A newly identified strain of H5N1 bird flu has been confirmed in dairy cattle in Nevada, marking the second known spillover of the virus from wild birds to cows in the United States. This latest discovery underscores growing concerns about the disease’s ability to spread between animals and its potential risk to humans.
Bird flu spillover to cattle occurs for the second time
The US Department of Agriculture (USDA) confirmed that dairy cows in Nevada have been infected with a new strain of bird flu, known as D1.1, separate from the B3.13 variant that has been spreading across US dairy herds since 2023. This new finding suggests that bird flu may have jumped from wild birds to cattle on multiple occasions, rather than stemming from a single spillover event.
This development raises critical questions about how extensively the virus has circulated undetected and the challenges it poses for containment. Dr. Richard Webby, an influenza expert at St. Jude Children’s Research Hospital, noted that a single spillover event was once considered rare but may now be more frequent than previously thought.
The H5N1 virus has already affected over 950 herds across 16 states, with milk samples collected under a USDA surveillance program detecting the new D1.1 strain last week. Experts stress that increased testing is essential for tracking the virus and understanding its spread.
Human infections and potential risks
Public health officials are closely monitoring human cases linked to the virus, particularly among individuals who work directly with infected cattle. At least 67 people in the US have been infected with bird flu, most of whom had close contact with dairy animals.
The D1.1 strain has been linked to two severe human cases in North America:
- A Louisiana resident who died in January after developing severe respiratory symptoms following contact with infected birds.
- A teenager in British Columbia, Canada, who was hospitalised for months after contracting the virus from poultry.
Experts warn that continued mutations could increase the virus’s ability to spread between humans. Michael Worobey, an evolutionary biologist at the University of Arizona, emphasised the importance of sharing genetic data quickly, calling it a “vital part of national and global security.”
Federal officials have announced that genetic sequences of the D1.1 strain will be made publicly available later this week, helping researchers determine whether this spillover event is recent or if the virus has been circulating undetected for months.
