A mystery virus infecting young black bears in the US states of California and Nevada makes the animals behave in a strangely friendly manner towards humans.

In one case a cute bear cub climbed fearlessly onto a snowboarder’s board at the Northstar resort near Lake Tahoe, showing none of the caution a wild animal would naturally show around humans.

But the bizarre condition, which vets believe is a type of viral encephalitis, is ultimately fatal. The condition, still un-named, has drawn comparisons with the “zombie virus” that causes strangely aggressive and uncoordinated behaviour in deer.

The disease is still rare in bears, says the California Department of Fish and Wildlife (CDFW), but appears to be on the increase.

In February, CDFW animal specialists responding to a call in El Dorado County, California found a bear too young to be out on its own, displaying “dog-like” behaviour.

The animal seemed completely comfortable around humans, and even picked up an apple and happily ate it up an apple to eat in front of a group of people on their backyard patio.

Physically and mentally, said the CDFW team, the bear “didn’t seem quite right”, with a shambling walk and a dull, unresponsive manner.

As yet, the precise cause of the condition is not known. The affected bears have been found to be carrying around half a dozen viruses unknown to science, but which of any of those viruses could be causing the encephalitis is unknown.

Encephalitis, an inflammation of the brain tissue, can also be caused by infectious agents such as fungi or bacteria.

In a CDFW statement, specialist wildlife vet Brandon Munk said that while normally the agency would try to treat the animals and return them to the wild, it was too dangerous to do that in these cases: “At this point, we don’t know what causes the encephalitis,” he said, “so we don’t know what, if any, health risks these bears might pose to other animals.”

As yet it's not clear whether whether this condition is directly related to the 'zombie disease' in deer, or if it represents a danger to humans or other animals as well.

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