Mysterious brain disease found in euthanized bear captured in south Eugene

A young black bear that wandered through southwest Eugene was living with severe neurologic disease before the Oregon Department of Fish and Wildlife captured and euthanized her.

After they had captured the female bear, biologists in the department’s health lab found the animal — about 1½ years old — in her carrier July 16, lying in a pool of drool and breathing rapidly and shallowly. After euthanizing her, they conducted a necropsy and ruled out rabies. Test results that came back Tuesday, July 28, confirmed the bear had viral encephalitis.

Encephalitis causes the brain to swell, and under that pressure, brain tissue can become damaged or stop working. Animals with the disease can develop head tremors, an abnormal gait, and unusual “doglike” friendliness — they lose their natural fear of humans.

Lookout Eugene-Springfield requested an interview with a Department of Fish and Wildlife biologist, but spokesperson Beth Quillian said the department doesn’t have the staff capacity right now for an interview.

Quillian also said staff did not have time to check historical records to determine whether this is the first time in Oregon a bear’s encephalitis has been confirmed by lab testing. She said, however, that it is the first such case “recently.”

“I can say that we don’t yet know what virus caused the encephalitis in this bear,” she said in an email. “There have been other cases in North America over recent years (California and Nevada). The root cause is still under investigation.”

The root cause remains a mystery in California and Nevada, where the disease in black bears was first observed in 2014 near Lake Tahoe on the state line. Since then, it has been reported in eight counties in California and two in Nevada, according to researchers at the University of California, Davis, School of Veterinary Medicine.

While trying to solve the mystery, scientists have discovered several previously unknown viruses and microscopic parasites in the sick bears. But because these same infections frequently show up in healthy bears, too, researchers still can’t prove they’re causing the brain swelling.

When examining sick bears, wildlife biologists often find them dangerously underweight and in poor physical condition — including one young female bear that arrived covered in ticks, weighing only a quarter of what she should have.

The neurologic condition can amplify human-wildlife conflicts, because brain-damaged bears won’t retreat and often approach humans directly, research shows.

Bear activity typically spikes in late spring and early summer, with more sightings in forested communities like the south hills of Eugene. During this time, young bears are pushed out to establish their own territory, adults roam in search of mates, and mothers with cubs forage together. This seasonal activity often leads bears beyond their usual home ranges, where they typically are more secluded from humans.

Last year in April, a man in the Trainsong neighborhood put a leash on a bear, caged it, and then released it into the wild. Christopher Yee, a biologist with the Oregon Department of Fish and Wildlife’s Springfield office, described the bear as having some “issues,” and said being able to leash it was not “normal.” But at the time, he said the bear could have been coming out of torpor — a lighter, less intense form of hibernation — which can cause hormonal imbalance.

Also, last year in June, a mother and three cubs were spotted ambling in the streets of south Eugene. Quillian said it is “very unlikely” that the sick bear was part of this bear family seen last year, based on its size and condition.

More bears have been spotted in the south hills since the sick bear died, according to camera recordings neighbors have posted on social media — though no close encounters have been reported.