A cause of death for the world-famous German polar bear Knut has been discovered: an autoimmune disease of the brain, which caused anti-NMDA receptor encephalitis (inflammation of the brain). It is the first recorded case of the disease in a non-human animal. The research is published today in the journal Scientific Reports.

Knut drowned on March 19, 2011, after suffering an epileptic seizure and falling into a pool in his enclosure. The polar bear, raised at the Berlin Zoological Garden after being rejected by his mother at the zoo at birth, had an estimated 15 million visitors in his almost 4.5-year lifetime. The exact cause of his dramatic and tragic death had remained a mystery until now, and the new research could shed light on other unexplainable deaths in animals.

The conclusion was reached by Dr. Harald Prüß of the German Center for Neurodegenerative Diseases (DZNE) and Charité, and Dr. Alex Greenwood of the Leibniz Institute for Zoo and Wildlife Research in Berlin. The potentially fatal disease, which affects one in 200,000 humans a year, was discovered by re-analyzing stored samples of the polar bear's brain.

"Until now, this autoimmune disease has only been known in humans," explained Prüß in a statement. "In this illness, the body’s immune system overreacts and produces antibodies which damage nerve cells instead of fighting against pathogens. Epileptic seizures, hallucinations and dementia are among the possible symptoms."

^{The disease causes antibodies to attack nerve cells. Kateryna Kon/Shutterstock.}

The reason for the late diagnosis of Knut is that autoimmune diseases of this type were not known of until 2007, and thus have not been extensively studied. And while cases have been recorded in humans, mostly women, no such diagnosis has been made for a non-human animal before. The research suggests that autoimmune diseases of the nervous system may be more widespread than thought but, importantly, the non-infectious illness is treatable. Spotting even some of the symptoms early in humans and other animals can prevent deaths from the disease.

"The anti-NMDA receptor encephalitis has been described only very recently in humans," said Greenwood. "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."

The researchers stressed that it was unlikely Knut's captivity caused the disease, and they expected many other deaths both in the wild and captivity could be attributed to it.

In a press briefing, Prüß added: "The symbolic nature of Knut, the world famous polar bear, should raise the public awareness necessary to ensure that clinicians and patients consider this treatable disease even in such encephalitis cases in which only a subset of symptoms is present."