When H5N1 bird flu began infecting US cattle in early 2024, the diagnosis was unclear because in cows, the disease looked completely different. Instead of affecting the lungs, as H5N1 does in other species of mammals, it caused severe infection in the udders of cows, largely causing the lungs.
A study by researchers at the University of Pittsburgh School of Public Health published today in the Advances in Science provides the first mechanistic explanation for this strange new guise for H5N1, which now affects more than 100 species of birds and mammals worldwide. The study also establishes a new way to help scientists spot the next bird flu surprise more quickly, saving valuable time in taking public health measures to contain the spread.
The disease first appeared in dairy cattle along the Texas Panhandle as persistent cases of severe, necrotizing mastitis, a painful inflammatory condition that destroys tissue in the mammary glands.
Mastitis is a classic disease in dairy animals, and veterinarians dutifully looked for all the usual suspects for the source, such as bacterial pathogens. When the real culprit turned out to be bird flu, everyone in the field was taken aback. We didn’t even think that cattle could harbor H5N1.”
Suresh Kuchipudi, Ph.D., senior author, chair of Infectious Diseases and Microbiology, Pitt Public Health
In the weeks before the virus was identified, it moved from herd to herd, sickening cattle and contaminating their environment.
“If a cow gets infected, it sheds a lot of virus into the milk,” Kuchipudi said. “This has raised concerns about occupational risk for farm workers. Also, there is a habit of feeding raw milk to pets such as cats, and there have been cases of cat deaths, which we have previously studied.” He stressed that fortunately, pasteurization is effective in killing the virus, stressing 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 known to function in a locking relationship with influenza, a subset of sugar-based molecules known as glycans.
In initial studies by other groups, such experiments suggested that flu-associated glycan receptors were present in the noses, tracheas and lungs of cows. The fact that the animals, however, did not develop respiratory infections told the team that there was more to the story.
“Glucan biology is very complex,” Kuchipudi said. “We realized that to understand what was really going on, we would need to use more innovative technologies and map the detailed architecture that allows the virus to bind to cells.” Kuchipudi collaborated on the study with Lauren E. Pepi, Ph.D. of Harvard Medical School, an expert in the methodology for the comprehensive cataloging of all glycan structures, called glycomics.
Using a multimodal approach that combined binding experiments, staining methods and ultra-high-resolution imaging, the team revealed that not all glycan receptors functioned the same in animals infected with bird flu. Only one specific subtype, known as N-linked sialic acid receptors, could bind to H5N1. These receptors were virtually absent in the cow’s airway tissue, but were widespread in the udders, making them “a perfect breeding ground for the virus,” Kuchipudi said.
The research provides a framework that other scientists can use to potentially predict not only whether H5N1 might jump to new hosts, but also how.
“We can proactively screen different species and different tissues within them for sensitivity,” Kuchipudi said. “For example, would they show respiratory symptoms? Would they only show mastitis, like in cows? Or would they show neurological disease, like our team showed in cats? The lessons learned could potentially help us not to be surprised again.”
Other authors on the study were Surabhi Srinivas, MS, Shubhada K. Chothe, Ph.D., Santhamani Ramasamy, Ph.D., Sougat Misra, Ph.D., Noel Chandan Nallipogu, MD, MPH, and Lindsey LaBella, all of Pitt. Yin-Ting Yeh, Ph.D., of Pennsylvania State University. May Wang, BS, 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 US Department of Agriculture’s National Institute of Food and Agriculture (FP00039373/AWD00010780).
