A mosquito bite was the culprit that led to the death of a 545-kilogram polar bear at the Toronto Zoo, a vet has confirmed. 

Kunik, one of two popular polar bears at the zoo, may be the first known case of a polar bear contracting the mosquito-borne West Nile virus.

Although polar bears are thick-skinned animals, zoo officials say Kunik could have been bitten on the nose. The 26-year-old animal had to be put down for humane reasons in late September after it began having difficulty getting up on its hind legs, which had become paralyzed.

Dr. Ian Barker, a professor of wildlife disease at the Ontario Veterinary College at the University of Guelph, conducted a necropsy and found Kunik had suffered from the same brain inflammation that West Nile-infected humans would develop.

Barker said that carnivores are hardly ever affected by West Nile and that it's unlikely wild polar bears would be exposed to it because mosquitoes that transmit the virus are not usually found in the bears' natural habitat.

Evidence of the virus has been found in black bears in New Jersey and in horses, dogs and cats. But rarely in those does the virus actually erupt into the full-blown disease, Barker said.

With files from the Canadian Press