Bird Flu Basics

Since 2022 a version of Highly Pathogenic Avian Influenza (HPAI, “Bird Flu”) virus has been circulating among migratory water fowl, such ducks, geese, shorebirds. When commercial poultry flocks (broilers, layers, turkeys) become infected, the virus can kill hundreds of thousands of birds in a matter of day. It is for this reason when the disease is detected in a commercial flock it is immediately depopulated. Over 100 million commercial birds in the U.S. have been destroyed so far.

Sometime in late 2023 or early 2024 the virus in migrating birds acquired mutations allowing it to attach and enter cells in the cows’ mammary gland. This “spillover” event occurred in the Texas panhandle and spread across the U.S. from there.

This webpage contains links to resources explaining how the HPAI virus circulates in birds, how we suspect it is transmitted from cow to cow and from herd to herd, how biosecurity may slow or prevent the spread of the disease. 

Questions & Answers About Avian Influenza on California Dairies – This short briefing document covers how we suspect the virus is transmitted from cow to cow and from herd to herd, how biosecurity may slow or prevent the disease and how to keep workers safe.    

CDFA H5N1 Q&A (1-13-2025) – This CDFA Question & Answer briefing also contains numerous links to guidance documents.