What happened when bird flu reached US dairy cattle in 2024, and what do the human cases show?

Bird flu reached American dairy cattle in 2024, a surprise because H5N1 was known as a bird virus. The H5N1 variant was first identified in cattle around 25 March 2024. By early December of that year it had been reported in 16 states. The most common version in cows was clade 2.3.4.4b, the same type commonly found in poultry and wild birds.

The human side is smaller and mostly mild. On 2 April 2024 a dairy worker in Texas became infected, which the World Health Organization (WHO) said appeared to be the first human H5N1 infection caught from an infected mammal. In May, a Michigan farm worker with regular exposure to infected cows had mild symptoms and recovered. The US Centers for Disease Control and Prevention (CDC) reported in November that 4 dairy workers had infections without symptoms. By early December, over 700 herds, predominantly in California, had been affected since March, alongside 57 human cases of mild symptoms across the country. A first US death from avian influenza was recorded on 6 January 2025, in a person linked to a backyard flock, not to dairy farms.

What do the cases show? H5N1 can persist on milking equipment, a probable route for spread between cows and to workers. Pasteurization effectively inactivates the virus, and the US Food and Drug Administration (FDA) holds that pasteurized milk is safe to drink. In April 2024, because the virus had not acquired mutations that help it spread between people, the WHO assessed the risk to the general public as low, and as low-to-moderate for people exposed through their work.