State veterinarian: H5N1 spillover into dairy herds is driving new risks for poultry and public health

Denver — Colorado’s state veterinarian warned lawmakers that highly pathogenic avian influenza (H5N1) has shifted in 2024 from episodic wild‑bird spillovers to sustained mammal transmission in dairy cattle, changing the outbreak dynamic and prompting new federal and state controls.

Dr. Steph Baldwin told the Water Resources and Agriculture Review Committee that H5N1 historically traveled seasonally with migratory waterfowl but that recent clusters — first identified in Texas and now seen in Colorado — involve sustained transmission among lactating dairy cattle and have produced very high viral loads in raw milk. Baldwin said diagnostic CT values from affected milk samples have sometimes been in the single digits (very high viral loads). She emphasized that pasteurization is effective at inactivating the virus and that commercial milk remains safe when pasteurized, but that the presence of virus in raw milk and on farms creates acute biosecurity concerns.

Baldwin updated the committee with national and state counts as of July 10: 12 states with dairy detections nationally since March 2024 and roughly 145 dairies testing positive across the outbreak period; Colorado had 27 confirmed positive herds (four released from quarantine), additional presumptive positives pending USDA confirmation, and multiple investigations underway. She said case‑management capacity is strained, citing staff shortages and the need for outside assistance (Wyoming veterinarians were deployed to help case management).

To limit spread, USDA issued a federal order on April 25 and Colorado adopted an emergency rule on April 30 requiring reporting, epidemiologic information from owners, and mandatory testing of lactating dairy cattle moving across state lines; clinically affected or test‑positive animals cannot move for 30 days. Baldwin said within‑herd spread is likely cow‑to‑cow (milking equipment and intramammary transmission) and that farm‑to‑farm spread has been linked to animal movements and fomites (vehicles, equipment, people). She described typical clinical signs as decreased rumination and a marked drop in milk production, with herd impacts from about 10% to more than 30% of animals affected in examples given.

Baldwin also highlighted spillover into other species: farm cats that consume virus‑laden milk puddles have become acutely neurologic and died, and recent dairy‑to‑poultry spillover in Weld County led to depopulation of nearly 1.8 million layer chickens at a commercial operation. She said USDA is sequencing all positives to build epidemiologic links, and that federal funds are available to producers for PPE, biosecurity planning and implementation, heat treatment of waste milk, veterinary costs and some production‑loss assistance through FSA’s emergency livestock program.

The state veterinarian urged producers to increase biosecurity, monitor animals closely for early signs, test lactating animals before movement (within seven days of movement to events), and isolate animals returning from fairs for 30 days. Baldwin stressed close coordination with USDA, CDC, CDPHE and local public‑health partners and said Colorado’s comparatively high reporting reflects proactive outreach and better compliance rather than uniquely higher incidence.

What happens next: Baldwin said USDA sequencing and epidemiology will continue to shape response priorities and that the state is seeking additional case‑management resources. Committee members asked about on‑farm rapid tests, vaccine prospects (complexity because vaccination on the poultry side affects trade), and trade implications. Baldwin said no USDA‑approved on‑farm quick test exists for a reportable disease in Colorado and that vaccination discussions are active but constrained by trade concerns on the poultry side.