H5N1 Targets Cows' Udders, Not Lungs, Because Receptor Biology Routes Virus Away From Airways

When H5N1 bird flu began tearing through U.S. dairy herds in early 2024, veterinarians didn't recognize it at first. The animals weren't coughing. They were going down with severe mastitis. A new peer-reviewed study now explains why, and the answer has real consequences for how scientists watch for the virus's next species jump.

The research, published in Science Advances and led by Suresh Kuchipudi at the University of Pittsburgh School of Public Health, traced the problem to a specific molecular attachment point. As reported in an EurekAlert release reviewed for this story, the team found that H5N1 can bind only to a particular subtype called N-linked sialic acid receptors, and in cattle those receptors are concentrated in mammary tissue while being nearly absent from airway tissue. That distribution, in effect, routed the virus away from the lungs and toward the udder.

The distinction matters because earlier work had already detected flu-related glycan receptors in the noses, tracheas, and lungs of cattle, which made the absence of respiratory disease harder to explain. According to coverage in The Scientist, Kuchipudi's team suspected the existing data was missing important detail, so they partnered with Harvard Medical School glycomics specialist Lauren E. Pepi and combined binding experiments, tissue staining, and ultra-high-resolution imaging to map where H5N1 could actually attach inside a cow. Wild-type H5N1 bound to mammary gland cells but not to respiratory tissue.

The result, as reported by phys.org, was severe necrotizing mastitis rather than the pneumonia-like illness researchers might have expected from a respiratory pathogen jumping into a new mammalian host. That clinical mismatch is precisely what delayed detection during the Texas Panhandle outbreak, where affected herds showed reduced milk production and thick, discolored milk long before anyone called a federal lab.

A separate study published around the same time, from Ohio State's Plant and Animal Agrosecurity Research Facility and covered by the Wisconsin State Farmer, adds an unsettling data point to the picture: just 10 viral particles of H5N1 are enough to establish infection in dairy cows. The virus then replicates to high concentrations in milk, while cow-to-cow transmission routes remain incompletely understood.

There are limits worth naming here. The Science Advances paper focuses on receptor mapping, a mechanistic explanation rather than an epidemiological one. Understanding why the virus homes in on udders doesn't yet tell us how it gets there in the first place, and the Ohio State infectious-dose finding is from controlled experimental exposure, not field conditions. Neither study settles the question of human risk, which remains a separate surveillance problem.

What the Pitt work does offer is a framework. If you can profile N-linked sialic acid receptor distribution across tissues in other candidate host species, you might predict atypical infection patterns before a novel outbreak catches everyone off guard. As Kuchipudi put it in a statement reviewed by The Scientist, the lessons learned could potentially help prevent scientists from being caught by surprise again.

More than 1,000 dairy herds across 19 states have been infected since March 2024, according to USDA figures cited in recent coverage. Two years into a documented spillover event, the receptor biology is only now being fully mapped. That timeline is itself worth noting.

Sources cited:
- EurekAlert (University of Pittsburgh press release) (https://www.eurekalert.org/news-releases/1131354)
- The Scientist (https://www.the-scientist.com/why-bird-flu-targets-cows-udders-instead-of-lungs-74649)
- phys.org (https://phys.org/news/2026-06-h5n1-bird-flu-hid-unrecognized.html)
- ScienceDaily (https://www.sciencedaily.com/releases/2026/06/260620100315.htm)
- Wisconsin State Farmer (https://www.wisfarmer.com/story/news/2026/06/23/study-even-trace-exposure-to-bird-flu-triggers-infection-in-dairy-cows/90644107007/)
- Areo Magazine (https://areomagazine.com/2026/06/24/h5n1-bird-flu-dairy-cows-udder-receptor-mystery-solved/)