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For those of us who grew up watching Saturday morning cartoons, Warner Bros.' character Taz, the omnivorous, whirling, swirling cartoon caricature of a Tasmanian devil, was a delight. Already long on the endangered species list, however, the real Tasmanian devil (Sarcophilus harrisii) now risks extinction because of a strange form of contagious cancer.
A carnivorous marsupial about the size of a small dog, the devil is found only on the island of Tasmania, located off the southeast coast of Australia. The disease, which was first documented in the 1990s, has been killing the animals at an alarming rate. It attacks young devils at 2 to 3 years of age — long before they have an opportunity to reproduce. In areas where the disease has been found, up to 90 percent of the population has been decimated. The devils' rapid decline has led to aggressive — but so far only marginally effective — efforts to save them.
We do not typically think of cancer as being contagious, but dogs, like the devil, suffer from a contagious canine tumor: transmissible venereal tumor, or TVT. The disease has been around for 11,000 years. Devil facial tumor, or DFT — the cancer affecting Tasmanian devils — emerged in 1996 and is believed to have decreased the number of devils in the wild by more than half, with extinction predicted by 2024 unless a cure is found. Devil facial tumor is highly metastatic and always fatal. The dog is more fortunate; its transmissible tumor spontaneously regresses, or disappears, and renders the dog permanently immune to the disease. If a TVT lesion should cause a dog problems, it can be successfully treated with chemotherapy.
Devil facial tumor arose in an unknown Tasmanian devil many generations ago. Scientists describe the cancer cells in both DFT and TVT as “clonal,” meaning that all the tumor cells are genetically identical, like maternal twins. However, the tumor cells differ vastly from normal cells. Normal Tasmanian devil cells have 13 chromosomes; devil facial tumor cells have 14 chromosomes. A similar situation exists in TVT in dogs. Normal dog cells have 78 chromosomes; TVT cells have 57 to 64 chromosomes.
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