Scientists provided the most complete view of bird flu's spread through wild bird populations across North America, explaining how the dominant strain advanced, maintaining the risk of human infections.

MEMPHIS, Tenn., April 15, 2026 /PRNewswire/ -- An international group of scientists mapped the spread of the current dominant strain of highly pathogenic avian influenza virus through North American bird populations in 2024. Led by scientists from St. Jude Children's Research Hospital, the collaboration provides a comprehensive view of this novel flu spreading through birds in North America. The investigators collected and sequenced samples from birds across the continent, letting them map the virus's spread. They also compared the virus from birds to those causing human infections, showing that current human vaccine stockpiles will likely work well against both. Closer evaluation of the viruses led the researchers to classify them as low risk for human-to-human spread in their current form. The study was published today in Nature Medicine.

A highly pathogenic H5N1 avian influenza virus entered North America in 2021, causing havoc in wild birds and the poultry industry. Since then, researchers across the continent have continued to test wild populations of waterfowl, such as ducks and geese (influenza's natural hosts), for any changes in the virus. In late 2024, the St. Jude group detected D1.1, a novel strain which became dominant in several sites. They coordinated with multiple institutions across Canada and the United States to map the strain's geographic penetration. The researchers observed that the strain first appeared in Alaska and British Columbia, then moved south and east via known migratory pathways.

"Combining information collected from multiple partners, we've documented the entire continental spread of a newly dominant strain of highly pathogenic avian influenza virus through wild bird populations," said corresponding author Richard Webby, PhD, St. Jude Department of Host-Microbe Interactions. "We've shown the value of connecting what are usually regional findings into a single comprehensive map to understand the strain's spread."

Understanding D1.1 H5N1's risk to humans

Starting in 2024, severe infections from avian influenza were reported in North America. Every severe case was of this newly dominant D1.1 strain, though at the time, the disparate geographical locations of those cases made understanding the source of the infections unclear. The new map provides a direct explanation.

"We could connect all the severe cases with the wild bird map," said co-first author Walter Harrington, PhD, St. Jude Department of Host-Microbe Interactions. "We had reason to suspect wild birds played a role, but the extent of spread in wild bird populations wasn't clear. Without that context, the human infections appeared geographically scattered; with it, the strain's emergence correlated with where we saw it becoming the dominant strain in wild birds."

While the map retroactively provided a plausible source of human infections from D1.1, it could not tell the researchers what the general risk of bird-to-human infections was, or most importantly, what the risk of the virus becoming human-to-human transmissible was.

"Fortunately, we saw that these viruses remained mostly avian, with none of the major mutations known to enable efficient human infections," said co-first author Lisa Kercher, PhD, St. Jude Department of Host-Microbe Interactions. "They did have a mutation that gives resistance to a common antiviral drug, but when we tested existing candidate vaccines, they showed significant cross-reactivity, suggesting they will likely be effective to help control the virus."

The study's results suggest that the strain poses a low risk to the human population in its current form. However, the severity of disease for those few who have become infected is a reminder that the virus is a risk to individuals and emphasizes the need to keep surveilling avian flu to understand its potential impact on human health.

"We were lucky enough to join groups from Canada and throughout the U.S. to understand this virus," Webby said. "That lets us put human cases in the context of the strain's greater spread and gives us a model to continue to monitor and assess the threat of these highly pathogenic influenza viruses in the future."

Authors and funding

The study's other co-first author is Anthony Signore, of the National Centre for Foreign Animal Disease, Canadian Food Inspection Agency. The study's other authors are Ahmed Kandeil, Tom Fabrizio, Trushar Jeevan, John Franks, Lance Miller, and Karlie Woodard, St. Jude; Yohannes Berhane, National Centre for Foreign Animal Disease, Canadian Food Inspection Agency; Jolene Giacinti, Ecotoxicology and Wildlife Health Division, Science and Technology Branch, Environment and Climate Change Canada; Christina Ahlstrom, Evan Buck, Laura Scott and Andrew Ramey, Alaska Science Center, U.S. Geological Survey; Sarah Bevins, Julianna Lenoch and Krista Dilione, Wildlife Services, National Wildlife Disease Program, U.S. Department of Agriculture, Animal and Plant Health Inspection Service; Beate Crossley and Daniel Rejmanek, University of California, Davis; Jacqueline Nolting, Mohammad Jawad Jahid, Madison Owsiany and Lauren Smith, The Ohio State University; David Stallknecht, Deborah Carter, Nicole Nemeth and Rebecca Poulson, University of Georgia; Trent Bollinger, University of Saskatchewan; Bradley Cohen, Cory Highway, Lydia Holmes and Nathan Steelman, Tennessee Technological University; Jamie Feddersen, Migratory Gamebird Program, Tennessee Wildlife Resources Agency; Dayna Goldsmith, University of Calgary; Chelsea Himsworth, British Columbia Ministry of Agriculture; Claire Jardine and Brian Stevens, University of Guelph; Paul Link, Louisiana Department of Wildlife and Fisheries; Margo Pybus, Alberta Environment and Parks; Christopher Sharp, Ontario Region Wildlife and Habitat Assessment Section, Canadian Wildlife Service, Environment and Climate Change Canada and Mia Torchetti, National Veterinary Services Laboratories, U.S. Department of Agriculture.

The study was supported by grants and other financial support from the Southeastern Cooperative Wildlife Disease Study (SCWDS), U.S. Fish and Wildlife Service National Wildlife Refuge System, the U.S. Geological Survey Ecosystems Mission Area, National Institute of Allergy and Infectious Diseases Centers of Excellence for Influenza Research and Response (co. 75N93021C00016) and the American Lebanese Syrian Associated Charities (ALSAC), the fundraising and awareness organization of St. Jude.

St. Jude Children's Research Hospital
St. Jude Children's Research Hospital is leading the way the world understands, treats, and cures childhood catastrophic diseases. From cancer to life-threatening blood disorders, neurological conditions, and infectious diseases, St. Jude is dedicated to advancing cures and means of prevention through groundbreaking research and compassionate care. Through global collaborations and innovative science, St. Jude is working to ensure that every child, everywhere, has the best chance at a healthy future. To learn more, visit stjude.org, read St. Jude Progress, a digital magazine, and follow St. Jude on social media at @stjuderesearch.

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