Because of the cow's age, it's most likely that she became infected with mad cow disease by eating contaminated feed before the 1997 ban on feed that contains cattle parts.

Investigators used DNA testing to confirm the source herd where the cow is believed to have spent most of her life. Officials declined to release more details on the location or size of the herd on the ranch involved, but the herd has been "put under a hold order" so US Department of Agriculture officials can find "more animals of interest," said John Clifford, chief veterinarian at the USDA's Animal and Plant Health Inspection Service.

Clifford said DNA tests had identified two animals in the herd with direct relationships to the diseased cow as either a dam or offspring. The USDA now wants to find any animals from the herd born within two years of the cow's birth, as well as find the cow's last two offspring.

Contaminated Feed Most Likely

When an animal tested positive for mad cow disease in Washington state in December 2003, the USDA euthanized more than 200 animals as it traced the animal's history.

Because of the cow's age, Clifford said it's most likely that the cow became infected with mad cow disease by eating contaminated feed before the 1997 ban on feed that contains cattle parts.

Eating contaminated feed is how the disease is transmitted.

Texas is the nation's largest cattle state with 13.8 million animals, compared with about 6.35 million cattle in Nebraska, the nation's third-largest cattle state.

Brain-Wasting Disease

US Secretary of Agriculture Mike Johanns announced last Friday that the cow had tested positive for mad cow disease at a lab in Weybridge, England. Mad cow disease, formally called bovine spongiform encephalopathy, is a brain-wasting disease that is linked to a human disease known to have killed about 150 people, mostly in England.

The Texas cow was unable to walk and was in poor health when it was initially tested for mad cow disease after arriving at a pet-food processor in Waco, Texas, last November. A rapid test at the rendering plant came up with an "inconclusive" test result. Initial confirmatory tests at the National Veterinary Services Laboratory in Ames, Iowa, ruled that the cow did not have mad cow disease.

Steve Sundlof, a veterinarian for the US Food and Drug Administration, said his agency will be tracing the history of feed and compliance records for firms that may have processed meat and bone meal from the cow's cohorts.

Sundlof said the FDA would not be able to track feed records at facilities before the feed ban went into effect.

Did Not Enter the Food Supply

Clifford noted that the animal was incinerated and did not enter the food supply. While Taiwan and Indonesia have renewed their bans on US beef because of the positive test, Clifford said he hoped no one would seek to ban Texas beef, saying such a ban would not be scientifically sound.

An "experimental test" on the animal last November in Ames showed a positive result as well, but those test results were not reported.

The situation with the contradictory test results caused Johanns to announce new testing procedures at the USDA testing lab in Ames, Iowa. The Ames lab will now use the Western blot to confirm test results and USDA lab scientists will have their work reviewed by scientists in England, who have more experience testing cattle for mad cow disease.