The Journal of Dairy Science has published a comprehensive review of the unprecedented spread of highly pathogenic avian influenza (H5N1) in U.S. dairy cattle, offering new insight into how the virus emerged, how it spreads, and what it means for animal health, public health, and food security.
The outbreak, traced to clade 2.3.4.4b genotypes B3.13 and D1.1, likely began with a single spillover from wild birds into dairy herds, with subsequent rapid cow-to-cow transmission. By February 2025, H5N1 had reached 17 states and nearly 1,000 farms. Within affected herds, up to 25% of cattle showed signs within days—fever, reduced feed intake, decreased milk production, and mastitis. While many recovered, others were culled due to severe illness or long-term production losses.
The virus has also spread beyond cattle, killing cats, peridomestic birds, and wildlife. Human cases, primarily among dairy workers, have presented with conjunctivitis and mild respiratory symptoms. This sustained spread in a mammalian livestock species is rare for avian influenza and raises concerns over mutation potential and pandemic risk.
Food safety investigations have brought some reassurance. Multiple studies confirmed that pasteurization effectively inactivates H5N1 in milk, and no viable virus has been detected in commercial dairy products. USDA testing has found no H5N1 in ground beef from affected states, and cooking meat to at least 145°F eliminates the virus. Poultry and eggs remain low-risk when properly handled and cooked.
Researchers have identified milk as the main route of viral shedding in infected cows, with RNA also found in nasal swabs, urine, and blood early in infection. Initial herd exposure is suspected to occur through contaminated feed or water, though milking equipment and teat contamination may also contribute to spread. Experimental studies suggest that localized mammary infection could be central to cow-to-cow transmission, but the full picture remains incomplete.
Beyond animal and human health concerns, the outbreak has had significant social and economic repercussions. Producers have endured sustained milk yield losses, culling costs, and fractured relationships within the dairy and poultry sectors. Fear of economic and regulatory consequences has made some reluctant to report cases, hampering early detection and control efforts.
The authors emphasize that controlling H5N1 in dairy cattle requires a coordinated One Health approach—bringing together veterinarians, producers, public health agencies, and researchers. Transparent communication, timely data sharing, and targeted surveillance are critical to preventing further spread and protecting the U.S. food supply.
Without unified action, the review warns, H5N1 could remain an ongoing threat to livestock production, public health, and food security in the United States.
