4 Years Later, Mystery of Knut's Death Solved

The sudden death four years ago of Knut, the celebrity polar bear, shocked his fans around the world and posed a riddle for veterinarians. The 4-year-old bear died in March 2011 after suffering an apparent seizure and collapsing into his enclosure's pool in front of hundreds of visitors at the Berlin Zoo. His short life came as a surprise—polar bears can live for up to 20 years in the wild and sometimes longer in captivity. A necropsy quickly established that Knut suffered from encephalitis, but the cause was unclear. Now researchers in Germany say they know what killed Knut—and the discovery could help save human lives. Alex Greenwood, who worked on much of the initial research into Knut's death, says that by early 2014 his team had shelved its work, figuring it might take decades to figure out why Knut died.

Then they got a call from neurologist Harald Pruess, who said he noticed that Knut's case showed similarities to some of his human patients who suffered from anti-NMDA receptor encephalitis. The autoimmune disease, in which the body attacks its own brain cells, was only discovered in humans eight years ago and never previously found in animals. "It was a bit of a long shot, but after six or eight weeks we saw that it really was that," Greenwood says. Had Knut's keepers known what their star attraction was suffering from, he likely could have been treated, adds Greenwood; humans with the condition are given cortisone. Pruess says Knut's case may help raise awareness of what is still a relatively unknown illness in humans. The disease, which affects at least one in 200,000 people each year and often involves sudden behavioral changes, can be detected with a simple procedure. (More Knut stories.)