What killed Knut, the beloved polar bear? Now we know

Story highlights

  • Knut's brain inflammation was a result of an autoimmune disease
  • The condition had never been detected in animals

(CNN)More than four years ago, Knut the polar bear suffered a seizure and drowned in his enclosure in Germany as horrified zoo visitors watched.

Lots of questions and theories followed on what killed the beloved polar bear-- only to be discounted later.
    How could the 4-year-old ball of fur, young by polar bear standards, suddenly drop dead? An autopsy determined it was an inflammation of the brain. But what caused it?
    In a study published Thursday in journal Scientific Reports, experts say they now have the answer.

    What caused brain to swell?

    Knut's inflammation was a result of an autoimmune disease of the brain, known as the anti-NMDA receptor encephalitis, they concluded.
    The immune system's job is to seek out and destroy foreign cells or organisms that enter the body and cause disease, such as a virus. But sometimes the immune system launches an attack on the body's own cells.
    And in the case of anti-NMDA receptor encephalitis, the immune system attacks nerve cells instead of disease-causing cells.
    Anti-NMDA receptor encephalitis was discovered in humans just eight years ago. Until Knut's ailment, the disease had never been detected in animals. This makes his case a rarity.
    "The more common anti-NMDAR encephalitis has only been considered a human disease leaving open the possibility that an entire field of encephalitis relevant research has gone unexplored in domestic and wild animals," the journal said.
    The disease responds to aggressive treatment in humans, and that information could help save other animals in future, scientists said.

    Zoo superstar

    Knut captured hearts worldwide when he was abandoned by his mother at Zoo Berlin after birth in 2007. A zookeeper took care of the cub until he was old enough to fend for himself.
    Although animal rights groups denounced it as unnatural, the cub and his caretaker, who slept next to his crate, became instant celebrities and attracted throngs of adoring fans to the zoo.
    Knut was so popular, his presence translated to the zoo's most profitable period in its 163-year history.
    He died unexpectedly in March 2011 after he collapsed in a pool in his enclosure.