Dairy cattle throughout the United States have been infected with the H5N1 virus.Credit: MediaMedium/Alamy
In the southwestern United States in the early weeks of 2024, a large-animal veterinarian pulled up to a dairy farm on his usual rounds and was greeted by an odd absence. The barn cats who normally came trotting out to meet his truck were nowhere in sight.
Over the next few weeks, at dairies across Texas and New Mexico, cows started to get sick. They were losing their appetites and producing less milk than usual — and what little they did produce was thick and gluey. Deaths of birds such as crows and pigeons were also being reported. And then there were the barn cats. They were disappearing or dying suddenly, after becoming blind or unable to walk (see ‘Is a lack of curiosity killing the cats?’).
Is a lack of curiosity killing the cats? The plight of domestic cats proved crucial to identifying the outbreak of H5N1 influenza in US dairy cattle in 2024. Companion animal species are an important part of the One Health approach, which recognizes the interdependency of human, animal and ecosystem health. But there has been little organized surveillance of H5N1 in companion animals anywhere in the world. When H5N1 is concerned, feline friends should be top priority. So far, about 100 domestic cats are known to have died from H5N1 in the United States. Most have been barn cats, those that live mainly outside on agricultural properties, or feral cats. They could have been exposed to the virus in multiple ways — through drinking infected milk, contact with wild birds or contaminated clothing, or potentially transmission from infected humans. Government agencies often lack the authority to track emerging infectious diseases in domestic animals. The US Department of Agriculture is responsible for the surveillance of livestock and poultry to ensure the safety of the food supply, whereas the Food and Drug Administration is responsible for food products, including milk and pet food. Some states have incorporated H5N1 testing into their rabies protocols for cats with neurological disease, but few cats are ever tested for rabies. Veterinarians should be made more aware of the possibility of H5N1 infection in pets in areas where the virus is circulating in birds, researchers say. They also suggest that contact tracing for the virus should include questions about companion animals, and the development of rapid tests for such species.
The bird and cat deaths were consistent with influenza — specifically, a type of highly pathogenic avian influenza (HPAI) known as H5N1 that had been circulating among wild birds and causing outbreaks on poultry farms across the country since early 2022. But H5N1 had never been known to infect cattle.
Nature Spotlight: Influenza
As the weeks went on, dairy veterinarians across the region compared notes, messaging back and forth and organizing conference calls. Affected cows were tested for dozens of likely diseases, but to no avail.
In late March 2024, milk samples from sick cows and tissue samples from barn cats that had died at a dairy in northern Texas were tested for avian influenza at the National Veterinary Diagnostic Laboratory in Ames, Iowa. The test was more to rule out the virus than anything else. “Nothing else was coming up positive,” says Meghan Davis, a veterinary epidemiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. But both species were positive for H5N1.
The results unmasked an outbreak of H5N1 in US dairy cattle that, by September 2025, had affected more than 1,000 herds across 18 states. That birds and cats played a part in the outbreak’s detection highlights the common vulnerabilities and intertwined fates of livestock, companion animals and wildlife species. In fact, the US dairy outbreak is only one element of a global H5N1 outbreak that has caused mass deaths among dozens of wildlife species and killed tens of millions of domestic poultry in the past several years.
Human deaths from H5N1 have been much rarer — the virus killed 12 people, globally, between January and August 2025. But there are fears that further evolution of the virus could spark a human pandemic. Arresting that possibility requires a public-health approach that recognizes the interdependency of human, animal and ecosystem health, known as the One Health perspective. But surveillance often fails to cover a broad enough variety of species in sufficient detail to keep tabs on the virus, researchers say.
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