Mar. 13. 2025, scientists who examined the impact of an H5N1 avian flu outbreak in an Ohio dairy herd that had about 3,900 cows found a milk production drop in clinically affected cows that lasted 60 days and extensive asymptomatic infections in other cows. The team, led by researchers at Cornell University, published their findings as a preprint study in Nature Portfolio.

The first illness in the herd was noted about 2 weeks after apparently healthy lactating cows from Texas were introduced into the herd. Decreased rumination and a decline in milk production appeared to decline about 5 days before clinical diagnosis.

The researchers observed clinical disease in about 20% of cows, with milk losses of about 900 kg per cow in the 60-day period that followed the outbreak. They estimated the economic loss at $950 per clinically affected cow.

Seroprevalance findings were positive for nearly 90% of the 637 animals that were on the farm during the clinical phase of the outbreak, suggesting high transmission efficacy. Notably, antibodies were seen in 17 of 42 cows that were in the dry phase, hinting that nonlactating cows can also be the source of the virus.

“Although the precise mechanism of transmission of HPAI H5N1 virus in dairy cattle remains unknown, this is consistent with infections with other influenza A viruses which can quickly spread
through susceptible mammalian populations including in humans, dogs and swine,” they wrote.