Tasmanian Devils Spread Cancer by Biting, Study Says
Richard A. Lovett
for National Geographic News
|February 27, 2006|
For ten years a facial cancer has threatened to wipe out Australia's Tasmanian devils. (See "Tasmanian Devils Decimated by Mystery Cancer.")
The cancer is spreading fast, and scientists now say the disease transferred in tooth-baring combat.
Pound for pound, the Tasmanian devil is reportedly the most powerful biter alive today. Once widespread in Australia, the 20-pound (9-kilogram) marsupials now live only on the Australian island of Tasmania (map).
With as many as 150,000 roaming the wilds, the devils long seemed safe from extinction. Then in 1996 a nature photographer took a picture of a Tasmanian devil with a grotesque facial tumor.
Now dubbed devil facial-tumor disease, the ailment produces enormous growths that push the animals' teeth out of line and make it difficult for them to eat. Afflicted animals generally die of starvation within six months.
The disease has spread rapidly. Today biologists report that few animals evade it long enough to live into old age, which for a Tasmanian devil means about five years.
Scientists have long known the disease is infectious, but nobody understood what caused it. Some suspected that it might be transmitted via a virus.
But Anne-Maree Pearse, a biologist at Tasmania's Department of Primary Industries, Water, and Environment, thought something more exotic might be at work.
Now she and coworker Kate Swift believe they've found the answer: The animals inject cancer cells into each other when they engage in mating battles.
It's a unique theory, because veterinarians know of only one other type of cancer that can be transmitted by physical contacta venereal disease that affects dogs.
During mating season, Tasmanian devils are notoriously feisty, and their duels often involve mouth-to-mouth combat.
When a cancerous animal bites a healthy one, Pearse reported this month of the journal Nature, cancer cells can break off the cancerous devil's face.
Some of these cancer cells are then implanted into the bite wound. There they thrive, growing into new tumors.
It's a process similar to that by which human tumors multiply within a single individual: by shedding cells that move through the bloodstream.
The idea that something like this could be happening via bites, however, is new.
The smoking gun lies in the tumor cells' chromosomes, structures that contain DNA.
Tasmanian devils normally have 14 chromosomes. But the cancer cells contain only thirteen. And those 13 are "grossly abnormal," Pearse wrote in the new study.
More important, she found that the abnormalities were identical in 11 sick animals collected from widely separated regions.
The tumor cells' chromosomes are so similar, Pearse wrote, that all of them must have arisen from the same source.
Presumably, one devil developed the disease several years ago then spread it to its neighbors.
Pearse noted that inbreeding, and the resulting lack of genetic diversity, may make Tasmanian devils particularly susceptible to this type of infection.
Since the animals are so genetically similar, their immune systems may not recognize the new cells as alien invaders that need to be fought.
It's like humans receiving organ transplants, she wrotethe transplant is less likely to be rejected by the body if the new organ is from a close relative. In the case of the devils, however, the transplanted material is cancer cells capable of taking on a parasitic life of their own.
A Devil of a Quarantine
On its Web site, Pearse's agency is trumpeting the study as having "global significance."
That might be an overstatement, but the study is definitely important, says Pat Morris, director of veterinary services for the San Diego Zoo.
Morris thinks that additional work may be needed to completely rule out a viral cause for the disease. "But I believe they are on the right track as far as identifying tumor transmission from animal to animal as the most likely reason," he said.
The Tasmanian government is currently attempting to quarantine healthy devil populations from infected ones.
Officials could not be reached for comment, but on their Web site they note that Pearse's study "provides a high degree of confidence that keeping healthy animals away from sick ones is sufficient to prevent infection."
The officials note, however, that there are "significant practical challenges" to attempting to enforce quarantine in the wild.
Morris believes that these practical difficulties make it wise to begin a captive breeding program as soon as possiblea safeguard in case the wild population is wiped out by the cancer.
Morris notes, though, that unless someone comes up with an easy way to distinguish healthy animals from those in the early, nonvisible stages of the disease, even captive breeding may be impossible. That's because apparently healthy animals could infect an entire population before anyone knows they have the disease.
He also recommends collecting fibroblasts (a type of skin cell) from healthy devils so that, if all else fails, the species might someday be resurrected by cloning.
"That's a dream now, but we expect that at some time science will make it possible," he said.
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