California Department of Food & Agriculture reports H5N1 surge in poultry and significant dairy detections; officials describe surveillance, testing and control

State agricultural officials told the Veterinary Medical Board that the Eurasian strain of avian influenza (H5N1) has produced the largest outbreak in California history and is behaving differently in dairy cattle than in poultry, prompting ongoing surveillance, targeted quarantine and research.

CDFA State Veterinarian staff said the virus first appeared in California dairy herds in late summer 2024 and that, as of their update, hundreds of dairies had tested positive or presumptively positive. Many infected herds are under state quarantine that limits animal movement and requires biosecurity. The department reported that some dairies have cleared quarantine after three consecutive weeks of negative bulk‑tank tests, and others remain in multi‑week testing protocols.

Poultry impacts in recent months were described as dramatic: CDFA presented figures showing more than 10 million commercial poultry (layers, broilers, turkeys and ducks) lost in the current surge and earlier waves combined. The department said the latest wave has produced higher weekly detections than prior waves and that whole‑genome sequencing in some poultry cases suggests spillover from infected cattle herds as well as the usual introductions…