The Oregon Department of Agriculture confirmed a positive test for highly pathogenic avian influenza at a dairy in Morrow County. The farm has been placed under quarantine, and officials said no other dairies have been identified as affected as of Thursday. Investigators are still working to determine how the cows became infected.

Until now, bird flu's impact in Oregon had been concentrated in commercial and backyard poultry operations. The virus has affected more than one million commercial and backyard chickens in the state since 2022, according to the U.S. Department of Agriculture.

The detection was made through Oregon's statewide milk surveillance program, which began weekly testing in late 2024. Dr. Ryan Scholz, the state veterinarian for the Oregon Department of Agriculture, said the program performed as designed.

"Oregon's milk surveillance program worked exactly as intended, identifying this infection quickly and allowing us to respond rapidly with our state and federal partners," Scholz said in a statement.

Scholz said the Oregon case is significant but not unexpected, noting that the virus is spreading among dairy cattle across the country. Bird flu was first detected in dairy cattle in March 2024 and has since appeared in multiple states, including Washington, Idaho and California.

Washington has seen rapid transmission in recent weeks, with at least 41 dairy farms testing positive over the past 30 days. Earlier this month, a farmworker in Yakima County died after contracting the virus. Washington health officials said the worker had underlying health conditions. Two other Yakima farmworkers also contracted the virus but were not hospitalized and did not experience severe illness. Human deaths linked to bird flu remain very rare.

Oregon health officials did not immediately disclose how many people were exposed to the sick cows in Morrow County. But they said farmworkers exposed to the infected animals are being monitored for symptoms.

Dr. Melissa Sutton, medical director of respiratory viral pathogens at the Oregon Health Authority, said the risk to workers remains low with proper precautions.

"With the use of appropriate personal protective equipment, the risk to farm workers remains low," Sutton said in a statement. "Our public health approach is focused on ensuring rapid testing and treatment for any individuals who develop symptoms to prevent severe disease."

Federal officials maintain that the nation's milk supply and dairy products remain safe to consume because pasteurization effectively inactivates the virus. Milk from sick animals is diverted or destroyed before it can enter the food supply. The Centers for Disease Control and Prevention says the current public health risk to the general population is low, though people in close contact with dairy cattle or poultry face higher risk.

The virus affecting dairy cattle appears to spread mainly from cow to cow, according to federal animal health officials, unlike in poultry operations where birds typically contract the virus from wild migratory waterfowl. Avian flu can also jump to other animals, including wild mammals. In Oregon, the virus has been detected in at least one pig at a noncommercial farm, in skunks, raccoons and rodents, and in at least five cats last year.