Nature Communications finds H5N1 cattle affinity

Nature Communications published a study on May 24 reporting that highly pathogenic avian influenza A(H5N1) shows a strong tropism for the bovine mammary gland in experimental work on dairy cows. The paper said the virus infected mammary tissue at low doses and described transmission barriers that the authors said help explain how the outbreak has behaved in cattle. The findings add laboratory detail to an outbreak first reported in U.S. dairy cows in March 2024. CDC said on March 6 that H5 bird flu is causing outbreaks in U.S. dairy cows and that the current public health risk remains low. ### What did the new study actually find in cows? The Nature Communications paper said H5N1 infected the bovine mammary gland at “remarkably low doses” in experimental infections, indicating a strong affinity for milk-producing tissue. The abstract said that result challenges previous assumptions about influenza A host range and tissue specificity. (nature.com) The researchers also reported transmission barriers. In the paper’s summary, those barriers help explain why the virus appears well suited to infect mammary tissue while still facing constraints on broader spread routes. The study focused on biological mechanisms rather than field policy. ### Why does mammary-gland infection matter for milk? (nature.com) CDC reported in 2024 that A(H5N1) viruses were found in high concentrations in unpasteurized milk from infected cows. That made the udder and milk pathway central to outbreak monitoring from the start of the U.S. dairy episode. A CDC Emerging Infectious Diseases paper published in February 2026 said U.S. retail milk monitoring during April 13 to May 3, 2024, found H5N1 viral RNA in 36% of retail milk samples, which the authors said pointed to widespread undetected infections in dairy cows at that stage. (nature.com) The same paper said later sampling from late December 2024 to late January 2025 aligned more closely with reported herd infections after federal testing measures were introduced. (cdc.gov) ### Does this change the risk to people? CDC said on March 6, 2026, that the current public health risk from A(H5) bird flu is low and that there is no known person-to-person spread at this time. The agency said 71 human cases had been reported in the United States since 2024, including cases linked to dairy herds, poultry farms and other animal exposures. (wwwnc.cdc.gov) WHO says human infections are mostly linked to close contact with infected birds, other animals and contaminated environments. WHO also says the virus does not appear to transmit easily from person to person and that sustained human-to-human transmission has not been reported. (cdc.gov) ### What does the paper suggest about surveillance? The CDC said the multistate outbreak in dairy cows was first reported on March 25, 2024, and described it as the first time these bird flu viruses had been found in cows. That novelty is part of why detection lagged early in the outbreak, according to the CDC Emerging Infectious Diseases paper on retail milk monitoring. (who.int) The Nature study’s low-dose mammary-gland finding supports closer monitoring of milk and lactating cattle because the tissue identified in the lab is the same site tied to milk production. That is an inference from the paper’s experimental results and CDC’s outbreak reporting, not a separate regulatory finding. (cdc.gov) ### Why does the India oxytocin example appear in discussion of this paper? The source briefing linked the paper’s call for stronger milk-chain biosecurity and animal monitoring to reporting on misuse of restricted oxytocin injections in milch animals in India. That comparison is about farm oversight and enforcement pressure in milk production systems, not about oxytocin causing H5N1. (nature.com) WHO updated its joint H5 risk assessment with FAO and WOAH on May 18, 2026, and CDC continues to post U.S. surveillance updates. The Nature paper is now part of that evidence base as animal-health agencies track infections in cattle, milk and exposed workers. (who.int) (nature.com)