On 25 March 2024, the U.S. Department of Agriculture confirmed HPAI A(H5N1) in a dairy cow in Texas, the first detection of a highly pathogenic H5 virus in cattle anywhere in the world [1]. By early 2026, the count had reached more than 1,000 affected dairy herds across at least 17 states, with 71 confirmed human cases recorded in the United States since the outbreak began [2]. Two of those human infections were fatal. The wider poultry epidemic that runs alongside the dairy outbreak has now cost the United States more than 168 million birds across all 50 states and Puerto Rico, a toll the USDA has classified as the largest and longest animal health disease incident in the country's history [2, 3].
From first detection to a national dairy reservoir
The initial Texas case was a single spillover from wild birds. The virus responsible, genotype B3.13, carries a PB2 M631L mutation that enhances how the viral polymerase interacts with the bovine ANP32A protein, a small but consequential adaptation that helps explain why it spreads so efficiently between cows [2]. Within months, B3.13 had moved into California's Central Valley, the heart of US dairy production. California alone has logged 759 confirmed dairy-herd detections, and in December 2024 the governor declared a state of emergency [3].
In early 2025, surveillance identified a second, independent spillover: genotype D1.1, found in dairy cattle in Nevada and Arizona [2]. D1.1 is a different lineage from B3.13 and arrived through a separate wild-bird introduction, rather than evolving from the cattle-adapted virus. The 6 January 2025 death of a Louisiana patient hospitalised with severe H5N1 illness was caused by D1.1, not B3.13, after exposure to wild birds and backyard flocks rather than to dairy cattle [6]. A dairy worker in Nevada later tested positive for D1.1 following contact with infected cattle, the first known cow-to-human jump of that genotype; symptoms were limited to conjunctivitis [7].
The April 2025 joint FAO/WHO/WOAH assessment formally declared H5N1 enzootic in US dairy cattle, an unprecedented mammalian reservoir for a highly pathogenic H5 lineage [14]. The May 2026 update reaffirmed the picture, rating public-health risk as low for the general population but low-to-moderate for occupationally exposed workers [4].
Takeaway: B3.13 has become a self-sustaining cattle virus in the US, and D1.1 has now spilled into cattle at least once, so the outbreak is no longer a single-lineage event.
How the virus actually moves through a herd
Experimental work by Burrough et al. (2024) showed that H5N1 replicates preferentially in mammary tissue and is shed at high titres in milk from infected cows [10]. The same study documented mastitis-like symptoms in inoculated animals and confirmed airborne contact transmission between lactating cows, though cow-to-cow spread on farms is driven primarily by contaminated milk and milking equipment rather than aerosols [18]. In practice, that means the milking parlour is the main transmission hub: shared inflations, milk residues on hands, and pooled milk moving between animals.
Human cases in dairy workers have been dominated by mild conjunctivitis, the eye being the entry point when splashed milk reaches a worker's face [1, 16]. The two Michigan farm workers described by Uyeki et al. (2024) recovered fully with no respiratory involvement [16]. The occupational risk profile therefore hinges on splash and contact exposure in the parlour, not on drinking retail milk.
Takeaway: the virus travels in milk, not in breath, and that shapes both farm biosecurity and consumer risk.
Pasteurisation works, raw milk does not
The most important consumer question is whether the milk on the shop shelf is safe. The answer from the US Food and Drug Administration, working with Iowa State University, is that standard high-temperature short-time (HTST) pasteurisation reliably inactivates H5N1 in milk [8]. By contrast, raw milk inoculated with live H5N1 retained infectious virus for hours under refrigeration. A separate study by Aguilera et al. (2025) found that infectious H5N1 remained detectable in raw-milk cheese aged at refrigeration temperatures for at least 28 days, with standard cheesemaking conditions (pH, starter cultures) failing to inactivate the virus reliably [11].
For US families, the practical guidance is to keep choosing pasteurised dairy and to treat raw milk and unpasteurised cheese as the principal avoidable exposure route. The International Dairy Foods Association has been consistent on this point: the pasteurised commercial supply is safe, raw milk is not [19].
Takeaway: pasteurisation is the public-health firewall, and the firewall is working.
Surveillance in 2026: a national picture at last
Until 2024, the US had no systematic way to see where H5N1 was circulating in dairy. The National Milk Testing Strategy (NMTS), launched by USDA APHIS, now covers bulk-milk surveillance in 49 states and uses aggregated milk samples to flag infected herds before clinical signs appear [9]. The 2024-2025 push, paired with the agency's HPAI Detections in Livestock dashboard, has replaced anecdote with a near-real-time map of the outbreak [5]. Canada and Mexico have logged only isolated spillover events into dairy cattle, with no sustained spread, and the Pan American Health Organization coordinates regional surveillance across the Americas [15].
The 18 May 2026 joint FAO/WHO/WOAH assessment is the international benchmark: low risk for the general public, low-to-moderate for occupationally exposed workers, and a clear recommendation for ongoing genomic surveillance of cattle-derived isolates [4].
Takeaway: the US is finally measuring the outbreak properly, and the international agencies agree the public-health ceiling is low for now.
What families in Australia, the UK, and the US should do
In the United States, the commercial pasteurised milk supply is considered safe; the avoidable risk sits with raw milk, raw-milk cheese, and on-farm contact. In the United Kingdom, the Food Standards Agency reiterates that pasteurised US dairy remains safe for UK consumers and advises against raw or unpasteurised milk and dairy products from any source [20]. The UK Health Security Agency's HAIRS group classifies the risk to the UK general public from H5N1 in US dairy cattle as very low, and the risk to UK dairy workers from imported US dairy genetics as very low, with active monitoring continuing [12].
In Australia, the picture is even more reassuring. The Department of Agriculture, Fisheries and Forestry confirms Australia remains H5-free in commercial poultry, with no detections of H5N1 in dairy cattle, and the risk of introduction via imported dairy products or live cattle from the US is considered low under existing import controls [13]. NSW Department of Primary Industries maintains that Australia's commercial dairy herds remain H5-free, with movement controls and surveillance testing as the primary mitigations [17]. For Australian families, the relevant habit is the one already in place: choose pasteurised dairy, avoid raw milk, and don't worry about the imported cheese in the chiller.
The converging message from regulators in all three countries is the same: pasteurised dairy is safe, raw dairy is not, and the people at meaningful risk are dairy workers, not breakfast tables.
Takeaway: keep choosing pasteurised milk, and the rest of the family risk picture is small.