Overview
- The H5N1 outbreak that began in March 2024 has swept through more than 1,000 U.S. dairy herds and prompted a national milk‑testing and herd‑movement response to control spread.
- A Nature Communications study found that as few as 10 H5N1 viral particles can produce productive infection in cows and that the virus replicates in mammary tissue, creating high‑titer infectious milk for more than a week.
- Controlled transmission tests using contaminated milking equipment, bottle‑feeding of infected milk, and intranasal exposure did not reliably infect healthy animals, leaving the main cow‑to‑cow transmission route unresolved.
- Federal authorities say pasteurization inactivates the virus in milk and the CDC assesses public risk as low, but 71 human cases tied to the cattle epizootic, including two deaths, have been reported.
- Experts are urging USDA and industry support to develop and deploy cattle H5N1 vaccines, expand surveillance and diagnostics, and address regulatory, cost and facility barriers to reduce future spillover risk.