A recent incident has emerged involving the H5N1 bird flu virus potentially transferring from wild birds to dairy cattle. On Friday, the Arizona Department of Agriculture reported that the virus was detected in milk from a herd in Maricopa County, which includes Phoenix, the state’s capital.

This marks the first confirmed case of H5N1 in Arizona’s dairy cows, making it the 17th state to report infected cattle. Since the outbreak began in late March 2024, nearly 970 herds across the U.S. have tested positive for the virus.

The identification in Arizona was part of the U.S. Department of Agriculture’s National Milk Testing Strategy, aimed at monitoring bulk milk for the presence of H5N1 virus. Last week, the USDA, alongside Nevada’s Department of Agriculture, reported a similar finding in dairy herds in Nevada.

Initially, it was believed that all H5N1 detections in cattle were linked to a single spillover event most likely occurring in Texas, either late in 2023 or early in 2024. This conclusion was drawn from continuous genetic analysis of the viruses, particularly those of clade 2.3.4.4b, genotype B3.13. Comprehensive studies had indicated this singular origin.

However, findings from January indicating the virus in milk from Nevada herds—results released only last week—revealed a distinct strain responsible for those cases. Although this virus also belongs to the same clade, it is of the D1.1 genotype, which has been associated with a serious infection in a teenager in British Columbia last November and a fatal case involving a Louisiana resident with a backyard poultry flock in January.

The virus found in the Arizona herd’s milk was also a D1.1 variant, but importantly, it appears to be a unique version.

According to the Arizona Department of Agriculture, “This avian influenza detection aligns with a D1.1 genotype and is not related to the recent Nevada finding of the virus.” The statement added, “This specific D1.1 variant shows no characteristics that would increase its likelihood of human infection.”

When the Nevada results were announced, experts cautioned that further spillover events into cattle were likely due to the widespread occurrence of H5N1 among wild birds in the U.S. Nevertheless, the rapid announcement from Arizona was unexpected.

“I anticipated more instances would be uncovered through milk testing,” noted Michael Worobey, an evolutionary biologist at the University of Arizona, in a text to STAT. “However, I must admit that I did not expect it to happen so swiftly, especially so close to home while observing what was occurring in Nevada!”

Before the recent outbreak in cattle, the United States had reported only one human case of H5N1, involving an individual engaged in culling infected poultry in Colorado in 2022.

Over the past year, there have been 69 confirmed human cases of H5N1. Additionally, some cases where local labs indicated positive results were not confirmed by the Centers for Disease Control and Prevention due to low viral load or sample degradation. Most of these human infections were linked to those in direct contact with infected dairy cows or poultry, which can also become infected through exposure to infected wild birds.