When H5N1 bird flu began infecting American dairy cattle in early 2024, veterinarians had difficulty identifying the cause. The virus was difficult to recognize because it behaved very differently in cows than in other mammals. Instead of primarily infecting the lungs, H5N1 caused severe udder infections and largely spared the respiratory system.
Now, researchers at the University of Pittsburgh School of Public Health have discovered the biological reason behind this unusual pattern. Their findings, published in Scientific advancesThey provide the first detailed explanation of why bird flu took such an unexpected form in livestock. The work could also help scientists better anticipate how H5N1 might behave if it spreads to new animal species in the future.
The unusual appearance of bird flu in dairy herds
The outbreak first emerged in dairy cattle in the Texas Panhandle, where the animals developed severe cases of necrotizing mastitis, a painful inflammatory disease that damages mammary gland tissue.
“Mastitis is a classic disease in dairy animals, and veterinarians were diligently looking for the source of all the usual suspects, such as 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 completely surprised. We hadn’t even remotely considered that livestock could be a host for H5N1.”
Before the virus was identified, it spread from one herd to another, infecting livestock and contaminating their environment.
“If a cow is infected, it sheds a lot of virus into the milk,” Kuchipudi said. “This raised concerns about occupational risk for agricultural workers. In addition, there is a custom of feeding raw milk to domestic pets, such as cats, and there have been cases of cats dyingthat we studied previously.”
Kuchipudi emphasized that pasteurization effectively destroys the virus and highlighted the importance of avoiding raw milk.
Searching for the biological explanation
Throughout his career, Kuchipudi has studied influenza viruses, focusing on how receptor biology influences which species and tissues can become infected. Influenza viruses attach to specific lock-and-key receptors on cells. These receptors belong to a group of sugar-based molecules known as glycans.
Previous studies by other research groups suggested that flu-related glycan receptors were present in the nose, trachea and lungs of cattle. However, cows infected with H5N1 did not develop the respiratory disease that the researchers expected.
That discrepancy suggests that a more detailed explanation is needed.
“The biology of glycans is very complex,” Kuchipudi said. “We realized that to understand what was really happening, we would need to use more innovative technologies and map out the detailed architecture that allows the virus to bind to cells.”
To do this, Kuchipudi partnered with Harvard Medical School researcher Lauren E. Pepi, Ph.D., whose expertise is in glycomics, the comprehensive study of glycan structures.
Why H5N1 attacks cow udders
The research team combined multiple techniques, including binding experiments, staining methods, and ultra-high-resolution imaging, to examine how H5N1 interacts with different tissues.
Their analysis showed that not all glycan receptors function the same when it comes to bird flu infection. The virus was only able to bind to a specific subtype known as N-linked sialic acid receptors.
These receptors were found in all udder tissue of cattle, but were almost absent in airway tissue. According to Kuchipudi, this made the mammary glands a “perfect breeding ground for the virus.”
The discovery helps explain why H5N1 caused severe mastitis rather than respiratory illness in dairy cattle.
Predict the next step of bird flu
The researchers believe their findings could do more than explain the outbreak in cattle. The same approach can help scientists identify which animals and tissues are vulnerable to H5N1 before future outbreaks occur.
“We can preemptively screen different species and different tissues within them for susceptibility,” Kuchipudi said. “For example, would they show respiratory symptoms? Would they show only mastitis, as in cows? Or would they show neurological diseases, as our team has shown in cats? The lessons learned could help prevent us from being caught off guard again.”
Other authors of 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).