DIANE SAWYER (ABC NEWS)(OC): And now, mad cow disease. Last night, we told you the news about the first animal with mad cow disease in the United States in six years. And tonight, we have the latest on the investigation of the farm where the cow got sick. ABC's senior national correspondent Jim Avila with the details.
JIM AVILA (ABC NEWS)(VO): The USDA, the meat industry, and even their critics believe the chances of mad cow infecting the American beef consumer today are small. In fact, it's never happened in the United States. But, diary farmers at ground zero of this latest outbreak, California's central valley are panicked that demand and prices will drop.
RALPH MENDOZA (DAIRY MANAGER): The demand on beef, with all the media coverage, is gonna take an effect and people are gonna hold back.
JIM AVILA (ABC NEWS)(VO): The heat is on here, USDA inspectors today scouring an undisclosed dairy farm near Hanford, California, deciding if it must kill any calves delivered by the mad cow and destroy what the inspectors call her cohorts, cows that were born in the same year and same place.
RICHARD RHODES (AUTHOR OF "DEADLY FEASTS"): Indeed, there may be other animals in the herd that are carrying the disease but haven't show symptoms yet.
JIM AVILA (ABC NEWS)(VO): The five-year-old mad cow was already dead when it was delivered to this California rendering plant, where it failed a random brain matter test. No visible symptoms, the stumbling and nervousness that give the disease its name. And that is bothersome. It means other diary cows could have the disease, not shown symptoms and been sold to a hamburger packer without ever being tested.
JIM AVILA (ABC NEWS)(VO): The USDA says its 40,000 random tests a year, less than 1% of the cows slaughtered are enough. And even if a diseased cow somehow slipped through, guidelines put in place since the British outbreak that killed 175 people prohibit slaughterhouses here from using brain or spinal material that carry the disease to humans.
DOCTOR JOHN CLIFFORD (USDA): Through the inspection of our meat and removal of risk materials, our food is safe.
JIM AVILA (ABC NEWS)(OC): Safe, that is, if the USDA rules are followed. And sometimes they are not. In 2008, a California meat packer the largest meat recall in history after it was caught slaughtering sick animals prone to mad cow. And in 200g, a watchdog group revealed that 131 plants in 36 states violated USDA rules on handling that high risk spinal material. Critics say there are still plenty of questions to be answered about this latest incident in California. Diane?