Scientists and regulators are racing to understand how a deadly bird flu strain is behaving now that it has reached cattle in North America and Europe. U.S. retail milk testing has already documented Influenza A(H5N1) in dairy products, and European authorities have confirmed exposure in at least one Dutch dairy cow and a sheep in Great Britain. The concern is that these scattered detections signal a virus that is quietly spreading across species and continents faster than surveillance systems were built to detect.
The immediate stakes are twofold: protecting people who work with animals and making sure food supplies remain safe as the virus moves beyond birds. At the same time, scientists are watching for early signs that H5N1 is adapting to mammals in ways that could change its behavior in humans.
Why scientists see cattle-linked bird flu risk escalating now
The core worry behind warnings that bird flu is spiraling is not a single explosive event but the pattern of repeated mammal infections tied to the same clade 2.3.4.4b H5N1 virus. A peer-reviewed case report describes detection of clade 2.3.4.4b high pathogenicity avian influenza virus in a sheep in Great Britain, with researchers using targeted sampling and PCR assays to confirm infection and link the virus to local avian cases, according to a peer-reviewed publication. That sheep infection sits alongside widespread infection in U.S. dairy cattle documented through retail milk monitoring and a serology signal in a Dutch dairy cow, showing the same virus family reaching cattle and small ruminants on at least three continents.
In the United States, a peer-reviewed analysis by CDC scientists describes how retail milk monitoring for Influenza A(H5N1) in dairy cattle during 2024–2025 used systematic sampling of commercial products and laboratory testing to track virus spread, according to the CDC. The authors link their findings to federal orders and a National Milk Testing Strategy, indicating that virus circulation in herds was broad enough to require coordinated national surveillance. That scale of testing suggests scientists are not dealing with a handful of isolated animal infections but with an established presence in cattle systems.
European agencies are now treating U.S. dairy cattle as a plausible source of introduction. The European Food Safety Authority has issued a scientific opinion assessing how H5N1 genotype B3.13 in U.S. dairy cows could reach European herds, according to a primary assessment on the EFSA Journal. EFSA’s experts conclude that introduction of bird flu from U.S. dairy cattle into Europe is “very unlikely,” yet they still recommend specific surveillance steps, indicating that even a low-probability route is being treated as a serious biosecurity concern.
This pattern of spread sets the stage for a next-step scientific question: whether H5N1 lineages circulating in cattle will begin to show measurable changes in the viral polymerase and in receptor binding that favor human-type sialic acids. The working hypothesis among many influenza researchers is that repeated replication in mammalian tissues, such as the bovine udder and respiratory tract, can select for mutations that improve virus replication at mammalian body temperatures and shift binding preference toward the receptors common in human upper airways. If that shift emerges, it should be detectable within about a year through repeated deep sequencing of outbreak herds, building on the kind of genetic analysis already described in the Great Britain sheep report and U.S. dairy investigations.
The evidence tying cattle, milk and H5N1 across regions
The most detailed picture of H5N1 in cattle so far comes from the United States. In the retail milk monitoring study, CDC scientists used a peer-reviewed protocol to test commercial milk samples collected across multiple states, according to the CDC. Their analysis suggests that infected dairy cows were shedding virus into milk that later entered the processing chain, which in turn prompted the National Milk Testing Strategy and federal orders aimed at tracking infected herds. Although that study focuses on detection rather than human illness, it shows how milk can act as a surveillance window into cattle infections that might otherwise go unnoticed on farms.
On the regulatory side, the U.S. Food and Drug Administration has built an “Investigation of Avian Influenza A (H5N1) Virus in Dairy Cattle” hub that pulls together testing data and policy actions from FDA, CDC and USDA. That hub explains how federal agencies are assessing the safety of pasteurized retail milk and other dairy products while tracing virus spread in herds, according to the FDA. The same investigation links to experimental work on virus inactivation and to biosecurity guidance for dairy and animal-food producers, showing how laboratory findings are feeding directly into market-facing decisions.
In Europe, EFSA’s scientific opinion on H5N1 genotype B3.13 lays out detailed pathways by which virus in U.S. dairy cattle could reach EU herds, including animal movements and trade in dairy products, according to the primary analysis hosted on the EFSA platform. While the overall conclusion is that such introduction is very unlikely, the opinion still characterizes dairy cattle in the EU as potentially susceptible and calls for targeted surveillance. EFSA’s experts explicitly recommend bulk milk testing as a surveillance tool for H5N1 in dairy cattle, arguing that pooled milk samples offer an efficient way to detect infection in large herds before clinical signs appear.
An official summary from EFSA translates this science into policy guidance, stating that the introduction of bird flu from U.S. dairy cattle to Europe is very unlikely but that vigilance is urged in Europe, including through enhanced monitoring and rapid reporting, according to an official EFSA communication. That combination of low estimated probability and strong recommendations reflects a risk-management stance that treats even rare cattle-linked incursions as important to detect quickly.
Within the EU, the European Centre for Disease Prevention and Control has already had to interpret a concrete cattle exposure. ECDC reports that avian flu antibodies were detected in a Dutch dairy cow, indicating prior infection or exposure, according to an assessment on the ECDC site. After reviewing that finding alongside evidence of cattle-linked human infections reported in the United States among exposed workers, ECDC states that its risk assessment remains unchanged: risk to the general public is still considered low, while people with occupational exposure to infected animals face higher exposure.
The Great Britain sheep case adds another piece to the cross-species picture. In that peer-reviewed study, researchers describe how they sampled the sheep, applied PCR testing, and sequenced the virus to place it within clade 2.3.4.4b, according to the Detection of clade 2.3.4.4b report. The authors also compare the sheep virus to sequences from local birds, concluding that the infection likely arose from nearby avian sources rather than from cattle. Even so, the presence of the same clade in a lactating sheep, U.S. dairy cows and a Dutch dairy animal suggests that ruminant species are repeatedly encountering this virus family in real-world farm settings.
What scientists still do not know about mammalian adaptation and human risk
Despite this growing body of evidence, several key pieces are missing. For the Dutch dairy cow with avian flu antibodies, ECDC notes that the finding comes from serology rather than direct virus isolation, and no full genomic sequence from that animal has been shared in the public record, according to the ECDC. Without full sequences, scientists cannot yet test whether cattle-linked viruses in Europe carry polymerase or receptor-binding changes that differ from those seen in birds or in U.S. dairy herds.
Similarly, EFSA’s opinion on H5N1 genotype B3.13 focuses on pathways of introduction and surveillance options but does not present longitudinal bulk milk testing data from EU member states, according to the primary analysis in the EFSA Journal. That means European risk estimates still rest largely on modeling and expert judgment rather than on continent-wide milk testing campaigns comparable to the U.S. National Milk Testing Strategy. Until such data exist, scientists cannot say how widely, if at all, H5N1 is circulating in European cattle.
There is also limited public documentation of cattle-to-human transmission outside the United States. ECDC explicitly notes that human infections linked to dairy cattle have been reported in the U.S. among exposed workers, but its unchanged risk assessment indicates that similar events have not been confirmed in Europe, according to the ECDC. Without a multi-country case line list that ties together U.S. dairy outbreaks, the Great Britain sheep detection and the Dutch cow serology, it remains hard to judge whether mammalian infections are mostly dead-end events or part of a broader adaptation process.
The latest publicly available updates from EFSA and ECDC were published before early 2026, so current conditions on farms may already have shifted. That time lag matters for readers because virus evolution can move faster than formal risk assessments. For people who work with cattle, sheep or poultry, the most immediate consequence is that occupational exposure remains the main concern: ECDC explicitly distinguishes low risk for the general population from higher risk for workers in contact with infected animals, according to its assessment. For consumers, existing evidence from FDA and CDC testing has guided assurances around pasteurized milk safety, but those assurances depend on continued monitoring.
The next things to watch are clear. First, whether expanded bulk milk testing in Europe, as recommended by EFSA, begins to surface hidden infections in dairy herds. Second, whether deep sequencing of viruses from cattle, sheep and exposed humans reveals consistent changes in polymerase activity or receptor binding that suggest better adaptation to mammals. If those genetic signals start to appear across multiple countries, the case that H5N1 is spiraling through cattle systems into a new phase of risk will grow much stronger, and public health agencies will face pressure to update their assessments and controls in real time.
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*This article was researched with the help of AI, with human editors creating the final content.
