Three cows declared infected with mad cow disease in preliminary testing in Tokyo and Aichi prefectures tested negative for the debilitating disease Saturday after secondary checks were conducted, government officials said.

Samples from the three cows -- two in Tokyo and one in Aichi -- were analyzed at designated national labs with greater precision after being found tentatively positive earlier this week in local examinations.

But experts said the initial test is so sensitive that positive results can occur even in samples free of the disease, which is thought to cause a new variant of the fatal human disorder Creutzfeldt-Jakob disease.