"It is bizarre because (polar bears) have very thick fur and thick skin, so a mosquito would have had to land on his nose or his eyeball or somewhere where it could get through," Shanna Young, executive director of communications at Toronto Zoo, said on Wednesday.
She said the zoo believed it was the first time that a polar bear had died of the tropical disease, although there has been evidence that the virus has turned up in black bears.
The zoo euthanized the bear, named Kunik, last month after it had trouble using its hind legs. The symptoms pointed to West Nile encephalitis, an inflammation of the brain, and Young said postmortem tests came up strongly positive.
A sample of the bear's brain tissue has been sent to Health Canada, which is responsible for national public health, and a confirmation of the cause of death is expected within weeks.
West Nile virus was first isolated nearly 70 years ago in Uganda. It is mainly transmitted through the bite of an infected mosquito, which itself became infected by feeding on the blood of infected birds.
Toronto has seen only six human cases of West Nile this year, down from 38 in 2005. The zoo, Canada's biggest, conducts mosquito eradication programs and tests vaccinations on its animals, but there is no vaccine for polar bears, Young said.
Kunik was 26 years old and came to the zoo after it was orphaned in Canada's Northwest Territories, Young said. "He was huge, over a thousand pounds. He loved to swim and was very playful."
Only one polar bear remains at the zoo. Another died earlier this year of old age.
Reference:reuters
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