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

Knut the polar bear was a star at the Berlin Zoo despite a rough start in life. As a cub, he was abandoned by his mother, but a zookeeper hand-raised him to adulthood. His death of encephalitis in 2011, when he was 4, shocked fans. "Knut was something very special," said a zoo board member. Knut isn't the first animal to become a cause celebre for just being himself.

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Cecil the lion probably never knew how beloved he was. The Zimbabwean lion, who was killed on a hunt in early July 2015, was a popular attraction among visitors to Hwange National Park thanks to his status in some studies run by Oxford University scientists. Now, he's world-famous -- and the men accused of killing him are being taken to task.

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Jumbo the elephant was among the most famous beasts of the 19th century, so popular that his name became synonymous with bigness. After being captured in India and spending a few years in Europe, the elephant was purchased by P.T. Barnum and became the centerpiece of Barnum's circus. He died in a train accident in 1885. "Long after his life was extinct," The New York Times reported, "his keeper, who brought him from the Zoological Gardens in London, laid on his body and wept."

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Silverback gorilla Willie B. helped spark change at Atlanta's zoo in the 1980s. At the zoo's nadir, Willie B. lived in a cramped cage with just a television. Thanks to poor conditions like Willie B.'s, the zoo's accreditation was revoked. But new director Terry Maple made major changes -- including a habitat for Willie B. -- and Zoo Atlanta was turned around. Willie B. died in 2000.

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It's not often that an octopus becomes widely admired, but when you can pick World Cup winners, you can write your own ticket. Paul the octopus, a resident of the Sea Life Centre in Oberhausen, Germany, correctly predicted the winner of every German match in the 2010 World Cup -- and then nailed the final, too. He died of natural causes a few months later.

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When she was 12, Koko the gorilla -- who is able to use sign language -- wanted a pet. She got a stuffed animal. That wasn't satisfactory, so for her 13th birthday in 1984, she got a kitten, which she named All Ball. When the kitten died six months later, Koko expressed great sadness at her loss. She got two more kittens in 1985. Since the second one, Smoky, died in 2005, she's been getting cat visits from the Humane Society at her home at the Gorilla Foundation in Woodside, California.

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SeaWorld's Shamu, the first orca to survive for more than a year in captivity, became a celebrated attraction at the San Diego park -- so much so that other orcas have been named Shamu. The orcas' life has not been without controversy; one named Tilikum killed a trainer in 2010, an event examined in the CNN documentary "Blackfish."

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Tama, a Japanese cat, became celebrated as the friendly stationmaster of the Kishi rail station in Kinokawa -- part of a railway line that she helped save from shutting down, thanks to her popularity, which brought in millions of dollars. Tama died June 22. She was 16. Her funeral was attended by 3,000 people.

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Beloved animals

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.

Knut’s half-brother is fully adorable