosecurity improving, but no cure-all

Endemic or not, countries where HPAI seemingly has gained a foothold won't necessarily have to live with it forever.

As Hansen, the epidemiologist in Washington, D.C., noted, endemic diseases can be eradicated. Smallpox among humans and rinderpest among cattle are cases in point. "We tend to think of endemic as meaning, 'OK, we're stuck with it at this rate for good,' but we could certainly work to try to eliminate it," she said.

All of the veterinarians contacted by VIN News said they consider adopting strict biosecurity measures — and applying them consistently — as key to controlling the virus's spread. Generally, biosecurity means making sure that everything that enters a farm — whether a person, their clothes, their shoes, and equipment like egg trays and motor vehicles — is either disinfected or changed. Tiensin of the U.N. noted, however, that there are varying levels of rigor.

"Everyone talks about biosecurity, but when you go to the actual implementation, it's different," he said, recalling that on visits to some countries, he was able to enter chicken farms too easily. "When I work in my home country of Thailand, where biosecurity measures have been introduced more consistently, you need to take a shower, you need to change your clothes, your boots. In some countries, you need to stay on the farms at least three days before they will allow you to enter into the chicken house."

One reason HPAI has spread so readily in Western Europe, Tiensin maintains, is the adoption there of more humane farming practices. Increasingly, chickens are allowed to roam outside, which raises the risk of exposure to infected wild birds. "There's a trade-off," he said. "If you want to have good animal welfare, you sometimes lower the biosecurity measures at the same time."

Farming practices for poultry in the U.S. tend to be less free-range than in Europe: Most birds typically are kept indoors, sometimes in gigantic houses occupied by millions of animals.

McComb, of the AAAP, said biosecurity is critical to getting rid of HPAI. "It is the single most important thing that we can do to minimize infection," she said. "It's not 100% effective, but it's definitely a huge component."

Over the past four years, McComb said, she has seen vast improvements in biosecurity across the U.S. poultry industry.

In a sign that good biosecurity can make a big difference, McKillop, the poultry veterinarian in Maryland, has found that farm-to-farm spread in her region — Maryland, Delaware and Virginia — has been limited, despite the fact that poultry farms in these states are among the most densely located in the U.S.

Practicing good biosecurity means learning the nuances, she suggests, offering a disinfecting tip as an example.

"After cleaning off organic material and washing, you have to apply disinfectant to a dry surface," she said. "If you don't wait for the surface to dry and just put disinfectant on a wet surface, it gets diluted and it's not going to be as effective."

At a global conference on HPAI organized by the U.N. FAO in Brazil last year, some of the discussion focused on foot baths. Among simple pieces of advice: Don't put foot baths facing a wall, since that will force users to step back into their old footprints. Also, Tiensin said, "If you keep a foot bath under the sun, it won't be as effective. If the water is very dirty, and you never change it? The foot bath isn't a foot bath anymore."

Another way that the virus potentially could enter poultry housing is via infected rodents, insects or other vermin. "I can say that the houses that I've been to have been in good repair, and all of the companies have rodent- and insect-control programs," McKillop said. "They are checking the rodent boxes and looking for signs of rodents, to deter them. So that's another level of biosecurity, as well."

Still, all the veterinarians who spoke to VIN News acknowledged that biosecurity measures aren't foolproof, noting that the virus might enter poultry houses through air vents or drainage pipes.

Dr. Thanawat Tiensin

FAO photo by Giorgio Cosulich de Pecine

Dr. Thanawat Tiensin, chief veterinary officer of the Food and Agriculture Association of the United Nations, is working on establishing a new international animal disease response center.

To vaccinate or not to vaccinate?

Several effective vaccines for HPAI have been produced by major pharmaceutical companies, but their use is controversial because it can disrupt cross-border trade. That's because vaccinated birds can still become infected with HPAI and shed the virus. Since those birds may not exhibit clinical signs, they're harder to identify than unvaccinated infected birds, which usually appear ill and die quickly. That increases the risk that vaccinated but infectious birds, or meat products from such birds, will cross borders undetected — or so the thinking goes.

However, some countries that have rolled out vaccines for HPAI, such as France (in its case, with promising results), have maintained trade relationships. For example, while the U.S. initially stopped importing French poultry products when the country started vaccinating ducks, trade has since resumed.

Still, the dynamic is complicated, said McKillop, whose background includes working for the USDA from 2021 to 2025 in a role that provided insider knowledge of international poultry trade complexities.

"I can say, at least in my experience, that when I have asked countries, "If the U.S. decided to vaccinate, would you accept products?' many of them automatically said, 'No,' or would require a long, extensive process that usually takes years before they would accept or decline products," McKillop said.

Hansen, the public health expert in Washington, D.C., wonders whether the risk of spread due to international trade is overblown. "It's sort of a specious argument, because not that many live birds are transported, and dead birds don't transmit the virus especially well," she said.

Trade issues aside, vaccination isn't perfect, partly because immunization doesn't fully prevent infection. Under current U.S. rules, entire flocks must be culled if just one bird tests positive.

"Just say you spend all that money to vaccinate [an egg] layer premises that holds a million birds in several houses," she said. "And then, one house at one end of the property tests positive, and other houses at the opposite end of the property are negative. The vaccine helps lower the risk of the virus being spread from house to house, but with the current rules, you still have to depopulate everything."

For her part, McComb of the AAAP invokes the Veterinarian's Oath when mulling the value of vaccination. "As veterinarians, we take an oath to ensure the health and welfare of our birds, as well as public health," she said. "And if you look at vaccination, it can be a critical tool in prevention and reduction of virus infection within the birds."

At the same time, she said, vaccination must be accompanied by continued disease surveillance.

"In some of the countries that have tried to vaccinate, they haven't been particularly successful fighting HPAI because they're just applying the vaccine and walking away without adequate surveillance," McComb said. "In order to have more effective biosecurity in certain areas, you need to know where the virus is."

A multifactorial — and multinational — challenge

In addition to biosecurity and vaccination, Hansen would like to see another element in the mix: improving the genetic diversity of commercial poultry.

"All of the chickens are pretty much the same," she said, because they're bred to produce as much food as possible. "They come from basically, the same stock, undermining the chance that some birds will be resistant to disease." Recalling a statement last year by Robert F. Kennedy Jr., the U.S. secretary of health and human services, that the USDA should consider letting the virus "run through the flock so that we can identify the birds and preserve the birds that are immune to it," Hansen said, "That's not how it works — unless you have biodiversity."

On the possibility of ending the outbreak, McComb is optimistic, even as she's under no illusions about its difficulty. "It's incredibly complicated," she said, but added, "I do think, in the United States, we are very on top of it. The good news is that case numbers in 2026 have been stable so far compared to last year and hopefully, that'll continue into the spring, and we may even see a decrease."

Duerr, the wildlife veterinarian in California, is pessimistic. "It's going to keep evolving into different clades," she said. "I don't know what's going to happen, but none of it's good for wild birds. I don't think it's going to go away."

Similarly, McKillop in Maryland doesn't imagine the disease disappearing any time soon, in part because of its prevalence in wild birds.

"Right now, it's just part of our reality," she said. "And things might not change much unless a majority of the countries in the world come together and accept vaccine programs."

Tiensin at the U.N. believes international cooperation in general is crucial for tackling HPAI, given that it's spread readily by migratory birds. To that end, the FAO is planning an initiative called the Global Partnership Program for Transboundary Animal Disease. The program could replace the FAO's Emergency Centre for Transboundary Animal Diseases, which was severely weakened after the Trump administration eliminated the U.S. Agency for International Development, a foreign aid agency that had provided some 90% of the center's funding. The new program would be supported by its member countries. The FAO is in discussions with prospective members and plans to launch the partnership later this year.

"I think it's very clear that since avian influenza is a transboundary animal disease, you cannot work alone, especially in countries that share borders," Tiensin said. "We need to work together and share information about surveillance systems, about the virus itself, to tackle this complex situation."