Tasmanian devils claw their way back from extinction


For decades a ghastly facial cancer has been decimating Tasmanian devils. Spreading from animal to animal when the stocky, raccoon-size marsupials bite each other, the transmissible cancer has killed up to 80% of the devils in Tasmania, their only home for millennia. Some researchers saw extinction as inevitable. Now, a new study in Science, suggests the remaining 15,000 devils have reached a détente with the cancer. Until recently it was spreading exponentially, like the pandemic coronavirus among humans in many parts of the world. But geneticists calculate that each infected devil now transmits tumor cells to just one—or fewer—other devils. That could mean the disease may disappear over time.

“It is a promising sign for the future,” says Gregory Woods, an immunologist at the University of Tasmania, Hobart, who was not involved with the work. Along with two other recent studies, the findings hint that changes in devil behavior—and possibly the emergence of less virulent tumor cells—may be taming the cancer’s spread, and that desperate efforts to breed the devils in captivity may not be needed. “[This] deeper understanding of the links between host behavior and infectious disease may help reveal new insights that can help both devils and other wildlife facing emerging disease threats,” says Vanessa Ezenwa, a disease ecologist at the University of Georgia, Athens.

Transmissible cancers are rare in mammals, and devils—whose nocturnal screams and growls earned them their name—are mostly solitary. But starting in 1996, researchers began to notice more and more devils with tumors. Sick animals infected others with cancer cells during mating season fights and scuffles over scavenged carcasses, triggering rapid spread of devil facial tumor disease (DFTD). Tens of thousands of devils died, and conservationists established captive breeding programs to create a reserve population for reintroduction.

The dynamics of the disease are complex, according to a recent study led by cancer geneticist Elizabeth Murchison of the University of Cambridge. Her team genetically analyzed more than 600 tumor samples collected between 2003 and 2018 and found five genomic versions, three of them widespread, with some devils contracting multiple types. That complexity could hamper efforts to develop vaccines to conquer the cancer, according to their 24 November study in PLOS Biology.

To further investigate the spread of the rogue tumor cells, Washington State University, Pullman, geneticist Andrew Storfer and his graduate student Austin Patton examined differences in tumor genomes through time, an approach routinely used to trace the spread of viruses including SARS-CoV-2, the pandemic coronavirus. A mammalian tumor genome is much larger than that of a virus, so Patton and colleagues had to work out ways to analyze their data, gleaned from 51 tumors collected from 2003 to 2018.

Focusing on 28 genes that seemed to be evolving at a consistent rate, they traced how specific mutations spread through the tumor samples over time. That enabled them to infer the rate at which the cancer itself was spreading among devils. “The application of these methods for transmissible cancer is very clever,” says Michael Metzger, a molecular biologist at the Pacific Northwest Research Institute.

At the disease’s peak in the early 2000s, each infected devil spread the disease to at least 3.5 others, the team reports today in Science. But transmission has slowed recently, with some infected animals not passing DFTD on at all. The reduced density of devils accounts for much of the decline, Patton suggests, as animals come into contact with fewer of their fellows. Remaining devils may also have better immune systems or altered behavior, speculates Patton, now a postdoc at the University of California, Berkeley.

A study out on 9 December in the Proceedings of the Royal Society B supports the idea that an animal’s behavior can slow transmission. Disease ecologist Rodrigo Hamede and behavioral ecologist David Hamilton from the University of Tasmania, Sandy Bay, did devil contact tracing: For 6 months they put radio collars on 22 devils that revealed when an animal came into close contact with another. The tracking showed that once infected, even dominant, aggressive devils withdrew from others as they became sicker. These individuals were “superspreaders” only early in the mating season, Hamilton, Hamede, and their colleagues report. “The fact that they behave in this way is likely to have a big impact on disease dynamics,” Hamilton says.

The Science study authors argue against plans to introduce captive-bred devils into remaining wild populations. Beefing up devil populations may increase their density and rev up transmission again, and captive-bred animals may lack resistance built up in wild populations, Storfer speculates.

Though this week’s news is good, “devils are still not out of the woods,” warns conservationist Max Jackson of Aussie Ark, which helps breed captive devils. Indeed, researchers detected a second transmissible facial cancer in devils in 2014. But the new findings offer hope, Hamilton says. “It looks extremely unlikely that we’ll be losing them any time soon.”

sciencemag.org, 10 December 2020
; https://www.sciencemag.org