BERLIN — Knut, a polar bear that became a global celebrity then mysteriously drowned at age 4 in his Berlin Zoo pen, died of a rare autoimmune disease, a scientist revealed Thursday.

The finding solves a mystery that has lingered since Knut’s sudden death in 2011. Knut, who had been rejected by his mother and hand-reared by a zookeeper, was the global face of 2007 with his fluffy fur and toddler antics.

Hand-reared as a cub by a zookeeper, Knut was a popular attraction at Berlin Zoo. He drowned in 2011. The Associated Press

Eleven million people visited the zoo to see him and tens of millions of people worldwide followed him on the web, though interest waned as Knut grew up into a moody adolescent bear who found female bears bothersome.

A German neuroscientist, Harald Pruess, said Knut was felled by anti-NMDA receptor encephalitis, an illness especially hitting children and women that was only discovered in the last decade and is now proven by Knut’s demise to affect animals too.

Though Knut was known to have succumbed to a form of encephalitis when he was found dead in his pool, many thought it had been caused by a viral or bacterial disease.

But Pruess, an expert behind a 2010 breakthrough in Berlin that linked many unsolved human deaths to the disease, said it is caused by antibodies. In Knut’s case, it triggered an epileptic fit and the cub tumbled into the water and drowned.

In effect, the body’s own system for fighting disease becomes over-excited and damages healthy nervous tissue in the brain. In humans, fits, hallucinations and dementia occur, but many people recover.

Pruess and colleagues published their findings in the U.S. journal Scientific Reports. Knut was stuffed and went on show in a Berlin museum last year.

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