What killed Knut? Riddle of polar bear's death solved

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Knut the Polar Bear Returns to Limelight

BERLIN (AP) -- Scientists in Germany say they have finally figured out what killed Knut, the polar bear who was a worldwide celebrity.

Knut drowned four years ago after swelling of his brain caused him to collapse and fall into his enclosure's pool at Berlin Zoo.

READ MORE: National Zoo says smaller of twin panda cubs has died

In an article published Thursday by the journal Nature Scientific Reports, researchers say Knut suffered from anti-NMDA receptor encephalitis. That's an autoimmune disease discovered in humans eight years ago and never previously found in animals.

Alex Greenwood of the Leibniz Institute for Zoo and Wildlife Research says the discovery paves the way for treatment of other animals with the disease.

He says the case may also help raise awareness of what he called "Knut's disease" and ensure early treatment for the one in 200,000 people affected annually.

Photos of the late polar bear, Knut:

14 PHOTOS
World-famous polar bear Knut
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What killed Knut? Riddle of polar bear's death solved
FILE - In this Dec. 5, 2010 file photo, polar bear Knut stands behind his 'birthday cake' to celebrate his 4th birthday in the Zoo of Berlin, Germany. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon March 19, 2011 while alone in his compound. He says the cause is not yet clear. (AP Photo/Michael Sohn, File)
Berlin's famous polar bear Knut reacts at the snow at the zoo in Berlin, Germany, on Friday, Jan. 8, 2010. Germany faces some heavy winter days with much snow and cold temperatures down to minus 15 degrees Celsius (5 degrees Fahrenheit) at night. A snow storm all over Germany is expected for the weekend. (AP Photo/Markus Schreiber)
Eisbaer Knut sitzt am Dienstag, 13. Oktober 2009, in seinem Freilandgehege im Zoo in Berlin. (AP Photo/Franka Bruns) ---Polar bear Knut relaxes in his outdoor enclosure of the Zoo in Berlin, Tuesday, Oct. 13, 2009. (AP Photo/Franka Bruns)
Polar bear Knut is seen as he plays in the water in his enclosure, Friday Sept. 4, 2009, in Berlin. Knut will soon be joined by a female companion from the Munich zoo. (AP Photo/Maya Hitij)
Eisbaer Knut spielt am Mittwoch, 16. September 2009, in seinem Gehege im Zoo in Berlin. Eisbaerin Gianna aus dem Muenchener Zoo ist mittlerweile in Berlin angekommen und soll Knut demnaechst Gesellschaft leisten. (AP Photo/Maya Hitij) --- Polar bear Knut is seen in his enclosure, Wednesday, Sept. 16, 2009 in Berlin, Germany. The female polar bear Gianna from the Munich zoo has arrived in Berlin and is supposed to join Knut soon. (AP Photo/Maya Hitij)
Polar bear Knut is seen as he plays in the water in his enclosure, Friday Sept. 4, 2009, in Berlin. Knut will soon be joined by a female companion from the Munich zoo. (AP Photo/Maya Hitij)
Ein Eisbaer schwimmt am Freitag, 10. April 2009 im Zoologischen Garten in Berlin im Wassergraben seines Geheges. Nach Polizeiangaben war eine Frau am Freitag in das Eisbaerengehege gesprungen und mehrfach von einem der vier Baeren gebissen worden. Bei dem Gehege handelt es sich nicht um das Gehege von Eisbaer Knut. (AP Photo/Gero Breloer) --- A polar bear swims in the moat of his enclousure at Berlin Zoo in Berlin, Friday, April 10, 2009. Police say a woman jumped into the polar bear enclosure and was bitten several times on her arms and legs by one of the four older polar bears in the enclosure. The woman was not bitten by the famous Knut, who took Germany by storm as a cub after he was hand-raised by a keeper. (AP Photo/Gero Breloer)
Polar bear Knut, seen in the snow, in his enclosure, at the zoo in Berlin, Germany, Feb. 17, 2009. (AP Photo/Michael Sohn)
ARCHIV - Eisbaer Knut haelt sich am 4. Dezember 2008, in seinem Gehege in Zoo von Berlin auf. Die Tierrechtsorganisation Peta Deutschland hat die Kastration des beruehmten Berliner Eisbaeren Knut gefordert. Knut und die Eisbaerin Olga, mit der er zurzeit im Berliner Zoo zusammenlebt, haetten "einen gemeinsamen Opa", erklaerte die Organisation am Dienstag, 2. Maerz 2010 in einer Pressemitteilung in Berlin.(AP Photo/Franka Bruns,file ) ---FILE - Polar bear Knut is seen in his outdoor enclosure at the Berlin Zoo, Thursday, Dec. 4, 2008. Knut will turn two years old on Dec. 5, 2008. (AP Photo/Franka Bruns,file)
Polar bear Knut is seen in his outdoor enclosure at the Berlin Zoo, Thursday, Dec. 4, 2008. Knut will turn two years old on Friday, Dec. 5, 2008. (AP Photo/Franka Bruns)
Polar bear Knut lies in his enclosure in Berlin on Tuesday, Sept. 23, 2008. Knut's zookeeper Thomas Doerflein was found dead in his Berlin apartment on Monday, Sept. 22, 2008.(AP Photo/Miguel Villagran)
** FOR USE AS DESIRED WITH YEAR END--FILE **In this Dec. 5, 2008 file photo, polar bear Knut sniffs at his birthday presents in the zoo in Berlin, Germany. Knut the superstar polar bear turned two and looking nothing like the button-eyed ball of white fluff who captured hearts around the world. (AP Photo/Miguel Villagran/FILE)
Eisbaer Knut streckt sich am Freitag, 3. April 2009, im Zoo in Berlin. Das Wetter soll auch in den naechsten Tagen warm und sonnig bleiben. (AP Photo/Franka Bruns) ---Polar bear Kunt stretches in the sun in the Zoo in Berlin, Friday, April 3, 2009. The weather is supposed to stay sunny and warm in the next few days. (AP Photo/Franka Bruns)
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