The Tasmanian devil, a marsupial native to Tasmania, faces a significant threat from an aggressive illness. This condition, known as Devil Facial Tumor Disease (DFTD), has had a severe impact on the wild devil population, raising questions about the nature of the disease and its potential reach.
Understanding Devil Facial Tumor Disease
Devil Facial Tumor Disease is a rare, clonally transmissible cancer, meaning the cancer cells themselves act as the infectious agent. Unlike cancers caused by viruses, DFTD spreads through the physical transfer of living tumor cells from one animal to another. This transmission primarily occurs when the animals bite each other, a common behavior during feeding and mating disputes.
Once an animal is infected, large, ulcerating tumors develop on its face and inside its mouth, which interfere with the devil’s ability to eat. As the tumors grow, they often destroy the jaw, and the affected animal succumbs to starvation. Most devils die within three to six months of the first appearance of these tumors, which has led to a population decline of over 70% in some areas.
The Risk of Transmission to Humans
There have been no recorded instances of Devil Facial Tumor Disease spreading to humans. It is not considered a zoonotic disease, which means it does not naturally transmit from animals to people. The primary reason for this is the “species barrier.” The cancer cells of DFTD are genetically Tasmanian devil cells, and the human immune system is adept at eliminating cells from a different species.
The genetic difference between humans and marsupials is immense. When foreign cells from a Tasmanian devil enter a human body, they are identified as “non-self” by the immune system. This triggers a defensive response designed to destroy the invading cells before they can replicate. The cancer cells cannot survive in a human host, whose biology is incompatible with their own.
Transmissible Cancers as a Biological Phenomenon
Contagious cancers are a rare occurrence in the natural world. DFTD is one of only a few known examples where the cancer cell line itself is the agent of transmission. Another documented case is the Canine Transmissible Venereal Tumor (CTVT) found in dogs. This cancer spreads between dogs during mating and has existed for thousands of years, having evolved to a state where it can be transmitted without always killing its host.
It is important to distinguish these contagious cancers from cancers caused by transmissible pathogens. For instance, the Human Papillomavirus (HPV) can be transmitted between people and may lead to cancer, but it is the virus that is contagious, not the cancer cells. In the case of DFTD and CTVT, the cancer itself behaves like a parasite, moving from one host to another. These instances remain exceptional cases.