Discovered: Influenza in American Livestock, Specifically Affecting Birds
The D1.1 genotype of the H5N1 bird flu has been confirmed in dairy cows in the United States, marking a significant development in the ongoing battle against the virus. This genotype, which has been spreading among wild birds and poultry in the U.S. and Canada, has been primarily transmitted to cattle through multiple spillover events.
The spread of the D1.1 genotype began with a B3.13 genotype spillover event in late 2023 or early 2024, but it shifted in 2025 to multiple detections of the D1.1 genotype in dairy herds. These detections represent distinct separate introductions from birds to cattle rather than sustained cattle-to-cattle transmission.
Confirmed cases of the D1.1 genotype have been reported in Nevada, Arizona, and California since early 2025. As of mid-2025, over 1,000 dairy cattle herds in 17 states have tested positive for H5N1, including cases of reinfections in previously cleared dairies, particularly in California.
To prevent further spread, key measures have been implemented. These include state quarantines and re-quarantines of infected dairy farms, such as the 43 California dairies recently re-quarantined after virus detection post-clearance. A statewide ban on all poultry and dairy cattle exhibitions at fairs and shows in California has also been enforced since January 2025 to minimize animal movement and congregation.
Regular surveillance and testing of dairy herds using whole-herd and bulk milk tank sampling are also crucial to detect ongoing infections or reinfections. The U.S. Department of Agriculture (USDA) and the California Department of Food and Agriculture (CDFA) have invested in specialized veterinary outbreak response teams and multiple research projects aimed at understanding virus evolution and transmission dynamics for better control strategies.
Given the severe human infections caused by the D1.1 genotype, public health awareness and monitoring are essential. The emergence of the D1.1 genotype in American cattle necessitates heightened surveillance and swift action to safeguard both animal and human populations from the potentially devastating consequences of avian influenza.
As of the latest data, 36 states are enrolled in the USDA's National Milk Testing Strategy (NMTS), covering a significant portion of the nation's commercial milk supply. Michael Osterholm, an epidemiologist at the University of Minnesota and director of the Center for Infectious Disease Research and Policy (CIDRAP), has highlighted the heightened risk of spillover events due to increased activity among waterfowl across the U.S.
Tragically, a person in Louisiana succumbed to a severe case of the D1.1 genotype of H5N1 bird flu. The D1.1 genotype has already been responsible for infecting individuals, including poultry workers in Washington state and a teenager in British Columbia. The detection in Nevada marks the first time this genotype has been found in this livestock population.
In response to this discovery, the USDA's Animal and Plant Health Inspection Service (APHIS) has issued a statement emphasizing the widespread prevalence of the D1.1 genotype in North American flyways during the recent fall and winter seasons. The USDA is collaborating with the Nevada Department of Agriculture to conduct thorough assessments on the affected farm to contain the spread of the virus.
Osterholm stressed the importance of vigilance and proactive measures in combating the persistent threat posed by the bird flu virus. As the situation continues to evolve, it is crucial that all relevant parties remain vigilant and take necessary precautions to protect both human and animal health.
- The detection of the D1.1 genotype of H5N1 bird flu in dairy cattle in the United States not only threatens animal health but also raises concerns for public health, as the genotype has already been responsible for infecting individuals.
- To mitigate the potential impact on both animal and human health, it's essential to implement preventive measures such as state quarantines, regular surveillance, and public health awareness, as highlighted by epidemiologist Michael Osterholm.
