AAAP: H5 avian influenza in dairy
The continued evolution of the H5 virus requires constant monitoring
17 November 2025
2 minute read
By: [Excerpts of presentations at the 2025 annual meeting of the American Association of Avian Pathologists]
In March 2024, the world seemed to stand still when H5N1 was detected for the first time in commercial dairy cattle in Texas. A virus primarily associated with the poultry industry, H5N1, was an unexpected threat for the dairy sector and its veterinarians. The weeks following this initial discovery presented a crucial window to contain the virus, but the absence of clear guidance from industry leaders, state, and federal authorities led to a concerning lack of action, noted Kay Russo, RSM Consulting, Fort Collins, Colorado, USA.
This gap in direction left both the dairy and poultry industries struggling to respond effectively to the rapid and unforgiving spread of H5N1 across multiple states and farms. Key questions still need answering to develop comprehensive and actionable strategies for managing H5N1 in cattle and other commercial species moving forward.
California dairy
Blaine Melody, Lander Veterinary Clinic, Turlock, California, USA, said that H5 influenza was introduced to California through the movement of lactating cows in late September 2024. As in other affected states, the virus spread rapidly through vulnerable dairy farms in the region, also impacting nearby poultry operations. Data from other states indicated that dairy cows shed detectable levels of the virus in bulk milk tanks before clinical symptoms appeared.
The detection of the virus in milk tanks provided an opportunity for herd screening, allowing the early identification of infections. This gave the industry a better understanding of how the virus spread both within and between herds. It is important to involve local practitioners in both preventive and control strategies for emerging diseases in production animal species.
H5N1: Human health risks
Close to 1,000 zoonotic infections with the A/goose/Guangdong/1/96-lineage A(H5) viruses have been detected globally with a mortality of over 50%. Despite these numbers, decades of circulation in birds, and sporadic detections of mammalian adaptive markers, the A(H5) viruses have maintained their avian influenza virus features.
Fears of this changing to include more mammalian influenza virus phenotypes were exacerbated with the detection of A(H5N1) viruses in dairy cattle in the USA, said Richard Webby, St. Jude's Children's Research Hospital, Memphis, Tennessee, USA.
Contrary to these fears, however, the ability of the bovine-sourced A(H5N1) viruses to bind to human virus receptors, transmit between ferrets, and evolve antigenically - all markers associated with elevated risk to humans - have not changed even with more than a year of continued circulation in dairy cows.
The occasional spillover of the virus from birds to other mammals has similarly led to limited evidence for mammalian adaptations. While these data are reassuring from a public health perspective, the continued evolution of the A(H5) viruses requires constant assessments and strong collaboration between One-Health partners.
