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By R. Scott Nolen
Published on May 06, 2026
The U.S. outbreak of highly pathogenic avian influenza (HPAI, more specifically avian influenza type A H5N1), is experiencing a resurgence among commercial poultry operations, driven largely by spillover from migrating wild birds, according to the U.S. Department of Agriculture (USDA).
The threat of infection to poultry operations remained constant throughout the winter and into spring. On the final day of March, the U.S. marked a grim milestone: the depopulation of the 1,000th commercial flock since the outbreaks began in early 2022, according to USDA's Animal and Plant Health Inspection Service (APHIS). Around 1,190 backyard flocks had also been affected during that time.
Cumulative losses exceed 206 million birds in 50 states and Puerto Rico, with table egg–laying hen flocks being the most impacted by the ongoing outbreak. They account for around three-quarters of total domestic poultry loss, but comprise less than 4% of the total domestic poultry population, according to a Congressional Research Service report published last year.
"Detections are higher in the fall and spring, because we continue to see wild birds spreading virus as they migrate to their seasonal homes," the USDA says. "APHIS continues to work closely with State animal health officials on surveillance efforts to look for the virus in commercial, backyard, and wild birds. We also continue to encourage all bird owners to practice strong biosecurity—that means reducing opportunities for wildlife to spread the virus to their birds and preventing the spread of the virus from one premises to another."
As of early April, large-scale outbreaks involving more than a million birds at each facility—all commercial table egg-layer operations—emerged in Colorado, North Carolina, and Wisconsin. Pennsylvania alone has seen 8.7 million birds affected since the beginning of the year.
History lesson
By contrast, dairy producers saw a welcome lull in HPAI infections over the winter. After no new infections were reported since a December 2025 detection in Wisconsin, the virus was confirmed in five dairy herds on April 13 in Idaho by APHIS.
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Initially, B3.13 remained the primary genotype in dairy herds throughout much of 2024 before being largely supplanted by the D1.1 variant. D1.1 was first detected in a wild duck in Alaska in October 2024. It then spread among ducks and geese across all North American migratory flyways. This variant also was detected in dairy cattle and was linked to 17 human cases, according to a paper published April 15 in the journal Nature Medicine.
Researchers investigated the rapid expansion of D1.1 that migratory season. They found that none of the mammalian-adaptive markers detected in human cases were found in wild bird viruses and concluded that these mutations likely arose de novo during infection.
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In February 2025, the USDA Center for Veterinary Biologics issued conditional approval for an H5N2 vaccine offering cross-protection against H5N1. Then in January, APHIS committed $100 million to the HPAI Poultry Innovation Grand Challenge to fuel new vaccines and transmission research.
No licensed vaccines currently exist for U.S. dairy cattle either, but researchers reported promising 2026 results in the journal Nature Communications on an experimental vaccine that produced strong immunity in dairy calves and full protection in mice. The USDA is overseeing field trials of several other bovine vaccine candidates.
continued: https://www.avma.org/news/surge-hpai...bird-spillover