Scientists have now traced why the H5N1 bird flu virus that jumped into U.S. dairy herds keeps turning up in cows’ udders and milk instead of their lungs. Controlled infections and lab work on cow tissues show the virus replicates efficiently in mammary glands while staying largely confined to the upper airways after respiratory exposure, a pattern that shapes how it spreads and how regulators monitor milk.
The findings carry direct stakes for dairy workers, raw-milk drinkers and farm operators, because they explain why milk has become both the key diagnostic sample and a likely driver of farm-to-farm transmission, even as respiratory disease in cattle remains relatively limited.
Why udder‑focused H5N1 biology matters now
Researchers working with ex vivo cow tissue have shown that H5N1 can actively replicate in bovine mammary gland and teat explant cultures, demonstrating that the udder itself is a permissive site for growth according to a Primary study. That direct replication in mammary tissue helps explain why field investigations have repeatedly detected viral RNA in bulk milk from affected herds.
In live-animal experiments, high-dose oronasal infection of calves produced only low to moderate viral replication, and this was largely confined to the upper respiratory tract according to a separate Primary infection study. By contrast, the same research reported that lactating cows subjected to intramammary challenge shed high levels of infectious virus in milk and udder tissue, confirming that the mammary route supports far more intense viral growth than the airways.
This biological split feeds directly into how the virus moves between farms. An experimental transmission study concluded that spread in dairy settings is likely driven by milk and milking procedures rather than respiratory routes, based on patterns of viral RNA detection in milk and limited positivity in other organs, according to a Primary dynamics analysis. That conclusion supports the working hypothesis that milking equipment and pooled milk pose a greater risk for herd-to-herd spread than coughing or nasal discharge from infected cattle.
For producers and regulators, the stakes are clear: a virus that concentrates in udders can silently contaminate milk lines even when cows show only mild respiratory signs. This udder-centric pattern is why surveillance programs and regulatory responses have focused on testing milk and adjusting milking protocols rather than treating the outbreak as a classic respiratory epidemic in livestock.
The evidence behind H5N1’s mammary tropism
The clearest quantitative picture of how strongly H5N1 favors the udder comes from pathology and virology work in dairy cows. A Foundational study of spillover into cattle documented high infectious virus titers in milk, reporting concentrations up to 10^8.8 TCID50/mL in samples from infected animals, and described a distinct tropism for mammary tissue with only sporadic detection in other organs according to research published in Nature. Those sampling data show that the virus can reach extremely high levels in milk while remaining comparatively scarce elsewhere in the body.
Receptor mapping adds a mechanistic layer to that observation. Influenza viruses use sialic acid receptors to enter cells, and both α2,3 and α2,6 sialic acids are distributed in the bovine respiratory tract and mammary glands according to a Direct lectin histochemistry study of receptor specificity in dairy cattle, which reported this pattern in Direct analyses. In that same work, viral antigen was shown to co-localize with α2,3-linked receptors in mammary tissue, indicating that the virus is binding and replicating in areas of the udder where those receptors are concentrated.
Genetic data from the outbreak strains connect this receptor environment to viral evolution. A Primary molecular study of hemagglutinin reported that a single mutation linked to dairy-cow outbreak viruses increases receptor binding breadth, based on glycan microarray analysis of HA variants associated with infected herds as described in Primary. The authors noted that mammary alveoli contain abundant α2,3-linked glycans, so a virus with broader binding could more efficiently use the receptor mix present in the udder.
Together, these lines of evidence support a simple chain: bovine mammary tissue offers dense α2,3-linked sialic acid targets, H5N1 carries a hemagglutinin mutation that broadens its receptor use, and in both tissue cultures and live cows the virus reaches its highest titers in the udder. That pattern fits the experimental findings that intramammary exposure in lactating cows leads to heavy shedding in milk, while high-dose respiratory exposure in calves yields only modest replication restricted to the upper airways.
Regulatory sampling has confirmed that this biology shows up along the milk supply. The U.S. Food and Drug Administration reported that infectious H5N1 virus was detected in a subset of positive raw milk samples, with concentrations calculated from cell culture assays according to an FDA investigation of avian influenza in dairy cattle. That finding links on-farm mammary infection to viable virus in unpasteurized milk leaving the farm.
Retail surveillance has extended the signal into grocery stores. A Primary analysis of national retail cartons found that H5 viral RNA appeared in retail milk during early outbreak windows, indicating that milk from infected herds had entered commercial distribution before on-farm infections were fully recognized, according to a Primary RNA study. The detection of RNA, rather than infectious virus, in those pasteurized products still tracks back to the same source: udders shedding virus into the bulk tank.
Because milk has become the most reliable specimen, federal surveillance has been built around it. The U.S. Department of Agriculture’s National Milk Testing Strategy describes how routine testing of bulk milk samples is being used as the primary evidence stream for detecting infections in dairy herds, according to an Evidence program description. That design choice reflects the scientific consensus that mammary shedding, not respiratory disease, is the main signal of H5N1 in cattle.
What remains unresolved about udder‑centric H5N1 spread
Even with this detailed picture of mammary tropism, key gaps remain. The controlled infection work in calves and cows shows clear differences between respiratory and intramammary routes, but it does not provide primary longitudinal pathology data on how udders recover or how long cows might keep shedding virus in milk after a natural farm infection, which the reporting brief lists as missing. Without those data, scientists cannot yet say how persistent the risk from a recovered herd might be.
There are also limits on what is known about virus levels in consumer products. The FDA has confirmed that infectious virus was found in some positive raw milk samples, but the agency’s public materials do not disclose exact infectious titers measured in retail milk, and the brief notes that direct statements on those figures remain unpublished according to the same FDA sampling record. The retail RNA analysis from CDC-linked researchers shows genetic material in cartons, yet it does not establish whether any viable virus survived processing, as described in the Primary RNA surveillance study.
On the viral genetics side, the Nature Communications work ties a specific hemagglutinin mutation to broader receptor binding, but the reporting brief notes that primary records detailing how many farms completed whole-genome sequencing to confirm this mutation in field isolates have not been released. That means scientists know the mutation is linked to dairy-cow viruses in sampled herds, yet they lack a complete map of how widely that variant has spread across all affected operations, even though the mutation’s effect on receptor breadth is clearly documented in the Primary analysis.
Human health risk from milk exposure is another open question. The brief explicitly states that controlled human or animal exposure studies connecting raw-milk consumption to infection are absent from the cited sources. Regulators have evidence of infectious virus in some raw milk and RNA in retail products, but no experimental data yet tie those findings to actual transmission events, beyond what is summarized in the Investigation of Avian Influenza situation materials.
Finally, the outbreak’s full trajectory in cattle remains hard to pin down. The latest publicly available experimental and surveillance updates come from recent Primary and institutional reports, but there is no unified, real-time dataset that links milk viral loads, farm biosecurity practices and genetic data on the hemagglutinin mutation across all affected states, a gap highlighted indirectly by the need for the National Milk Testing Strategy described by National Milk Testing Strategy materials.
For readers, the practical takeaway is that H5N1’s focus on the udder changes where risk concentrates. The strongest evidence so far points to milk and milking equipment as the main conduits for spread within the dairy sector, while respiratory illness in cattle appears limited in controlled settings. The next things to watch are how long infected cows shed virus in milk, whether more detailed sequencing confirms the reach of the receptor-broadening mutation, and how surveillance of raw and retail milk evolves as agencies refine their response to a bird flu virus that has learned to thrive in the bovine udder.
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*This article was researched with the help of AI, with human editors creating the final content.
