H5N1 bird flu is advancing east, with roughly 10 million birds affected in the past month

The highly pathogenic avian influenza (HPAI) virus has continued spreading across US commercial and backyard poultry operations in recent weeks, with federal officials confirming new H5N1 detections in Pennsylvania, Maryland, Michigan, and Indiana. The cases reflect ongoing nationwide activity that has affected approximately 10 million birds in the past 30 days alone.¹

So far this month, the largest impact has been at a commercial table egg layer farm in Lancaster County, Pennsylvania, where 870,000 birds were affected.^1,2 In Cecil County, Maryland, a table egg pullet operation also experienced an outbreak involving 354,100 birds. A commercial turkey farm in Kent County, Michigan, saw 51,700 birds affected. Indiana was also impacted, with 7600 birds at a commercial duck meat facility confirmed infected for the second consecutive week.²

The commercial sector is not the only concern. The Animal and Plant Health Inspection Service (APHIS) detected H5N1 in a backyard flock in Portage County, Ohio,² a reminder that the virus does not discriminate by operation size.

Backyard flocks present their own distinct set of challenges, Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology), explained in an interview with dvm360 last year.³ "In backyard settings, chickens often have names—like Susie or Jill—so the term 'depopulation' sounds very harsh when applied to them," he said. The disease itself moves fast, as birds can progress from mild symptoms to death within a day or 36 hours, often before lab results can confirm what producers already suspect.³ "When we confirm it's HPAI, it only confirms what we suspected," Lee said.

Wild birds carry the virus into urban areas

Wild bird detections, though slowing compared with the previous week, continue across the country. APHIS confirmed additional cases of H5N1 in wild birds, including Canada geese in the Bronx, New York,² evidence that H5N1 can spread beyond rural, agricultural areas. Detections in wild birds are often an early warning sign of increased risk to farms. Migratory birds play a central role in the virus’ spread, and their proximity to commercial and backyard flocks has often preceded outbreaks.

The challenge, experts say, is that the wild bird problem has no clean solution. "Unfortunately, nothing can be done about wild birds," Lee said. "They are the reservoir." Many migratory species congregate in the Arctic and similar regions, where different viral strains can intermingle, producing entirely new influenza variants by the time birds return south. That process involves 2 distinct mechanisms: antigenic drift, in which the virus accumulates small genetic mutations as it replicates; and antigenic shift, in which 2 different viruses exchange genetic material—a more abrupt and potentially dangerous change.

On farms, surface water is often the biggest risk. "Much of it centers on surface water around poultry areas because it attracts waterfowl," Lee said. "Wild birds come to these water sources next to where other poultry is kept." Even enclosed facilities carry risk, as people can carry the virus on clothing or equipment between areas. "It's damage control—minimizing spread rather than magically curing it," Lee added.

A season of sustained losses

The current wave is unfolding against a backdrop of persistent H5N1 activity. Since the outbreak began in 2022, the virus has affected more than 200 million birds in the United States, based on US Department of Agriculture (USDA) APHIS data,¹ a figure that includes birds culled as part of standard disease control protocols.

When a flock tests positive, depopulation is swift and often massive. "You're looking at depopulation or euthanasia of thousands—sometimes even millions—of birds, depending on the virus," Lee said.

The downstream effects reach beyond the farm. "All of a sudden, it's going to take quite a while to regain that population and get eggs back into circulation," Lee said, a dynamic that he noted is a key driver of the soaring egg prices consumers have felt at grocery stores in recent months.

Recent activity has continued into 2026, with multiple states reporting new detections in both poultry and livestock. Wisconsin has experienced a particularly damaging stretch: A September 2025 outbreak affected more than 3 million commercial poultry, and a second Jefferson County detection in early October confirmed the virus in a flock of more than 520,000 birds.⁴ In the past 30 days, the state has reported 3 affected commercial flocks and 1 backyard flock, totaling more than 4 million birds affected.¹

The damage in Wisconsin does not stop there. In December 2025, federal officials confirmed the state's first known case of H5N1 in a dairy cattle herd.⁵ The detection, confirmed by the National Veterinary Services Laboratory in mid-December 2025 via polymerase chain reaction and enzyme-linked immunosorbent assay testing, was identified through the USDA's National Milk Testing Strategy—a mandatory surveillance program established in December 2024 that screens milk samples for the virus.⁶ The case belonged to the H5 clade 2.3.4.4b strain.⁵

Related: Wildlife linked to H5N1 spread in Wisconsin dairy farm

Wisconsin joined a list of states grappling with the dairy dimension of the H5N1 outbreak. Since the virus first appeared in US dairy cattle in March 2024, the virus has been confirmed in herds across 19 states, showing an ability to move beyond avian hosts. In September 2025, Nebraska reported its first dairy cattle case.⁸

Biosecurity remains the frontline defense for veterinary professionals

Veterinarians working with poultry or livestock are advised to talk with clients about minimizing contact between domestic animals and wild birds, which is a key driver of transmission, and to follow personal protective protocols.

The risk to the general public remains low, according to federal health officials. However, veterinary teams, farm workers, and others with direct animal contact face higher exposure risks and should adhere closely to protective guidelines, including proper personal protective equipment and sanitation practices.⁹

USDA and state officials say spring migration increases HPAI risk as infected wild birds move across regions. Seasonal spikes during spring and fall are well documented, so the shift is expected to heighten risk and keep animal health systems on alert in the weeks ahead.

References

1. Confirmations of highly pathogenic avian influenza in commercial and backyard flocks. US Department of Agriculture Animal and Plant Health Inspection Service. Accessed March 26, 2026. https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/commercial-backyard-flocks
2. Soucheray S. Avian flu hits farms in Pennsylvania, Maryland, Michigan, Indiana. University of Minnesota. March 19, 2026. Accessed March 26, 2026. https://www.cidrap.umn.edu/avian-influenza-bird-flu/avian-flu-hits-farms-pennsylvania-maryland-michigan-indiana
3. Lee C, Alaburda R. Highly pathogenic avian influenza and biosecurity. dvm360. March 28, 2025. Accessed March 26, 2026. https://www.dvm360.com/view/highly-pathogenic-avian-influenza-and-biosecurity
4. Bautista-Alejandre A. New bird flu outbreaks reported in poultry across 3 Midwestern states. dvm360. October 6, 2025. Accessed March 26, 2026. https://www.dvm360.com/view/new-bird-flu-outbreaks-reported-in-poultry-across-3-midwestern-states
5. Bautista-Alejandre A. Federal officials confirm Wisconsin’s first case of bird flu in a dairy cattle herd. dvm360. December 15, 2025. Accessed March 26, 2026. https://www.dvm360.com/view/federal-officials-confirm-wisconsin-s-first-case-of-bird-flu-in-a-dairy-cattle-herd
6. USDA announces new federal order, begins National Milk Testing Strategy to address H5N1 in dairy herds. News release. US Department of Agriculture Animal and Plant Health Inspection Service. December 6, 2024. Accessed March 26, 2026. https://www.usda.gov/about-usda/news/press-releases/2024/12/06/usda-announces-new-federal-order-begins-national-milk-testing-strategy-address-h5n1-dairy-herds
7. Avian influenza virus type A (H5N1) in U.S. dairy cattle. American Veterinary Medical Association. Updated December 16, 2025. Accessed March 26, 2026. https://www.avma.org/resources-tools/animal-health-and-welfare/animal-health/avian-influenza/avian-influenza-virus-type-h5n1-us-dairy-cattle
8. Bautista-Alejandre A. USDA confirms first case of bird flu in Nebraska dairy cattle. dvm360. September 16, 2025. Accessed March 26, 2026. https://www.dvm360.com/view/usda-confirms-first-case-of-bird-flu-in-nebraska-dairy-cattle
9. Reducing exposure for workers to avian influenza A viruses. CDC. May 6, 2025. Accessed March 26, 2026. https://www.cdc.gov/bird-flu/worker-safety/index.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fbird-flu%2Fprevention%2Fworker-protection-ppe.html