Zoology: The curious case of Knut

Knut, the famous hand-reared polar bear from the Berlin Zoological Gardens who drowned in the pool of his enclosure following epileptic seizures in 2011, suffered from anti-NMDA receptor (NMDAR) encephalitis according to new research published in Scientific Reports this week. This diagnosis makes Knut the first reported non-human case of this treatable disease.

Following Knut’s death, pathological analysis revealed he had suffered from encephalitis (inflammation of the brain), which caused the seizures leading to his death. However, investigations to determine a bacterial, viral or parasitic cause were inconclusive and a diagnosis of “encephalitis of unknown etiology” was reported.

Using the diagnostic criteria applied to human patients, Harald Pruss, Alex Greenwood and colleagues conclude that Knut suffered from an autoimmune disease called anti-NMDAR encephalitis, which is the most common non-infectious form of encephalitis in humans. In a post mortem examination, the authors detected high concentrations of antibodies which specifically bind to the NR1 subunit of the NMDA receptor, found in nerve cells, in Knut’s cerebrospinal fluid. Using an immunostaining technique, the authors found that these antibodies bound to rat brain sections in tests, in a nearly identical labelling pattern to human anti-NMDAR encephalitis patients, suggesting that these antibodies may have caused the encephalitis that resulted in Knut’s death.

The authors suggest that Knut’s anti-NMDAR encephalitis may mean that antibody-mediated autoimmunity can be found in mammals besides humans.