Knut the polar bear died of autoimmune illness usually found in humans

When a four-year-old celebrity polar bear named Knut died suddenly at Berlin zoo in 2011, vets were at a loss to explain the death.

Knut rose to fame as bear cub when he was rejected by his mother at birth, along with his twin brother who died within days.

To ensure the bear got his bottle every two hours, his main keeper, Thomas Doerflein, camped out at the enclosure. Knut became so famous he appeared on magazine covers.

The day Knut died, shocked visitors saw him turn around a few times before slumping into the water at his enclosure. A postmortem revealed the bear had encephalitis, or a swollen brain, but the cause of the illness was a mystery.

The first hint of an answer came when Harald Prüss, a researcher at the German Centre for Neurodegenerative Disorders in Berlin, read the postmortem report and noticed similarities between Knut and human patients with an autoimmune disease that caused brain swelling.

Prüss noticed that antibodies in Knut’s blood looked similar to those he had seen in patients in 2010 whose encephalitis had yet to be explained. He contacted Alex Greenwood, an expert in wildlife diseases who was working on Knut’s case, and the two decided to investigate.

Writing in the journal Scientific Reports, the researchers describe how they went on to uncover how Knut died. They took sections of Knut’s brain that had been kept in storage and stained them to show the presence of antibodies. These revealed patterns that closely matched those seen in the brains of people with a rare autoimmune disorder known as anti-NMDA receptor encephalitis or anti-NMDAR, which causes seizures, cognitive problems and even coma.

Knut is the first animal to be diagnosed with the disease, and the discovery has opened the door to new realm of research. “For domestic, wild and zoo animals this represents the birth of a new field of research and understanding of encephalitis of unknown origin,” Greenwood said.

Antibodies that normally defend against viruses can turn on the body’s own tissues and attack its cells. Knut’s antibodies had attacked nerve cells in the brain important for memory and cognition, causing seizures that led to his drowning in the pool.

Knut’s postmortem showed he had been exposed to flu viruses, but none of the bugs scientists found could explain his death. At the time, researchers did not consider that his death might have been caused by anything other than bacteria or viruses, because anti-NMDAR had only been discovered in humans four years earlier, in 2007.

People who are affected by the disease are treated with steroids and have antibodies removed from their blood.

Knut showed no symptoms before his seizures and sudden death. Even had he shown symptoms, it may have been too late to treat him. Greenwood added that the discovery of the autoimmune disease in Knut could help explain a large proportion of currently undiagnosable cases of encephalitis in animals.

“In the future, because treatment options are available in humans, that could easily be transferred to animals,” he said. “The death of many animals (particularly [those] of conservation concern) of encephalitis in the future may be preventable.”

Knuts lives on as a life-size model, which has been on display at Berlin’s Natural History Museum since 2013.