An Unknown Disease Is Giving Bears Dog-Like Friendliness – And It's Dangerous, Not Cute


Tom Hale

Senior Journalist

clockMar 30 2021, 15:10 UTC
Black bear.

The total number of American black bears (Ursus americanus) in North America is likely within the range 850,000–950,000. Image credit: NaturesMomentsuk/

California's black bears are being gripped by a mysterious brain disease that makes them behave in a friendly and fearless way. As the neurological disease progresses, however, it's also linked to lethargy, a dramatic loss of weight, a distinct head tilt, tremors, and an unusual gait. Left untreated, the disease can be deadly.

California Department of Fish and Wildlife (CDFW) documented some of the first cases back in 2014 after they were alerted to a number of young black bears in the Tahoe Basin with apparent neurological abnormalities. A number of similar cases have since been documented in a range of California counties.


One report from Pollock Pines in El Dorado County last month appears to describe the situation quite well. A small black bear was spotted alone at a utility worksite. Along with appearing lethargic and possibly sick, the animal was almost totally unfazed by people trying to shoo it away with claps and yells. Eventually, the CDFW received another call about a similar report in their backyard. 

“When a CDFW wildlife biologist and warden went to investigate, they encountered a situation becoming more common in the Tahoe Basin and elsewhere around the state. They found a bear too young to be out on its own, "dog-like" in its behavior, completely comfortable around people, picking up an apple to eat in front of them on the backyard patio. Physically and mentally, the bear just didn’t seem quite right, walking oddly, dull, and not responsive like a normal bear should be,” the CDFW said in a statement.

The black bear was taken in for observation and evaluation by veterinarians, which revealed it was undersized and drastically underweight for its age. After a week, they were forced to euthanize the bear.


The cause of the unusual disease is baffling scientists. Preliminary post-mortem results and CT scans of the affected bears suggest they are suffering from encephalitis, an inflammation of the brain tissue typically caused by infectious agents like a virus, fungi, or bacteria. Wildlife biologists at the CDFW say the root cause of the disease is currently unknown, but they have uncovered some intriguing clues, namely the presence of five never-before-seen viruses within the bears. 

Reporting the discovery in the journal PLOS One, the researchers at the CDFW and the University of California Davis say novel viruses belonged to five genera: Circoviridae, Parvoviridae, Anelloviridae, Polyomaviridae, and Papillomaviridae. It’s also noteworthy that the virus was detected in the bear’s liver, spleen, and – critically – brain tissue. 

The symptoms of the disease all hold some similarities to chronic wasting disease seen in deer, elk, and moose across many parts of the US. Also known as the “zombie disease”, it causes a range of species to suffer from drastic weight loss, stumbling, drooling, lack of coordination, aggression, and other abnormal behavior. This neurological disease is thought to be caused by prions, a misfolded protein that acts as an infectious agent.


While the bid to understand this bear outbreak continues, the public is being warned to keep a respectable distance from any black bear, especially those displaying suspiciously friendly behavior. After all, it’s not yet clear whether this disease is a threat to humans or other wildlife.