The first case of bird flu in cattle was reported on March 25, 2024. In less than a year, the virus has hit 973 herds, according to the U.S. Centers for Disease Control and Prevention.
The United States Department of Agriculture (USDA) first issued an order to test cows that farmers intended to move between states last April. Then, in December 2024, the agency issued a federal order for milk testing. The order stipulated that unpasteurized milk samples be collected from dairy processing facilities nationwide and tested, with the results being shared with the USDA.
Since then, the virus has been detected in 17 states: 747 herds in California, 64 in Colorado, 35 in Idaho, 31 in Michigan, 27 in Texas, 13 each in Iowa and Utah, nine each in Minnesota and New Mexico, seven each in Nevada and South Dakota, four in Kansas, two in Oklahoma, and one each in Arizona, North Carolina, Ohio, and Wyoming.
Monitoring the Spread of the Virus
Complicating matters, it is unclear how many CDC employees are still employed to monitor the spread of the virus and how many USDA workers are still on the job to conduct the milk testing orders.
The good news is, that, for cows, the disease is seldom deadly (or as public health officials say, highly pathogenic). The bad news is that there is now more than one form circulating among dairy herds. Until early 2025, only one genotype showed up in milk. Since then, there has been another.
Many farmers and scientists have remained unsure about just how the virus spreads: direct contact with milk, eggs, or meat? Is it in animal excrement? Droplets from sneezes?
If any or all of those transmission methods were effective, strategies like sequestering farm animals could, in theory, slow the spread. But a new study, if true, is a bearer of bad news. A preprint biology paper suggests that the virus could be spread by the wind. If that theory holds up to scrutiny, sequestration strategies will be less effective.
Read More: Humans Remain at Low Risk to Spread of the Latest Strain of Bird Flu
Human-to-Human Transmission?
The virus has been spreading in humans as well — but not as rapidly as in poultry or dairy cows, according to the American Veterinary Medical Association. So far, 69 people in the U.S. have tested positive for the virus. Of those, 23 cases involved poultry farm workers, and 41 involved dairy farm workers.
Since the outbreak in dairy cattle began, infection with avian influenza virus type A (H5) has been confirmed in 69 people in the United States. Twenty-three of these cases involved poultry farm workers exposed to infected poultry in Washington (11 cases), Colorado (9 cases) and Iowa, Oregon, and Wisconsin (1 case each). Forty-one other cases involved dairy farm workers exposed to sick or infected cows — 36 in California, two in Michigan, one each in Colorado, Nevada, and Texas.
The virus has also been detected in other animals, including at least one domestic cat, two pigs, and two ferrets. The CDC continues to maintain that, thus far, there has been no signs of human-to-human transmission.
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
