Investigators are trying to identify offspring and herd mates born within a year of the infected cow's birth.
The U.S. Department of Agriculture (USDA) says the infected animal was a 12-year-old Brahma cross beef cow who had spent its life at the ranch, which the government has not identified.
The USDA says the cow was sold through a livestock market in November, but was dead on arrival four days later at a slaughterhouse. The animal was not used for food, and its brain tissue was removed for testing.
The rapid test that was conducted on the infected cow indicated the presence of the disease, but results from a different test were negative, and the department announced the animal was free of the brain-wasting disease.
Last month, the USDA's internal watchdog ordered a third type of test that came back positive, and a laboratory in England confirmed the results last month - it was the second case of mad cow disease in the United States.
The new case prompted a change in the government's testing policy. Now, both additional tests will be done if rapid tests indicate a sample is suspect.
Meanwhile, U.S. Treasury Secretary John Snow says he expects the American border to be reopened soon to Canadian cattle imports.
Imports from Canada have been banned for more than two years after mad cow disease was found in an infected Canadian cow.
Both governments agree enough safeguards are now in place to reopen the border. However, a lower court ruling that is delaying the border from opening back up must first be overturned.
That appeal will be heard July 13, and Canadian officials say the ruling will be followed "very, very closely" by the Canadian government.
Canada's beef industry has already suffered more than $5.6 billion in export losses.
