🟠 Moderate Evidence
Researchers have identified why H5N1 influenza virus targets dairy cattle’s udders rather than their respiratory tract—a finding that may reshape how scientists monitor and predict zoonotic spillover events. The discovery reveals that viral receptor distribution in mammary tissue creates an unexpected infection pathway, fundamentally altering understanding of how avian influenza can adapt to mammalian hosts.
Key takeaways
- H5N1 preferentially infects dairy cow udders because viral receptors are concentrated in mammary tissue, not lungs
- This tissue-specific tropism explains the unusual presentation of bovine H5N1 infection and suggests the virus has adapted to exploit host-specific vulnerabilities
- Understanding receptor distribution may help predict future zoonotic spillover events and guide surveillance strategies in livestock populations
- The findings highlight the importance of monitoring non-respiratory manifestations of avian influenza in animals and humans
Where H5N1 Receptors Concentrate in Cattle Tissues
Comparative receptor density across respiratory and mammary tissues
Source: Research findings on H5N1 receptor distribution | Georgian Medical Journal News
The Puzzle of Udder-Specific Infection
When H5N1 avian influenza began appearing in dairy cattle in 2024, veterinary epidemiologists noted an anomaly: infected cows developed severe mastitis—inflammation of the udder—rather than the respiratory disease typical of avian flu in birds and most mammals. This presentation puzzled researchers because H5N1 has historically targeted the lungs and upper respiratory tract as its primary sites of replication.
The newly published research explains this discrepancy through receptor biology. Influenza viruses, including H5N1, require specific cellular receptors to attach and infect host cells. In birds, the virus binds to alpha-2,3 sialic acid receptors, which are abundant in respiratory epithelium. Humans and terrestrial mammals typically express alpha-2,6 sialic acid receptors, which is why avian flu normally struggles to establish human-to-human transmission. However, dairy cattle mammary tissue appears to possess a unique receptor landscape that facilitates H5N1 binding and replication, according to the research findings on viral tropism.
Tissue-Specific Receptor Expression as an Infection Gateway
The breakthrough centres on understanding that mammalian tissues do not uniformly express the same receptors. While a cow’s lungs may lack the optimal receptor configuration for H5N1 attachment, its mammary gland epithelial cells—which line the milk ducts and alveoli—express high concentrations of receptors that facilitate viral entry. This tissue-specific heterogeneity explains why the virus has adapted to exploit a biological niche that few other respiratory pathogens target with such precision.
This discovery has immediate implications for infection control. Because infected udder tissue sheds high viral loads into milk, dairy cattle with H5N1 present a significant transmission risk through unpasteurized milk and direct contact with infected secretions. The research suggests that conventional respiratory surveillance protocols—which focus on lung tissue and nasal swabs—may miss or delay detection of H5N1 infections that are actively replicating in mammary tissue. For a comprehensive review of epidemiological patterns, see our coverage of emerging zoonotic threats in Global Health.
Predicting Future Spillover Events
Understanding receptor distribution across tissues opens new pathways for predicting how H5N1—and potentially other influenza viruses—might adapt to new hosts. If researchers can map receptor profiles in livestock and companion animal populations, they may be able to identify tissues and species at highest risk before widespread infection occurs. This proactive approach differs fundamentally from reactive surveillance, which typically begins after cases appear.
The research suggests that similar tissue-specific vulnerabilities may exist in other animal species and organs. For instance, if pigs or poultry possess tissues with high concentrations of receptors favourable to H5N1, these could become new infection sites that traditional surveillance misses. The World Health Organization emphasizes the need for enhanced surveillance systems that extend beyond respiratory sites, particularly in agricultural settings where multiple animal species interact with humans.
H5N1’s selective tropism for bovine mammary tissue, driven by concentrated receptor expression, explains the virus’s unusual presentation in dairy cattle and offers a model for predicting tissue-specific spillover in other species.
— Research findings on viral receptor adaptation in mammalian tissues
Implications for Surveillance and Public Health Response
The practical implications of these findings extend across veterinary, agricultural, and public health sectors. Dairy farms must now consider that H5N1 surveillance requires milk testing and udder tissue sampling alongside conventional respiratory swabs. This broadens the diagnostic toolkit but also increases the cost and complexity of monitoring. Additionally, because infected milk can transmit the virus, pasteurization standards and milk handling protocols take on renewed importance for H5N1 prevention, though pasteurization efficacy for H5N1 remains under active investigation.
For clinicians and veterinarians, recognizing that H5N1 may present as mastitis rather than pneumonia is critical for early detection. A cow with acute mastitis that tests negative for bacterial pathogens should raise suspicion for viral infection, particularly in regions with documented H5N1 circulation. This diagnostic reframing requires education and awareness campaigns across farm workers, veterinarians, and dairy processors. Access to curated clinical updates on emerging infections is available through our Clinical Updates section.
What this means
Frequently asked questions
Why does H5N1 prefer cow udders to lungs?
The virus preferentially infects mammary tissue because of the high concentration of viral receptors (sialic acids) in udder epithelial cells. These receptors facilitate viral attachment and entry far more efficiently than receptors in respiratory tissue, which explains why infected cows develop mastitis rather than pneumonia.
Can H5N1 spread from infected milk to humans?
While unpasteurized milk from infected cattle could theoretically transmit H5N1, pasteurization—heating milk to specific temperatures—inactivates the virus. The risk is substantially reduced for pasteurized dairy products, but unpasteurized or raw milk products carry higher theoretical risk and should be avoided during periods of documented bovine H5N1 circulation.
How should farms monitor for H5N1 in dairy cattle?
Farms should implement a multi-tiered surveillance approach including regular nasal and throat swabs for respiratory monitoring, milk testing for viral presence, and clinical monitoring for unusual mastitis cases that do not respond to standard antibiotic treatment. Rapid reporting of suspected cases to veterinary and public health authorities is essential.
As H5N1 continues to circulate globally, understanding its tissue tropism and receptor preferences will guide the next generation of surveillance, diagnostic, and control strategies. The discovery that mammary tissue serves as an infection site reshapes our conception of how avian influenza can adapt to terrestrial mammals and underscores the critical importance of integrating molecular virology with epidemiological field investigation.
Source: Scientists finally solved how H5N1 bird flu hid in dairy cows
Was this article helpful?
Disclaimer. This article is health journalism intended for general information and education. It is not medical advice and is not a substitute for professional diagnosis or treatment. Always consult a qualified healthcare provider about your individual circumstances. Full disclaimer →
Related Coverage
Editorial standards. This article was produced under the GMJ News editorial process, with oversight by the GMJ Editorial Board. Our editorial process. Spotted an error? Contact the editorial team.
