type: timeline_event
On March 25, 2024, highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b was confirmed in a dairy cow in Texas, marking the first detection of H5N1 in U.S. dairy cattle. The USDA, FDA, and CDC jointly announced this unprecedented development after a multistate investigation into unexplained milk production losses. Veterinarians at Cornell University College of Veterinary Medicine were among the first to sequence the virus using samples from sick cattle in Texas.
The initial outbreak began on March 18, 2024, when researchers were notified of epidemics of illness among Texas dairy cattle showing transient respiratory and gastrointestinal signs. This represented the first reports of HPAI of any subtype causing viral mastitis in lactating dairy cows, introducing a novel transmission pathway for the virus into the human food supply chain.
Immediately following the first detection in the Texas panhandle region, USDA and HHS experts took action to trace animal movements, began sampling to assess disease prevalence in herds, and initiated testing activities to confirm the safety of meat and milk supplies. This initial detection would prove to be the beginning of a nationwide outbreak that would eventually spread to 989 dairy herds across 17 states by early 2025.
The detection in dairy cattle created new public health concerns because of the potential for human exposure through close contact with infected animals and contaminated milk. Within days of the Texas detection, on April 1, 2024, Texas reported the first confirmed human H5N1 infection associated with this outbreak, confirming that the virus could jump from infected dairy cattle to farmworkers.
This event marked a critical turning point in the H5N1 outbreak, transforming what had been primarily a poultry disease since 2022 into a multi-species agricultural crisis affecting the dairy industry. The failure to contain the initial outbreak in Texas dairy cattle would lead to widespread geographic spread, ultimately affecting hundreds of dairy herds across multiple states and creating sustained human exposure risks for agricultural workers throughout 2024 and into 2025.