A Puzzling Brain Disease Is Killing Black Bears in the Western United States

Some animals showing signs of a neurological disorder had brain inflammation, but the cause is still unknown

A small black bear cub is see standing on all fours on a tree branch.
Bears with intensifying symptoms either die or are require euthanasia. Those that survive require lifetime treatments and can't return to the wild. U.S. Fish and Wildlife Service Southeast Region, Public domain, via Wikimedia Commons

In California and Nevada, vets and biologists are receiving an increasing number of reports of bears exhibiting signs of an unknown neurological condition. The bears appear lethargic, underweight, experience head tremors, and have a subtle head-tilt. The condition also makes the bears unwary of humans, posing a threat to their safety, reports Ryan Sabalow for the Sacramento Bee.

In the last year, four cases of the strange illness were identified in California, reports Ed Cara for Gizmodo. One incident found a 21-pound female cub, which should have weighed at least 80 pounds for its age, climbing into the trunk of a car after humans fed the bear, reports Gizmodo. In many cases, it turns out the bears developed encephalitis, or severe brain swelling, which can be caused by a variety of sources, even viral infections. In a December 2020 study, researchers identified five new viruses in the bears showing the symptoms, but whether any of these viruses cause the brain-swelling illness and odd behaviors remains unclear, Gizmodo reports.

In what seems to be a classic display of the disease, a now-viral 2019 video showed a young bear cub in Lake Tahoe walking up to a snowboarder in a "dog-like" way with its head tilted. When officials further examined the bear and scanned its brain, they found that it had encephalitis, the Sacramento Bee reports. The video is one of the only documented cases of a bear showing symptoms of encephalitis.

The California Department of Fish and Wildlife (CDFW) first received reports of the condition back in 2014. In many of the cases reported, there are similarities. Often officials find a bear cub that is too young to be on its own, shows no fear towards humans, walks oddly, and does not respond as expected. After a closer look, wildlife officials eventually conclude that the bears had encephalitis. Bears with worsening symptoms either died or were euthanized. Those that survived require lifetime treatments and can't return to the wild, the Sacramento Bee reports.

"Any time a wild animal comes into our care, the best possible outcome is a release back to the wild. That's just not possible for these neurologically impaired bears. At this point, we don't know what causes the encephalitis, so we don't know what, if any, health risks these bears might pose to other animals," said CDFW wildlife veterinarian Brandon Munk in a statement.

While new cases of young bears with encephalitis keep showing up, it is currently not affecting growing black bear populations in the West. In 1982, black bear populations in California were between 10,000 and 15,000 bears. Now, the population is currently between 30,000 and 40,000 bears, reports the Sacramento Bee.

While the cause of the bear's illness has not been identified, it is not a risk to humans, Jaime Sherman, an expert in bear diseases at UC Davis' One Health Institute, tells the Sacramento Bee. Still, officials say people shouldn't encourage bears to go near them even if a bear cub appears friendly—mainly for the bears' own safety.