It’s been nearly 18 months since the H5N1 bird flu virus was first detected in the US dairy herd. The disease affects mostly lactating cows, leading to a sharp decrease in milk production and animal welfare. California, the US’ biggest milk-producing state, experienced the worst of the outbreak, with 75% of its dairies affected affected.
Nearly a year and a half since the first detection, the number of reported cases in the US dairy herd has fallen, but the outbreak’s impact on farmers’ bottom line is only starting to become apparent.
For one, the economic losses have been ‘striking’, according to new Cornell University research.
This study focused on a single Ohio-based herd, but even with that caveat, the herd was the typical total-mixed-ration-fed, free-stall type prevalent in US dairy, making the findings relevant to the wider industry.
The research found that milk losses caused by H5N1-induced mastitis were more than 50 times greater than those resulting from typical bacterial mastitis.
Bird flu-hit milk cows produced around 945kg less milk during the outbreak (a 67-day period). Meanwhile, a cow suffering from bacterial mastitis would produce up to 18kg less milk and recover quicker in comparison.
Not only did milk production decrease significantly, but this loss was also swift – clinically diagnosed animals produced nearly 73% less milk within two weeks of the virus being reported in the herd; with daily values dropping from around 35kg per day to just over 10kg.
The study also estimated that the Ohio farm operator lost around $950 per cow – i.e. about $737,500 for the entire 776-head herd. (The calculations were based on a 67-day observation period and assumed a milk price of $21.50 per 100 lbs (45.4 kg) and a replacement cow cost of $3,000.)
The estimate includes mortality-related costs of around $166; milk losses of around $222, and the cost of early herd removal and/or replacement, to the tune of $448.
Monitoring key to spotting subclinical infections
But there is a silver lining – farms that utilize monitoring systems may be better equipped to spot early signs of the virus circulating in the herd undetected.
This is because both rumination time and milk production start to drop in the run-up to the first clinical symptoms.
“[W]hen examining adjusted individual animal means for rumination time (min/day) and milk production (kg/day) in relation to when each animal was diagnosed with clinical influenza by farm personnel, we observed that both parameters begin to decline ~5 days before clinical diagnosis,” the researchers explained.
“Therefore, farms utilizing monitoring systems should closely track individual cow rumination times and milk production, as decreases in these parameters can serve as early warning indicators of influenza A H5N1 virus introduction into the herd.”
Generally, the study found that rumination time starts decreasing around 7 days prior to clinical diagnosis, returning to pre-outbreak lengths within 14 days.
Long-term milk production drop
The same cannot be said about milk production, however – where even animals that had fully recovered from the disease struggled to hit pre-outbreak levels.
On a typical day, milk cows in the Ohio herd would produce around 35-36kg of milk. But those that became infected started to produce less in the five days before they were diagnosed – dropping to around 11kg two days after diagnosis. And two weeks later, production had only climbed to between 20.8kg and 24kg per day – remaining significantly lower when compared to what those milk cows had yielded before becoming sick.
“The abrupt and long-term drop in milk production could be a direct result of the virus replication in milk-secreting epithelial cells in the mammary gland, which results in necrosis and destruction of these cells,” the Cornell researchers explain in the paper.
“Future studies, evaluating milk production in subsequent lactations in affected cows, will be critical to determine whether regeneration of the mammary gland epithelium that occurs during the dry period is sufficient to re-establish pre-infection milk yields in clinically affected H5N1 cows.”
How does bird flu spread on dairy farms?
USDA identified several potential risk factors for the introduction and spread of HPAI H5N1 B3.13 in dairy herds, including cattle movement, shared equipment and vehicles, presence of wild and domestic animals, and frequent contact with external personnel such as veterinarians and haulers.
Transmission within farms is primarily driven by contaminated milk and milking procedures, while farm-to-farm spread is mainly linked to cattle movement and shared equipment.
Certain farm management practices – like proper waste handling and wildlife deterrence – may reduce risk.
How is the US government helping dairy farms?
Since July 1, 2024, US dairy producers have been able to claim financial aid for milk losses, testing or containment measures as part of the Emergency Assistance for Livestock, Honey Bees, and Farm-Raised Fish Program (ELAP) administered by the USDA.
Non-profit Farm Forward, through a Freedom of Information request, found that the federal government has spent at least $80m in relief payments by January 2025 alone.
Large-scale producers from major dairy-producing regions like California and Colorado received the largest payments (some being of nearly $1m), with Colorado-based Prado Dairy awarded more than $1.5m.
And 43% of payments went to dairies that were reimbursed multiple times, some receiving up to five payments in a six-month period; with the likes of Meadowvale Dairy LLC, Wolf Creek Dairy LLC, Sierra View Dairy, 4K Dairy Family Partnership, and Parreira-Gaspar Dairy collectively receiving nearly $6m in payments.
The USDA payments are based on a per cow milk production losses over a four-week period; with repeat payments suggesting re-infections or stubborn positive tests, underlining challenges with prevention and detection of the disease on farms.
EU keeps a watchful eye
Over in Europe, the European Food Safety Authority (EFSA) is assessing how HPAI A/H5N1 could spread to the EU.
In the US, the virus is believed to have jumped to dairy cattle from wild birds; it has also infected humans (e.g. farm workers who had been in close contact with sickened animals). But no human-to-human transmission has been reported so far, with the CDC maintaining that risk to the public remains low.
The virus that affects dairy cattle has also remained confined within North America, but the EU is taking precautions, including bolstering monitoring of migratory birds and considering ways the virus could enter the bloc via trade.
In a report released in July 2025, EFSA concluded that migratory birds pose the biggest threat, with trade a lesser concern.
The regulator reported that there is some risk of the virus to be introduced in Europe via US raw milk or non heat-treated dairy imports, and all but ruled out the likelihood of transmission via meat or poultry imports.
These EU concerns could become more relevant in the near future, however, as the bloc and the US iron out barriers affecting trade in food and agricultural products, including streamlining requirements for sanitary certificates for US pork and dairy products.
For now, migratory birds are the ones to watch. According to EFSA, if the H5N1 virus is to spread to Europe, this would likely happen during the migration season (April to May) and the non-breeding season (September-April).
And depending on the migratory route, infected birds could fly in through Iceland, Scandinavia, Eastern Europe and the Mediterranean, says the body.
Monitoring would be key for prevention, with the body recommending early detection efforts to prioritize areas with the biggest concentration of such birds, e.g. Iceland, Britain, Ireland, western Scandinavia and large wetlands like the Wadden Sea.
Why is EFSA taking these measures? The regulator says that the virus has infected a wide range of species, from birds to cattle and humans. It has mutated and adapted in the process, suggesting there’s scope for it to evolve enough to become even more transmissible.
EFSA’s report says that “mutations of HPAI A(H5N1) B3.13 virus associated with mammalian adaptation (e.g. PB2 E627K) have been detected in both human and cattle samples, underscoring the need for strict biosecurity and protective measures for farm workers”.
By the end of 2025, the EU regulator will assess the potential impact of this HPAI genotype entering Europe and recommend measures to prevent its spread.
Source:
Peña-Mosca, F., Frye, E.A., MacLachlan, M.J. et al. The impact of highly pathogenic avian influenza H5N1 virus infection on dairy cows. Nat Commun 16, 6520 (2025). https://doi.org/10.1038/s41467-025-61553-z