An outbreak of a previously unknown virus that causes fatal brain cancer in raccoons has been detected in northern California and southern Oregon.

Tumors and the new virus were found in 10 raccoons autopsied between March 2010 and May 2012. Nothing like them had been seen before in raccoons, in which tumors are very rare.

There's no reason to think the virus could be contagious to humans. Its emergence does, however, raise fascinating questions about how it evolved and whether patterns of suburban development actually fueled its rise.

"We need to understand how infectious pathogens are empowered by global changes," said veterinary pathologist Patty Pesavento of the University of California, Davis, leader of the team studying the new disease, which was reported in the January issue of Emerging Infectious Disease. "If there's a new niche, pathogens will find it."

Nine of the raccoons came from around Marin County, just north of San Francisco, and the 10th was sent from southern Oregon. The raccoons had been spotted wandering in daylight, approaching humans, falling unconscious and generally displaying signs of neurological distress.

Tumors appeared to have formed in their olfactory tracts, spread to their frontal lobes and compressed their mid-brains (see picture below). Reviews of scientific literature and calls to veterinary pathologists across North America found no precedents.

In each of the tumors, but not in brain tissue from raccoons tested for comparison, Pesavento's team found an unknown form of polyomavirus, one of a group of viruses known to cause a rare form of skin cancer in humans and tumors in other animals, including mice and birds. Pesavento's team called it raccoon polyomavirus.

"The connection between the novel polyomavirus and these raccoon brain tumors is strong," said disease ecologist Richard Ostfeld of the Cary Institute for Ecosystem Studies, who was not involved in the research.

The exact virulence and contagiousness of the new virus is unknown, but there's reason to think it's high. Raccoons killed by the tumors accounted for more than one-fifth of all the raccoons Pesavento's group autopsied between March 2010 and May 2012, and the cases they saw are likely the disease's tip.

"Raccoons go and hide when they're sick," said Pesavento. "The reason we're seeing this at all is because they don't know what they're doing. They're neurological."

Also unknown is whether the virus is unique to raccoons, or if that species is a so-called dead-end host for a disease transmitted between other animals, such as skunks or opossums.

It's also possible that the virus is an opportunistic pathogen signaling some deeper problem in raccoons, just as outbreaks of Kaposi's sarcoma, a once-rare type of cancer that thrived in the compromised immune systems of people with AIDS, signaling the HIV epidemic's beginning.

Though much remains unknown about raccoon polyomavirus, preliminary examination by Pesavento's team has turned up some interesting information. Unlike other polyomaviruses, it doesn't seem to fuse with the DNA of its host cells, but instead floats outside the chromosomes, potentially representing a new mechanism by which the virus induces cancer.

"That's known to have happened in a dish, but nobody believed it happened in an animal," said Pesavento.

The new virus also appears to be more closely related to human than animal polyomaviruses, suggesting a possible origin in our own species. Raccoons are known to frequent sewage drains, and exposure to polyomavirus-laden human waste is almost inevitable.

That contact creates opportunities for a mammalian species-hopping polyomavirus to thrive. If the raccoons are physiologically stressed, or are isolated from other populations, it may become even easier for viruses to cross the species gap.

Autopsy cross-section of an unaffected raccoon's head (top) compared to an affected raccoon (below). Tumors have grown in the olfactory tract, extending into the frontal lobe and midbrain. Image: Dela Cruz et al./Emerging Infectious Diseases

"Their immune systems are not as rich, not as deep," said Pesavento. "All of a sudden, we've created an evolutionary petri dish" for viruses that would otherwise have died out.

Fragmented suburban ecologies and stressed animal populations "create an environment where a virus can work towards species-jumping," said Pesavento.

Ostfeld cautioned against jumping to conclusions about the new virus's origins, which could instead be in rodents, bats or some other animal.

"There's really nothing in this paper to indicate what might have caused the outbreak," he said.

But disease ecologist Peter Daszak, president of the EcoHealth Alliance, sees the raccoons as a potential sentinel for changing conditions. "Urban development drives changes in ecology that promote the emergence of disease," he said.

A variation on this type of evolution is the lethal Hendra virus in Australia, which jumped to humans as a consequence of development-driven changes in the habits of flying foxes, its traditional animal hosts.

Unlike the Hendra virus, it's extremely unlikely that the raccoon polyomavirus could infect humans. And unlike influenza viruses, it's unlikely that genes from the new virus will be transferred into human-infecting strains, said Pesavento.

"Polyomaviruses have never been shown to recombine like influenza does," said Pesavento. But, "as humans, we have the responsibility to these animals to understand how we're affecting them," she said.

Daszak echoed her reassurances that the new virus won't infect people, but warned that creating reservoirs of any new disease is an unnecessary risk. "The message from this is not that wildlife are scary," Daszak said. "The lesson is that we need to protect wildlife."

Citation: "Novel Polyomavirus associated with Brain Tumors in Free-Ranging Raccoons, Western United States." By Florante N. Dela Cruz, Federico Giannitti, Linlin Li, Leslie W. Woods, Luis Del Valle, Eric Delwart, and Patricia A. Pesavento. Emerging Infectious Disease, Vol. 19 No. 1, January 2013.