A longitudinal study of influenza A viral detection in bulk tank and pen-level milk collected from dairy farms in California affected by Highly Pathogenic Avian Influenza H5N1

were seen 1-3 weeks after initial detection.In PLMtested herds, virus was detected in samples from all pens of cattle soon after BTM detection, and in some cases was seen in all pens on the first day of PLM sampling.Overall, these results support BTM testing as an effective early detection tool for dairy H5N1 outbreaks but suggest that surveillance strategies that do not test aggregate milk from all lactating cows on an operation, or have a long interval between BTM testing days, may fail to identify all affected herds.Future investigations should study farm-level factors associated with long-term viral detection in some herds and assess detection sensitivities of the IAV rRT-PCR assay used on aggregate milk samples.