The first calls that Dr. Barb Petersen received in early March were from dairy owners worried about crows, pigeons and other birds dying on their Texas farms. Then came word that barn cats — half of them on one farm — had died suddenly.

Within days, the Amarillo veterinarian was hearing about sick cows with unusual symptoms: high fevers, reluctance to eat and much less milk. Tests for typical illnesses came back negative.

Petersen, who monitors more than 40,000 cattle on a dozen farms in the Texas Panhandle, collected samples from cats and cows and sent them to Dr. Drew Magstadt, a friend from college who now works at the veterinary diagnostic laboratory at Iowa State University.

The samples tested positive for a bird flu virus never before seen in cattle. It was the first proof that the bird flu, known as Type A H5N1, could infect cows. As of Wednesday, 36 U.S. herds had confirmed infections, according to the U.S. Agriculture Department.