“It’s called an ataxic gait,” Karen Coleman said, referring to the jerky, staggering walk she often sees in the cows she owns with her husband, Tony. The couple, who run a cattle operation on 315 acres of property owned by Karen’s mother, Patsy Schultz, in Grandview, about a half-hour south of Fort Worth, had been bewildered by the behavior when it first appeared. They never saw obvious signs of injury—the animals just suddenly and mysteriously lost control of their legs. Sometimes the ataxia was preceded by excessive drooling and weight loss and accompanied by other strange behaviors. One day a young steer that had never shown signs of aggression just started charging at Tony; cows would mysteriously lose weight and would look years older than…