PITTSBURGH— When H5N1 bird flu first began infecting U.S. cattle in early 2024, diagnosis was elusive, because in cows, the disease looked completely different. Instead of affecting the lungs, as H5N1 does in other mammalian species, it caused severe infection in the cows' udders, largely sparing the lungs.
A study by University of Pittsburgh School of Public Health researchers published today in Science Advances provides the first mechanistic explanation for this peculiar new guise for H5N1, which now affects more than 100 bird and mammal species globally. The study also establishes a new way to help scientists spot bird flu's next surprise move more quickly, saving precious time in mounting public-health measures to stem the spread.
The disease first appeared in dairy cattle along the Texas Panhandle as stubborn cases of severe, necrotizing mastitis, a painful inflammatory condition that damages tissues in the mammary glands.
"Mastitis is a classic disease in milk-production animals, and veterinarians were dutifully looking to all the usual suspects for the source, like bacterial pathogens," said senior author Suresh Kuchipudi, Ph.D., chair of Infectious Diseases and Microbiology at Pitt Public Health . "When the real culprit turned out to be bird flu, everyone in the field was caught completely by surprise. We hadn't even remotely considered that cattle could be a host for H5N1."
In the weeks before the virus was identified, it moved from herd to herd, sickening the cattle—and contaminating their environments.
"If a cow is infected, it sheds a lot of virus into the milk," said Kuchipudi. "This raised concerns about occupational risk for farm workers. Also, there is a habit of feeding raw milk to domestic pets, like cats, and there have been instances of cats dying , which we studied previously." He stressed that fortunately, pasteurization is effective at killing the virus, underlining the importance of avoiding raw milk.
Kuchipudi has been studying influenza viruses for his entire career, with a particular focus on how receptor biology determines which species—and which tissues—can be infected. Typically, such studies involve staining cells for the presence of receptors that are known to work in a lock-and-key relationship with influenza, a subset of sugar-based molecules known as glycans.
In initial studies by other groups, such experiments suggested that flu‑related glycan receptors were present in the noses, tracheas and lungs of cows. The fact that the animals were nonetheless not developing respiratory infections told the team there was more to the story.
"Glycan biology is very complex," said Kuchipudi. "We realized that, to understand what was really going on, we would need to use more innovative technologies and map out the fine‑detailed architecture that enables the virus to bind to cells." Kuchipudi collaborated on the study with Harvard Medical School's Lauren E. Pepi, Ph.D., an expert in the methodology for comprehensively cataloging the entirety of glycan structures, dubbed glycomics.
Using a multimodal approach that combined binding experiments, staining methods and ultra‑high‑resolution imaging, the team revealed that not all glycan receptors were functioning the same in animals infected with bird flu. Only a particular subtype, known as N‑linked sialic acid receptors, could bind to H5N1. These receptors were virtually absent in cow airway tissue, but pervasive in udders, making them a "perfect breeding ground for the virus," Kuchipudi said.
The research provides a framework other scientists can use to potentially predict not just whether H5N1 can jump to new hosts, but also how.
"We can preemptively screen different species and different tissues within them for susceptibility," said Kuchipudi. "For example, would they exhibit respiratory symptoms? Would they show only mastitis, as in cows? Or would they show neurological disease, as our team has shown in cats? The lessons learned could potentially help prevent us from being caught by surprise again."
Other authors on the study were Surabhi Srinivas, M.S., Shubhada K. Chothe, Ph.D., Santhamani Ramasamy, Ph.D., Sougat Misra, Ph.D., Noel Chandan Nallipogu, M.D., MPH, and Lindsey LaBella, all of Pitt; Yin-Ting Yeh, Ph.D., of Pennsylvania State University; May Wang, B.S., of Harvard University; and Heidi L. Pecoraro, Ph.D., and Brett T. Webb Ph.D., of North Dakota State University.
This research was supported by Pitt Public Health, and the U.S. Department of Agriculture's National Institute of Food and Agriculture (FP00039373/AWD00010780).
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About the University of Pittsburgh School of Public Health
Founded in 1948, the University of Pittsburgh School of Public Health is a top-ranked institution of seven academic departments partnering with stakeholders locally and globally to create, implement and disseminate innovative public health research and practice. With hands-on and high-tech instruction, Pitt Public Health trains a diverse community of students to become public health leaders who counter persistent population health problems and inequities.
