Neuroscientists Unlock the Secret of Beloved Berlin Polar Bear’s Death

Knut, Berlin Zoo's first polar bear in more than 30 years, attracted hordes of fans to the zoo but died from a hitherto mystery illness when he was only four years old.

The untimely death of Knut the polar bear was due to an autoimmune brain disorder which caused him to develop encephalitis, researchers in Germany announced on Thursday, solving the mystery of his death four and a half years ago.

Knut, Berlin Zoo's first polar bear to survive past infancy in more than 30 years, was born in December 2006 and raised by zookeepers after being rejected by his mother at birth. 

Knut shot to worldwide fame and quickly became the zoo's star attraction; in March 2007, the month when he was first shown to the public, 541 newborn babies in Berlin were named 'Knut.' 

In the wild, polar bears have a life expectancy of 15 to 18 years, and in captivity can live to their mid- or late-30s; the world's oldest polar bear was Debbie, who died in Canada in 2008 at the age of 42.

However, Knut was just four years old when he died in March 2011. He experienced a seizure and then drowned in his enclosure at the zoo, watched by upset visitors who had come to see him.

A post-mortem revealed that the unfortunate white bear was suffering from encephalitis, a brain inflammation, but puzzled veterinarians could not figure out the reason for the disease; pathogen screening for infection gave no signs of the cause.

On Thursday, German neuroscientists published an article in the journal 'Scientific Reports' with their diagnosis of Knut's disorder, which had never before been diagnosed in an animal. 

The polar bear died of a rare autoimmune disorder known as anti-NMDA receptor encephalitis, which causes seizures, cognitive problems and even comas.

Harald Pruss, a researcher at the German Centre for Neurodegenerative Disorders in Berlin, and his colleagues suspected the diagnosis after reading Knut's postmortem report, where Pruss noticed similarities between Knut and human patients with an autoimmune disease that caused brain swelling.

A woman touches a monument to polar bear Knut after its unveiling at the Zoo in Berlin, Germany, Wednesday, Oct. 24, 2012

© AP Photo / Michael Sohn

"We conclude that Knut suffered anti-NMDA receptor encephalitis, making his the first reported non-human case of this treatable disease," wrote the scientists, who came to their conclusion after examining the antibodies in Knut's brain.

"The results suggest that anti-NMDA receptor encephalitis may be a disease of broad relevance to mammals that until now has remained undiagnosed."

The autoimmune disease, which was first discovered in humans eight years ago, responds to aggressive medical treatment, and scientists hope the diagnosis can help other animals found to be suffering from the illness.