Revealed: Polar bear Knut had encephalitis, died from drowning

She said it was unclear specifically what caused the brain swelling, but that experts had ruled out rabies, botulism and bovine spongiform encephalopathy, also known as mad cow disease.

Szentiks also rejected allegations from animal-rights groups that he was traumatized by living in a zoo environment, saying that there were no indications of any chronic stress.