LAKE TAHOE, Calif. / Nev. - Post-mortem pathology results have ruled out public speculation of a vehicle strike as the cause of a bear's deterioration when she was found with her yearling in a public parking lot near Northstar in March. The results show no signs of blood, broken bones or other injuries consistent with blunt force trauma from a vehicle, but instead reveal severe inflammation in multiple areas of the brain and surrounding area.
This brain inflammation, clinically called encephalitis, is something wildlife agencies have detected in bears over the last 10-plus years, but was somewhat of a surprise in this case.
While the California Department of Fish and Wildlife (CDFW) had suspected some type of central nervous system condition, Dr. Brandon Munk, the state's lead bear veterinarian says, "This one, it wasn't high on our list, because the animal was an adult."
The vast majority of cases in Tahoe bears have been in juvenile bears (three years or younger).
Dr. Peregrine Wolff with the Nevada Department of Wildlife first identified this unexplained encephalitis in 2014 that was associated with cubs of the year, or yearlings coming out of hibernation early.
One sign of the condition is abnormal -- dog-like -- behavior exhibited in an afflicted bear's comfortability around humans. Other signs are subtle neurologic deficits like a head tilt, tremors or inability to climb trees.
At times, the department brings these bears in for observation with some showing what appears to be decreased mental capacity. "They're not all there," as Munk describes.
The Northstar bear mentioned earlier exhibited the abnormal behavior of not moving away from people as they approached as well as other neurological dysfunctions. CDFW removed the female bear and her yearling from the crowded public parking lot, tagged and released the two into the wilderness and continued to monitor them. Three days later, the department euthanized the mother bear to minimize her suffering since her condition had worsened. Staff had determined her yearling was old enough to live on its own.
Since regional detection in 2014, wildlife biologists and veterinarians have conducted ongoing studies on this unexplained encephalitis in Tahoe bears, providing some answers and raising other questions.
What is causing it?
Encephalitis is a general term for brain inflammation, but the condition has many different causes.
Identifying the cause hasn't been easy. When investigating, researchers hope their analysis points towards one thing, offering evidence towards the cause.
That hasn't been the case with this unexplained encephalitis.
So far, research has identified multiple different types of protozoa spanning a majority of cases. In other cases, signs point to a viral source. Still, others show evidence of both.
Although many people may be familiar with viruses, they may be less familiar with protozoa. Protozoa, which are single-celled microscopic organisms, sometimes find themselves in places they shouldn't be -- like a bear's brain.
"They run amok because they're in the wrong place," Munk says. This can lead to inflammation and depending on where the inflammation occurs in the brain, encephalitis can cause seizures. In some cases, they have evidence that suggests the seizures can impact the entire brain.
"That's one of the reasons why our prognosis is pretty bad on these guys," Munk explains. "Repeated seizures is not fun for anybody. So I would assume that is terrifying, at a minimum, for any animal."
A presumptive protozoal cause is nearly not transmissible (requires cannibalism). A viral cause would likely be transmissible; however, it depends on the virus with many specifics to viral transmission and susceptibility that come into play and are unknown at this time.
A very big question
The behaviors of bears with encephalitis are often difficult to distinguish from bears that are accustomed to human presence (habituation) or conditioned to human food, which raises a very big question regarding human-bear conflicts.
"So, is it the habituation, food conditioning, that maybe puts them at risk to exposure to whatever the pathogen is or the disease is?," Munk says. "Or is it the effects on the behavior that makes them more likely to maybe be in an area where people are more likely to be?"
He stresses the question mark as they don't know if there is a relation. There's also the variable of detection bias in which the agency is more likely to be informed about encephalitis cases where there are more people.
"It could also be a prevalent condition wherever there are bears in the wild," he explains, "and we just don't hear about it because they're in the wild."
Ongoing studies continue to work on deciphering details and potentially allude to how bears are getting exposed.
If anyone does observe a bear showing dog-like behavior, it's important to notify the local wildlife department and apply the standard practices when encountering a bear.
Those include keeping your distance, not running away, making loud noises, ensuring the bear knows you're around and providing it with an escape route. It's also vital to secure trash and avoid leaving food out.
"For the most part, black bears don't want anything to do with us. If they're behaving oddly, definitely inform the department," Munk says.
In California, report incidents to CDFW at 916-358-2917 or report online using the Wildlife Incident Reporting system at apps.wildlife.ca.gov/.
In Nevada, you can contact NDOW at 775-688-BEAR (2327).
If the issue is an emergency, call the local sheriff's department or 911.
For more information about what to do if you encounter a bear and to keep bears wild, visit TahoeBears.org and BearWise.org