UC-Davis veterinarians identify new raccoon polyomavirus

Discovery could help explain how viruses cause cancer in animals and humans
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Raccoon in a tree

Rare brain tumors emerging among raccoons in Northern California and Oregon may be linked to a previously unidentified virus discovered by a team of researchers, led by scientists from the University of California-Davis. Their findings, published in December 2012 in the journal Emerging Infectious Diseases, could lead to a better understanding of how viruses can cause cancer in humans and other animals.

Necropsies conducted since March 2010 by scientists at the UC-Davis School of Veterinary Medicine and UC-Davis–led California Animal Health and Food Safety Laboratory found brain tumors in 10 raccoons, nine of which were from Northern California, the article reports. The 10th was sent to the university by researchers at Oregon State University.

The common factor found in all the tumors was a newly described virus, dubbed raccoon polyomavirus. Researchers suspect the virus contributes to tumor formation. Polyomaviruses are common but rarely cause cancer, nor do they typically cross species, so the outbreak is not expected to spread to people or other animals.

Two more raccoons with the tumor and the virus have been found in Yolo and Marin counties since September 2012, when the article was submitted to the journal for publication.

“Raccoons hardly ever get tumors. That’s why we take notice when we get three tumors, much less 12,” said study author Dr. Patricia Pesavento, a pathologist with the UC-Davis School of Veterinary Medicine.

Polyomaviruses are known to cause cancer under laboratory conditions. Less is known about their ability to cause cancer in people under natural conditions, because such cancers often takes decades to develop. With their short lifespans of two to three years, raccoons can provide a model for studying how these viruses spread outside the laboratory, how they cause cancer, and how easily they can jump from species to species.

Of the 12 raccoons affected, 10 were collected from Marin County. This does not mean the virus is limited to that county or even to Northern California, according to Dr. Pesavento. Marin County is home to WildCare, an animal rescue and rehabilitation center that routinely submits animal remains for diagnostic testing, which might result in a sampling bias.

Other California raccoons were submitted by Lindsay Wildlife Museum in Contra Costa County and Sonoma Wildlife Rescue. Dr. Pesavento’s laboratory is collecting specimens and data from other sources across the country, looking for the virus and for raccoon exposure to it.

Dr. Pesavento said more research is needed to understand whether an environmental toxin, genetics, or something else is contributing to the cancer. The study notes that raccoons are exposed daily to human waste, garbage, environmental toxins, and environmental pathogens as the animals travel along sewer and water lines.

“This is just the beginning of a story,” Dr. Pesavento said, adding that high rates of cancer are found among animals that live in proximity to humans. “Wildlife live in our fields, our trash cans, our sewer lines, and that’s where we dump things. Humans need to be guardians of the wildlife-human interface, and raccoons are important sentinel animals.

They really are exquisitely exposed to our waste. We may be contributing to their susceptibility in ways we haven’t discovered.”

Infectious pathogens are associated with 15 to 20 percent of all human cancers worldwide, according to the American Cancer Society. For example, papillomavirus can lead to cervical cancer in women, and feline leukemia virus can cause cancer in cats.

“This work to investigate natural associations of cancer verifies the importance of our one-health approach to addressing complex biomedical problems, such as viral causes of cancer,” said Dr. Michael Lairmore, dean of the School of Veterinary Medicine, of which the UC-Davis One Health Institute is a part. “Understanding how infectious agents may contribute to cancer in animals has provided fundamental new knowledge on the cause of cancer in people.