February 18, 2025

 

Arizona, US, confirms virus in milk from cows in Maricopa County

 

 

The Department of Agriculture of Arizona, the United States, announced on February 14 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 US 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 US Department of Agriculture's (USDA) National Milk Testing Strategy, which samples bulk milk looking for presence of H5N1 viruses.

 

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.

 

USDA milk testing shows different strain of H5N1 bird flu in Nevada dairy herds

But the discovery of the virus in milk from Nevada herds in early January — results that were only released recently — 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.

 

- STAT