On Capitol Hill. Republican and Democratic Congressional leaders yesterday accused the Agriculture Department of misleading the public about a central fact in the nation’s first known case of mad cow disease. For the past two months the department has claimed that the animal that had tested positive for mad cow disease was a downer which meant it was sick and could not walk. But an investigation by the House Committee on Government Reform found that not to be true. The Committee interviewed three eyewitnesses to the slaughter of the cow who said the cow did not appear sick at all. In a letter to Agriculture Secretary Ann M. Veneman, Congressional leaders wrote that this “could have serious implications for both the adequacy of the national [mad cow] surveillance system and the credibility of the USDA.”