Poultry losses alone surpass 200 million since outbreak started in 2022.
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.
The first H5N1 infection in dairy cattle was confirmed in March 2024 in Texas. The virus quickly spread to dairy operations across 20 states, with 1,093 herds confirmed positive. In April 2024, the novel genotype B3.13 emerged as the dominant strain driving the outbreak. It marked several historic firsts: sustained cow-to-cow transmission and the first documented cow-to-human transmission.
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.
The Centers for Disease Control and Prevention (CDC) had confirmed as of this April a total of 71 cases of human H5N1 infection in the United States, two of them fatal. Nearly all were linked to exposure to infected poultry or dairy cattle.
Although no human cases have been linked to consuming unpasteurized dairy products, the FDA strongly advises against consuming raw milk or products made from it. The AVMA recently updated its policy on “Raw Milk” to include potential health risks.
That said, multiple domestic cat infections have been traced to raw milk, often the B3.13 variant. As of April, more than 150 cases of HPAI in cats had been documented in 25 states.
Dairy surveillance
Meanwhile, federal agencies continue to reassure the public that the U.S food supply is safe. The Food and Drug Administration (FDA) has repeatedly confirmed that standard pasteurization completely inactivates HPAI in milk. Extensive testing of pasteurized dairy products found no trace of a viable virus.
Jamie Jonker, PhD, chief science officer for the National Milk Producers Federation, attributed the integrity of the U.S. food supply to the USDA’s National Milk Testing Strategy (NMTS) and similar state-level surveillance programs. Launched in late 2024 and adopted in most U.S. states, the NMTS systematically tests bulk milk for HPAI to track the virus in herds.
When it comes to preventing large HPAI outbreaks in dairy cows, Dr. Jonker says nothing is better than detecting the virus before it gets out of hand.
“If we can pick up the virus in the milk of infected animals before we see the big clinical outbreak, then we can isolate affected animals and reduce the outbreak’s severity within the herd,” he explained.
Migratory flyways
Dr. Maurice Pitesky says the relationship between migratory waterfowl movements and HPAI outbreaks in U.S. poultry is “strong, and consistent” since emerging in North America. Dr. Pitesky is a professor in cooperative extension specializing in poultry health and food safety epidemiology at the University of California-Davis School of Veterinary Medicine.
By analyzing national data on commercial poultry cases of H5N1 since late 2021, Dr. Pitesky has found temporal and spatial links to waterfowl migration whereby HPAI poultry infections align with fall and spring migrations.
For example, in California’s Central Valley, a region dense with hundreds of poultry and dairy facilities, winter waterfowl populations swell to approximately 6 million birds, 10 times the summer count of about 600,000. These birds roost and feed for 10 to 14 hours daily in wetlands, lagoons, and crop fields immediately adjacent to farms, creating repeated opportunities for viral spillover.
“Operational and physical biosecurity inside the fence line is essential to managing the virus, but they do not fully mitigate the risk of HPAI infection, especially when the virus is just outside the door,” Dr. Pitesky said.
Current USDA wildlife assessments stop at the perimeter fence, which is only part of what needs to be considered, Dr. Pitesky continued. Farms are stationary while waterfowl populations surge and recede around them, often driven by weather events such as freezes or heavy rains. These sudden surges dramatically increase the chances of HPAI, sometimes triggering regional outbreaks within days. Dr. Pitesky cited recent HPAI poultry cases in the Dakotas, Michigan, Minnesota, and Missouri linked to such migration-driven events.
To address these risks, Dr. Pitesky advocates for strategic wetland reflooding away from production zones; habitat management to reduce attractive roosting sites near farms; and remote-sensing technologies.
Radar and telemetry models, already piloted with USDA support, can deliver daily, farm-specific risk rankings by integrating real-time waterfowl abundance, feeding patterns, and wind-dispersed feather dander data.
Nationwide implementation for every commercial poultry and dairy operation would cost roughly $30 per farm annually, Dr. Pitesky said.
Vaccine development
On April 21, Moderna began dosing participants in the third phase of its large-scale clinical trial of a vaccine to combat HPAI in humans. The company is aiming to enroll 4,000 volunteers, mostly in Britain and including roughly 1,000 in the United States, with an emphasis on high-risk groups.
The messenger RNA (mRNA) vaccine candidate trial is supported by the Coalition for Epidemic Preparedness Innovations after the U.S. Department of Health and Human Services cut funding for several mRNA influenza projects in 2025.
For poultry, HPAI vaccination raises questions about trade. Dr. Michelle Kromm and others in the University of Minnesota’s Secure Food Systems Team recently penned an article, published April 17 online in JAVMA, that explores the nuances of vaccinating poultry against HPAI.
They acknowledge that doing so could negatively affect the nation’s ability to export poultry and poultry products, but propose that a vaccination program could be implemented with enough support from stakeholders.
While there are some commercially produced, fully approved HPAI vaccines available, none are an exact match for the strain of the virus that is currently circulating in the U.S. and no HPAI vaccine has been authorized for use at this time, except on a case-by-case basis in certain endangered or zoo species.
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.
