Necropsy results show that Knut, the world's most famous polar bear from the Berlin zoo, drowned when brain swelling caused him to collapse, zoo director Bernhard Blaszkiewitz said on Friday.
"The cause of Knut's death was viral infection, which led to brain inflammation," he said.
The nature of the infection, which severely damaged the animal's brain and spinal cord, is yet to be established. Experts have already ruled out rabies, botulism and a prion infection.
Knut carried the disease for several months, and it had developed to a stage when it would have killed the animal sooner or later.
Knut, the first bear born in the Berlin Zoo in 30 years, died unexpectedly on March 19 at the age of four after falling into the water in his compound. Drowning was declared the immediate cause of his death.
Male polar bears normally live to their mid-30s in zoos and between 15 and 18 years in the wild.
Neurologist Christian Elger of the University of Bonn earlier said that Knut's convulsionary movements, which preceded his collapse, were symptoms of an epileptic seizure.
Knut was born in December 2006 and his mother rejected the cub shortly after his birth. Knut immediately captured the international community's heart and became a world celebrity - a popular brand, a symbol of the fight against global warming and a "poster-cub" for the polar bear campaign.
BERLIN, April 1 (RIA Novosti)
