By SHERRY BUNTING
Special for Farmshine
ITHACA, N.Y. — The highly pathogenic avian influenza (HPAI) H5N1 virus has slowed in dairy herds — just three cases in California over the past month. However, a new Cornell University study shows the disease can still be very costly when it hits a dairy herd.
Researchers looked at one Ohio dairy herd — the only confirmed HPAI case in that state — back in 2024. They chose it to study the economic impact of the virus in cows.
This herd had over 3800 adult cows. About 20% of them, 777 cows, showed clear signs of illness over about three weeks. In the two months after the outbreak, those sick cows lost almost 2000 pounds of milk production per cow.
Blood tests found that almost 90% of the herd had the virus, even though many never looked sick. About three-quarters were “subclinical” — infected but without obvious symptoms.
Cows that were sick had six times higher risk of dying and were 3.6 times more likely to be culled early compared to healthy herd mates.
Cornell’s team estimated the losses — from less milk, deaths, and early removals — at $950 per sick cow. For this herd, that added up to roughly $737,500 in losses.
“Our results show that HPAI can hurt production for at least 60 days after diagnosis and cause serious financial losses for dairy farms,” the report stated.
HPAI has been around in wild birds since 1996 and has caused severe outbreaks in poultry ever since. The current strain, H5N1 clade 2.3.4.4b, began in 2022 and has become the costliest animal disease outbreak in U.S. history.
By July 9, 2025, it had killed or led to the culling of over 174.8 million birds. It has also resulted in deaths among 28 species of wild and domestic mammals.
In March 2024, this virus was identified in sick dairy cattle, first in Texas, then spread to other states. The main type found in cows is called genotype B3.13, which spreads from cow to cow. Another type, D1.1, has shown up twice but does not seem to spread between cows.
As of Aug. 12, 2025, there have been 1078 confirmed infected dairy herds in 17 states.
When dairy cows get HPAI, the first signs are often less feed intake and less rumination. Milk production drops sharply, and the milk can look abnormal and similar to colostrum or milk from a cow with mastitis.
Some cows get a fever and mild respiratory signs, such as clear nasal discharge. Death and early culling losses have been higher on some affected farms.
The Cornell report stressed that the impact on milk production can last well beyond the visible sickness, adding to the financial toll on affected dairies.
