The cows were not coughing. That was the first thing that threw everyone.

When H5N1 reached American dairy herds in early 2024, it did not behave like a respiratory virus is supposed to. The cows went off their feed, their udders swelled with a severe necrotizing mastitis, and milk production dropped. To any veterinarian who saw it, that picture has an ordinary name, and they treated it like one, running down the usual bacterial suspects while a highly pathogenic avian influenza virus multiplied in the one tissue nobody thought to watch.

H5N1 in dairy cattle accountability arc
2024Outbreak confirmed in U.S. dairy herds; early receptor evidence published in CDC's Emerging Infectious Diseases2026University of Pittsburgh publishes detailed receptor map – the same answer, with more resolution, two years later
The receptor map was visible in the first months of the outbreak. The institutional response moved on a different timetable. Source: Emerging Infectious Diseases (CDC), July 2024; Science Advances (Pitt), June 2026

Two years on, a team at the University of Pittsburgh School of Public Health says it can explain why the virus chose the udder, and the answer is almost insultingly simple. In a study published in Science Advances, the researchers mapped the sialic acid receptors that influenza uses to enter cells across cattle tissue. The virus is picky. The receptor subtype it binds best, the N-linked variety, turns out to be pervasive in the mammary gland and virtually absent in the airway. In cattle the lungs offered the virus almost nothing to grab onto; the udder offered everything. “A perfect breeding ground for the virus,” is how senior author Suresh Kuchipudi put it.

It is a useful piece of biology. Read against the calendar, it is also a quiet record of how long the obvious took to land, because the shape of this answer was on the table in 2024.

That June, weeks into the outbreak, a separate team reported in the CDC’s Emerging Infectious Diseases that the mammary glands of two naturally infected Texas dairy cows were rich in the sialic acid receptors influenza needs, and that the virus co-localized there, predominantly with the avian-type receptor. They had looked at two cows. From a sample that small, the basic answer, the virus likes the udder because the udder offers what the virus wants, was already visible. What the Pittsburgh group added two years later was resolution and confirmation. What it did not add was surprise.

The biology was never the slow part. The response was.

The receptor map carries one consequence for anyone near a dairy. If the binding sites live in the udder, the virus lives in the milk. “If a cow is infected, it sheds a lot of virus into the milk,” Kuchipudi said. Pasteurization handles that: the FDA tested 167 retail dairy samples in the summer of 2024 and found no live virus in any of them. Raw milk is the open door, and Kuchipudi was blunt about it, pointing to earlier reports of cats that died after drinking it.

None of the institutional response moved at the speed of the virus. By May 2024 Nature was reporting that the sluggish official reaction worried scientists who could watch the virus jump between herds faster than the surveillance chasing it. By the time JAMA called the outbreak widespread and a public-health concern that June, the early window had closed. And by February 2025 the picture had tipped into something close to farce: as Canada moved to stockpile half a million doses of H5N1 vaccine, the United States was scrambling to rehire the bird-flu staff it had laid off, trying to rebuild expertise it had decided, at some point, it could spare. A virus hiding in an unfamiliar tissue is a hard problem. A government that fires the people who study it and then goes looking for them mid-outbreak is a different kind of problem, and not one a receptor map can fix.

The official reassurance has held steady the whole way. The CDC still rates the public risk as low, with 71 confirmed human cases since February 2024, almost all of them dairy and poultry workers. That is true. It is also the same register of confidence that carried the first weeks of an outbreak everyone now agrees was missed.

The receptors were there the whole time. They were there in the two cows examined in 2024, and they are there in the detailed map drawn in 2026. What was missing was never the molecule. It was anyone at USDA or the public-health agencies willing to treat an unfamiliar pattern in a familiar animal as the warning it plainly was.

Sources

  1. EurekAlert / University of Pittsburgh – “How H5N1 bird flu hid unrecognized for weeks in dairy cattle” (June 2026)
  2. ScienceDaily – “Scientists finally solved how H5N1 bird flu hid in dairy cows” (June 2026)
  3. Emerging Infectious Diseases (CDC) – Nelli et al., sialic acid receptor specificity in dairy cattle mammary gland, July 2024
  4. FDA – Investigation of Avian Influenza A (H5N1) Virus in Dairy Cattle (retail dairy sampling)
  5. Nature – “Could bird flu in cows lead to a human outbreak? Slow response worries scientists” (May 2024)
  6. JAMA – “Bird Flu Outbreak in Dairy Cows Is Widespread, Raising Public Health Concerns” (June 2024)
  7. BMJ – “Canada buys 500 000 H5N1 vaccines as US scrambles to rehire bird flu staff” (Feb 2025)
  8. CDC – A(H5) Bird Flu: Current Situation Summary