A new study sheds light on how bird flu is hitting dairy farms — and it’s worse than expected.
A recent study published in Nature Communications reveals the serious impact of highly pathogenic avian influenza (HPAI) H5N1 virus on U.S. dairy cows, highlighting its toll on cow health, milk production, and farm operations.
What Happened?
In the spring of 2024, an Ohio dairy farm with nearly 3,900 cows experienced an outbreak of H5N1, traced back to newly arrived cows from Texas. In just a few weeks, 20% of the herd (777 cows) showed signs of illness — including reduced appetite, fever, and dramatic drops in milk output. The infection primarily caused a severe form of mastitis (inflammation of the udder), damaging milk-secreting cells in the mammary gland.
Key Findings in Simple Terms
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High infection rates: 89% of the herd showed evidence of exposure to the virus. Most had no obvious signs but were still infected.
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Visible sickness: 777 cows got clinically sick. These cows faced a 6-times higher risk of dying and were 3.6 times more likely to be culled early compared to healthy herd mates.
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Milk loss: Affected cows lost an average of 900 kg (nearly 2,000 lbs) of milk per cow over a 60-day period.
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Rumination down: Infected cows ruminated (chewed their cud) 160 minutes less per day at peak illness — a clear sign of poor health and feed intake.
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Early warning signs: Milk production and rumination time started declining 5 to 7 days before visible illness appeared, suggesting these could be early warning signals for farmers.
Who Was Most at Risk?
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Older cows and those in mid-to-late stages of lactation were more likely to become sick.
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Parity and days in milk (DIM) were important risk factors. Cows with more than one lactation and those over 100 days in milk were more susceptible.
Long-Term Effects
Even after the outbreak was under control, milk production in affected cows remained lower for at least 77 days. Losses ranged from 8 to 14 kg of milk per cow per day compared to their healthier counterparts.
Why It Matters
This is the first large-scale, data-driven look at how the H5N1 virus affects dairy cows. Unlike poultry, where the disease causes swift and visible death, dairy cows often experience a slower, production-based impact — making the virus harder to detect early and potentially more damaging over time.
Takeaway for Dairy Producers
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Watch for early signs like reduced rumination and milk yield.
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Pay attention to cows in later stages of lactation and those with multiple calvings.
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Isolation and monitoring of new animals is critical, especially those arriving from regions with known HPAI activity.
As the H5N1 virus continues to spread, staying ahead with biosecurity, herd monitoring, and rapid response will be key to protecting both animal welfare and milk production.
