A group of raccoons forage in a municipal park. (K. Schneider )
An eerie new disease is cropping up among raccoons in Northern California and Oregon: brain tumors that may be linked to a previously unidentified virus discovered by a team led by UC Davis veterinarians and researchers.
Necropsies conducted since 2010 have found brain tumors in 11 raccoons from Northern California and one from Oregon, the researchers said. All of the animals with tumors also had the virus scientists know as raccoon polyomavirus.
"Previous to this, there had been two reports of a raccoon with a brain tumor over the past two decades," Patricia Pesavento, a pathologist at the UC Davis School of Veterinary Medicine and principal author of a study on the malady published earlier this month in the journal Emerging Infectious Diseases, said in an interview. “Now, we're getting one every month.”
Researchers are trying to determine whether the tumors are triggered by the virus alone, or a combination of variables including co-pathogens and environmental toxins that make raccoons susceptible.
"Of all wildlife, raccoons are especially good sentinels about how we are disturbing the environment because they have always thrived in neighborhoods," Pesavento said. "So when raccoons get sick, we should listen and see how we might have contributed."
This much is clear: The disease is restricted to raccoons and not expected to spread to other animals or humans, Pesavento said.
The problem first caught the attention of wildlife rehabilitation centers that had responded to reports of normally nocturnal raccoons foraging near homes during the day, passively and without fear.
"That kind of behavior isn't right for raccoons," Pesavento said. “Raccoons are out at night, and afraid of humans. When confronted, they respond aggressively."
Now, scientists are trying to figure out how the disease started, how many raccoons have it, and what percentage may host the virus without developing tumors.
As the number of raccoons with tumors continues to grow, they also want to know whether the disease is turning up elsewhere, such as Southern California, where the inquisitive mammals with a black fur band across their eyes and hand-like forepaws are ubiquitous.
The research may lead to a better understanding of viral causes of cancer. "Understanding how infectious agents may contribute to cancer in animals has provided fundamental new knowledge on the cause of cancer in humans," Michael Lairmore, dean of the UC Davis veterinary school, said in a statement.
In the meantime, Pesavento said, “it is important to remember that there is nothing about this disease that makes humans susceptible. It is still very rare, and I don't think raccoons will ever become an endangered species because of it.”