It appears that there may have been another spillover of H5N1 bird flu virus from wild birds into dairy cattle. The Arizona Department of Agriculture announced Friday that it had found the virus in milk from a herd of cows in Maricopa County, which contains the state capital, Phoenix.

This is the first detection of H5N1 in dairy cows in Arizona, making it the 17th state in which affected cows have been found. Nearly 970 herds have tested positive since the outbreak was first identified in late March 2024.

The Arizona detection occurred as part of the U.S. Department of Agriculture’s National Milk Testing Strategy, which samples bulk milk looking for presence of H5N1 viruses. Last week, USDA and Nevada’s Department of Agriculture announced a detection there of H5N1 in dairy herds. 

Since it was first discovered that bird flu viruses were infecting cows and spreading among herds, it was thought that all of the detections were connected — that there had been a single jump of the H5N1 virus into cows, in either late 2023 or early 2024, likely in Texas. That assessment was based on ongoing analysis of the genetic sequences of the viruses, which belong to a family of H5N1 known as clade 2.3.4.4b, genotype B3.13. 

But the discovery of the virus in milk from Nevada herds in early January — results that were only released last week — showed that a different version of the virus was responsible for those infections. That virus belonged to the same clade, but was of the D1.1 genotype, a version of the virus responsible for a severe infection of a teenager in British Columbia, Canada, in November, and the death of a person who owned a backyard poultry flock in Louisiana in January.

The virus isolated from the milk of the Arizona herd was also a D1.1 virus, but apparently a different version of it.

“This detection of avian influenza is consistent with a D1.1 genotype and unrelated to the recent Nevada detection of this virus,” the Arizona statement said. “This D1.1 genotype bears no features that would make it more likely to infect humans.”

When the Nevada detection was made public, flu scientists warned that more spillovers into cows were likely, given how prevalent H5N1 is in wild birds across the country. But Arizona’s announcement still came as a surprise.

“I definitely thought there would be more jumps found through milk testing. But I have to confess I did not think it would happen quite so fast, nor in my own backyard as I looked north, over Maricopa County, to what’s happening in Nevada!” Michael Worobey, an evolutionary biologist at the University of Arizona, told STAT by text.

Prior to the outbreak in cows, the United States had only ever detected one human H5N1 infection, in an individual who was involved in culling infected poultry in Colorado. That infection occurred in 2022.

But over the past year, there have been 68 confirmed human cases, and a number more where state laboratories have seen a positive result but additional testing done at the Centers for Disease Control and Prevention could not confirm the case, often because of low levels of viral material or degradation of the sample during transport. The majority of these cases have occurred in people with direct contact with dairy cows or poultry flocks, which are also susceptible to contracting the virus from wild birds.