Another Spillover of H5N1 Bird Flu Virus Detected in Dairy Cattle in Arizona
The Arizona Department of Agriculture recently made a concerning announcement regarding the detection of the H5N1 bird flu virus in milk from a herd of dairy cows in Maricopa County, Arizona. This marks the first time H5N1 has been found in dairy cows in Arizona, making it the 17th state in the U.S. to report affected cows since the outbreak began in late March 2024.
The discovery in Arizona was 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. Interestingly, just last week, the USDA and Nevada’s Department of Agriculture also reported a detection of H5N1 in dairy herds in Nevada.
Initially, it was believed that all cases of H5N1 in cows were linked to a single jump of the virus from wild birds, likely occurring in Texas in late 2023 or early 2024. This assumption was based on genetic sequencing analysis of the viruses, which belong to the clade 2.3.4.4b, genotype B3.13.
However, the discovery of a different strain of the virus in milk from Nevada herds revealed a new twist in the situation. The virus isolated from the Nevada herds belonged to the D1.1 genotype, which differs from the previously identified strain. Surprisingly, the virus found in the Arizona dairy cattle was also a D1.1 virus, albeit a distinct variant.
The Arizona Department of Agriculture emphasized that the D1.1 genotype of the virus found in their dairy cows does not possess features that make it more likely to infect humans. This reassurance comes amid concerns about potential spillovers of H5N1 into cows due to its prevalence in wild birds across the country.
Experts in the field had anticipated more instances of the virus jumping to cows, given the widespread presence of H5N1 in wild birds. Nonetheless, the detection in Arizona came as a surprise to many, including Michael Worobey, an evolutionary biologist at the University of Arizona.
Prior to the outbreak in cows, the U.S. had only recorded one human H5N1 infection in 2022, involving an individual who had contact with infected poultry in Colorado. However, in the past year, there have been 69 confirmed cases of human H5N1 infection, mostly linked to direct contact with dairy cows or poultry flocks.
The ongoing spread of the H5N1 virus among dairy cows and the subsequent human infections highlight the importance of continued surveillance and preventive measures to mitigate the risks associated with bird flu outbreaks in livestock. Vigilance and prompt action are crucial in controlling the spread of zoonotic diseases like H5N1.
