What Killed Knut? Researchers are Finally Able to Explain the Polar Bear's Sudden Death

Knut was a beloved polar bear at the Berlin Zoo, who died a little over four years ago from drowning in a pool in his enclosure. While scientists were able to determine he drowned as a result of having epileptic seizures, the cause of this illness remained unknown.

Recently scientists solved this zoological mystery. They discovered that the 4-year-old polar bear had an autoimmune disease known as encephalitis, which causes the brain to swell, according to their study published in Scientific Reports. While Knut is the first animal that this disease has been observed in, symptoms that he experienced are also seen in human patients.

"Until now, this autoimmune disease has only been known in humans. In this illness, the body's immune system overreacts and produces antibodies which damage nerve cells instead of fighting against pathogens," Dr. Harald Prüß, researcher at the Berlin site of the DZNE and a specialist in neurology at the Charité, said in a statement. "Epileptic seizures, hallucinations and dementia are among the possible symptoms."

Dr. Prüß worked alongside Professor Alex Greenwood, leader of the Department of Wildlife Diseases at the Leibniz Institute for Zoo and Wildlife Research (IZW). The two examined Knut's autopsy reports along with a sample of his brain that the IZW had preserved for analysis. They then discovered that the polar bear developed anti-NMDA receptor encephalitis, according to their study. 

"We were quite intrigued by this result," Greenwood said in the statement. "The anti-NMDA receptor encephalitis has been described only very recently in humans. Clearly it is also of importance for other mammals. We are relieved to have finally solved the mystery of Knut's disease, especially as these insights could have practical applications. If the current therapy for human patients is also suitable for wild animals, many cases of fatal encephalitis in zoos may be prevented in future."

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