Wild birds are driving the current U.S. bird flu outbreak
Penn Vet’s Louise Moncla has tracked H5N1 viruses in North America to better understand the role wild birds play in the current bird flu outbreak
Since late 2021, a panzootic, or “a pandemic in animals,” of highly pathogenic bird flu variant H5N1 has devastated wild birds, agriculture, and mammals. Unlike previous outbreaks, aggressive culling of domestic birds has not contained it, and the viruses continue to infect a broad range of species, including wild birds and mammals rarely affected before, suggesting that transmission patterns have shifted since 2022.
Now, in a new study looking at how these viruses were introduced and spread in North America, Louise H. Moncla from the School of Veterinary Medicine and her team have found that wild birds are critical drivers of the ongoing bird flu outbreak in the United States. Their findings are published in Nature.
Highly pathogenic avian influenza (HPAI) viruses—those responsible for bird flu outbreaks—continue to pose challenges for human and animal health.
“The picture for HPAI influenza has really changed for North America and the U.S. in the last couple of years,” says Moncla. “This used to be a virus that primarily circulated in Asia, Northern Africa, and domestic birds. But in more recent years, we've seen increasing outbreaks across Europe, associated with wild birds, and since 2022, we’ve also had similar outbreaks in our North American birds.”
Using publicly available databases from the Canadian Food Inspection Agency, Environment Canada, the Public Health Agency of Canada, the Canadian Wildlife Health Centre, and the United State Department of Agriculture (U.S.D.A) Animal and Plant Health Inspection Service, the researchers traced the introduction and spread of highly pathogenic H5N1 viruses during the first 18 months in North America using genomic sequencing and migratory flyway analysis.
“The main conclusion from this study is that that outbreak was really different from all of the past ones we have had in North America because these viruses were spread primarily by wild migrating birds,” says Moncla. “Our data pinpoint the Anseriformes, which are ducks, geese, and swans.”
She notes that the since 2020, when an evolutionary shift occurred, H5N1 has become better adapted to infect wild birds, meaning that it can be spread much more efficiently when wild birds migrate. “This was happening in Europe—Europe had almost the exact same thing happen that we had in 2022. They just had it 2 years earlier.”
However, Moncla explains, H5N1 viruses in North America are still classified foreign animal diseases. “Our policy is based on the idea that these viruses come from elsewhere and don't circulate continuously in our birds here,” she says. “Our study shows that this is no longer the case, and so we need to update our policy to align with this reality.”
This study also found that agricultural outbreaks were the result of repeated introductions of the virus from wild birds, says Moncla. In addition, backyard birds—populations of fewer than 1,000 domestic birds as defined by the U.S.D.A. and World Animal Health Organization—on average, were infected approximately nine days earlier than commercial poultry, suggesting these populations might serve as an early warning signal.
“These populations have a lot of different epidemiological features,” says Moncla. “The farms are smaller. They tend to have less biosecurity. These birds have a much higher likelihood of being raised outdoors with potentially more access to wild birds.”
Previous viruses transmitted really well between domestic chickens and turkeys, says Moncla, so stopping transmission in commercial farms would stop the outbreak. But transmission by wild migrating birds presents a challenge.
The solution? “A series of boring things,” says Moncla.
“We need to keep investing in biosecurity—biosecurity does work—making sure that people have good biosecurity plans, both to prevent transmission to other farms but also to prevent wild birds from interacting with their domestic birds,” she says, adding that a layered approach to encouraging adherence to these physical and/or behavioral protocols that prevent introduction of these viruses would also be needed.
“At some point we probably will need to investigate vaccinating domestic birds as a possibility,” she continues, adding that investing in novel ways to keep domestic and wild bird separated would also help reduce spillovers. Finally, continuous surveillance in wild birds, especially Anseriformes [waterfowl], would help with viral tracking and outbreak reconstruction.
“Our lab is really interested in risk modeling,” says Moncla. “If we had a better understanding of how these viruses are circulating in wild birds and the kind of degree to which different migratory birds are driving transmission, could we have something like a forecasting system for risk over time?” For example, she continues, if risk is highest in a particular region in September, people with backyard birds in that area could be told to make sure that they are adhering fully to their biosecurity plan during that month.
While Moncla says it is unlikely that the disease will ever completely go away or be solved, she says what we can do, however, “is try to manage it from getting into agricultural animals.”
Louise H. Moncla is an assistant professor in the Department of Pathobiology in the School of Veterinary Medicine at the University of Pennsylvania.
Other authors are Lambodhar Damodaran and Anna S. Jaeger of Penn Vet.
This work was supported by NIH R00-AI147029-05 and by funding from the Centers of Excellence for Influenza Research and Response (CEIRR), funded by NIH 75N93021C00015. LHM is a Pew Biomedical Scholar and is supported by NIH R00-AI147029-05. LD is supported by NIH 75N93021C00015 and ASJ is supported by NIH R00-AI147029-05.
Journal
Nature
Method of Research
Observational study
Subject of Research
Animals
Article Title
Ecology and spread of the North American H5N1 epizootic
Article Publication Date
12-Nov-2025
