Another Spillover of H5N1 Bird Flu Virus Detected in Arizona Dairy Cows
In a concerning development, the Arizona Department of Agriculture has reported another spillover of the H5N1 bird flu virus, this time affecting a herd of dairy cows in Maricopa County. This marks the first detection of H5N1 in dairy cows in Arizona, making it the 17th state to be affected by this outbreak. Nearly 970 herds have tested positive for the virus since the initial identification of the outbreak in late March 2024.
The detection in Arizona came as part of the U.S. Department of Agriculture’s National Milk Testing Strategy, which aims to identify the presence of H5N1 viruses in bulk milk samples. Just last week, the USDA and Nevada’s Department of Agriculture announced a similar detection of the virus in dairy herds in Nevada.
Initially, it was believed that all the detections of the H5N1 virus in cows were connected, stemming from a single jump of the virus into cows in late 2023 or early 2024, most likely originating in Texas. This assessment was based on genetic sequencing analysis of the viruses, which belong to the clade 2.3.4.4b, genotype B3.13.
However, the recent discovery of a different strain of the virus in milk samples from Nevada herds revealed a new version of the virus, belonging to the D1.1 genotype. This strain has been associated with severe infections in humans, including cases in British Columbia, Canada, and Louisiana.
The virus isolated from the Arizona dairy cows also belonged to the D1.1 genotype, but it was a distinct version of the virus. According to the Arizona Department of Agriculture, this strain does not possess features that make it more likely to infect humans.
Experts had warned that more spillovers into cows were likely, given the widespread presence of H5N1 in wild birds across the country. Still, the announcement of the virus in Arizona came as a surprise to many.
Michael Worobey, an evolutionary biologist at the University of Arizona, expressed his surprise at the rapid spread of the virus, stating that he had expected more jumps to be found through milk testing but did not anticipate it happening so quickly.
Prior to the outbreak in cows, the U.S. had only detected one human H5N1 infection in 2022. However, over the past year, there have been 69 confirmed human cases, with many more suspected cases that could not be confirmed due to low levels of viral material or sample degradation. The majority of these cases have occurred in individuals with direct contact with dairy cows or poultry flocks.
The detection of the H5N1 virus in dairy cows highlights the urgent need for continued surveillance and monitoring of both wild bird populations and domestic livestock to prevent further spread of the virus and protect public health.
