Knut, the star polar bear who was hand-reared at Berlin zoo after his mother rejected him, had a type of auto-immune inflation of the brain that is found in humans, scientists said on Thursday.
Knut, who was just four when he drowned at the zoo in 2011, was reared by his keeper Thomas Doerflein. Fans from around the world came to watch them play together, earning the zoo millions and inspiring a dizzying range of merchandise.
Knut had an epileptic fit and drowned in a pool in his enclosure. While a postmortem revealed he had encephalitis, or swollen brain, scientists had remained puzzled by the exact cause of the illness.
Now animal and disease experts have tested samples of Knut's brain for a condition known as anti-NMDA receptor encephalitis, which affects around 200,000 people a year.
They say high concentrations of NMDAR antibodies in Knut's cerebrospinal fluid resolve the puzzle of his death.
"Knut died in 2010 due to encephalitis which is an inflammation of the brain," said Alex Greenwood, head of the department for wildlife diseases at the Leibniz Institute for Zoo and Wildlife Research.
"The inflammation caused him to lose his motor control and he then lost his balance and fell into the water in the enclosure and drowned," Greenwood said.
The results of their findings were published in the journal Scientific Reports on Thursday. It is the first-ever diagnosis of the condition in an animal.
Bears can live much longer than Knut did in captivity and his mother, Tosca, died in June when she was nearly 30.
By the time Knut had reached adolescence, he had generated 5 million euros ($7 million) for Berlin zoo. The German post office produced a stamp in his honor. Other German zoos have tried in vain to create celebrity animals to match his fame.