A necropsy of the four-year-old bear who died suddenly two weeks ago showed he was suffering from encephalitis, an irritation and swelling of the brain that was likely brought on by an infection, pathologist Claudia Szentiks said.
It remains unclear what that infection was, but Achim Gruber, a professor of veterinary medicine at Berlin's Free University, said it likely was a virus.
"We believe that this suspected infection must already have been there for a long time ... at least several weeks, possibly months," Gruber said, although he added that there had been no sign of anything amiss in the bear's behavior.
Knut died March 19 in front of hundreds of visitors at Berlin zoo, turning around several times and then falling into the water in his enclosure. Polar bears usually live 15 to 20 years in the wild and even longer in captivity.
Experts who examined Knut found massive quantities of fluid in his lungs, supporting the conclusion that the immediate cause of death was drowning. But they said that even if he hadn't fallen into the water he likely wouldn't have survived.
"Given the massive scale of the inflammation, Knut would probably have died sooner or later – it wouldn't really have been possible to save him," said Szentiks, a pathologist at the Leibniz Institute for Zoo and Wildlife Research in Berlin, which led the examination.
Experts ruled out rabies, botulism and bovine spongiform encephalopathy, also known as mad cow disease, as causes of the encephalitis that led to the animal's spasm and collapse, said Szentiks.
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