200 dead cows were discovered an a farm near Stockton, Wisconsin on Friday. According to the local Sheriff's Dept., the cows suffered from IBR - infectious bovine rhinotracheitis, a highly contagious cow disease.
As Whitley's Place points out, this is the first case of larger animals being affected by the phenomenon of sudden, mass animal deaths.
Beef and dairy farmers usually vaccinate their cows against IBR. What happened in this case is still unclear. According to the Dept. of Animal Sciences, IBR can persist in clinically recovered animals for years, and remain inactive until the animal is placed under stress. The Department's website about the virus also claims that there is no definitive clinical diagnosis for IBR. Apparently this has not prevented authorities from promptly announcing the cause.