Tasmanian Devils and Devil Facial Tumor Disease (DFTD)
The Sad Case of an Endangered Marsupial
The Tasmanian devil is the largest carnivorous marsupial in the world. It’s known for its loud screams, shrieks, and snarls. Sadly, it’s an endangered animal. A major reason for this population status is a form of cancer that produces tumors on the animal's face. The disorder is known as devil facial tumor disease, or DTFD. The cancer is contagious and is transmitted when one animal bites another’s face, as may happen during mating and feeding. There is no known cure for the disease. It's always fatal.
In 2017, researchers reported that immunotherapy helped a small group of Tasmanian devils by either preventing tumor development or causing existing tumors to regress. More research and trials are needed, but immunotherapy might eventually be a helpful treatment or prevention strategy for devil facial tumor disease.
An Introduction to the Animal
Tasmanian devils generally don't attack people. Like most wild animals, however, they would probably do so in self defence and could inflict a nasty injury.
The Tasmanian Devil
The Tasmanian devil has the scientific name Sarcophilus harrisii. It lives only in Tasmania. The animal has a stocky build and is about the size of a small dog. It’s about 0.3 meters or twelve inches high at the shoulder when fully grown and about 0.6 meters or two feet long. Adult males weigh around fourteen kilograms or thirty pounds and adult females a little less.
The devil is mostly black in color but often has a white patch on its chest and side or on its rump. It may also have patches of brown hair. The animal has a big head, powerful jaws, and a large nose. Its ears are often noticeably pink or red on their inner surface. They may turn an even deeper shade of red when the animal is upset.
The devil has a reputation as a fierce creature with respect to its behaviour towards other members of its species. Its growls, barks, and screams while feeding support this reputation. The sounds may be bone-chilling for humans. A loud and threatening sneeze is also part of the animal's repertoire and is used to establish dominance.
The devil's behavior is sometimes misunderstood. Some of the sounds that it produces prevent a fight with other animals instead of triggering one. Parks and Wildlife Service Tasmania says that most bites are linked to reproduction and that bites during feeding are rare.
Tasmanian devils vary enormously in personality. Some individuals are very calm and tolerant, others very excitable.— Parks and Wildlife Service Tasmania
The Save the Tasmanian Devil Program
The Tasmanian devil is often referred to as simply a "devil". It's affectionately known as a Tassie or a Tassie devil. (Yes, some people do look upon the animal with affection.)
Tasmanian devils live in a wide variety of habitats. They seem to enjoy water and are good swimmers. They are usually nocturnal. During the day they hide in places such as burrows, thick bush, hollow logs, and caves. A large rock or stone may also provide suitable shelter. The animals may leave their shelter during the day to sunbathe, though they try not to attract attention while they do this. During the night the devils search for food. They may travel ten to twenty kilometres (six to twelve miles) in a night. They have a home range but don't maintain a territory. They are solitary animals but sometimes meet other devils while feeding.
The animals are mainly scavengers but also hunt prey, including frogs, lizards, ground birds, small mammals, and insects. Their sense of smell is excellent and very helpful in the hunt for food. They produce a strong and unpleasant smell themselves when they are stressed. They also have good hearing. The strong jaws and teeth of the devils enable them to eat the entire body of many animals, including the bones. When they are well fed, fat is deposited in their tail. The animals play a useful role in their environment because they remove the carrion that attracts insects.
An open mouth display is often not what it seems. The animal above is likely displaying uncertainty or fear instead of aggression.
The Tasmanian devil is a marsupial, which means the babies are born in a very immature stage and develop in the mother’s pouch. The gestation period is around three weeks. The babies are called imps or joeys. When the imps are born, they are as small as a grain of rice. They are pink in color and have no hair. They have to crawl over their mother's body to reach the pouch, whose opening faces backwards. Up to fifty imps enter the pouch, but only four teats are available. An imp grabs hold of a teat with its mouth and stays attached to it as it grows. The imps that don't reach a teat die.
The successful youngsters stay in the pouch for about four months as they complete their development. When they emerge, their mother often carries them around on her back until they get too big for this form of transport. The devils can climb trees while they are small. This task is difficult for the adults, however. The youngsters become independent from their mother around five months after leaving the pouch.
The animals are reproductively mature at around two years of age. Their normal lifespan appears to be five to eight years. Wild devils generally live for a much shorter time than this at the moment due to devil facial tumor disease.
Although the wild devil population is in serious trouble, captive and healthy animals exist in various facilities. Breeding programs have been established in some places.
Moments in a Tasmanian Devil's Life
Devil Facial Tumor Disease
Devil Facial Tumor Disease was discovered in 1996. An affected animal develops large, irregularly shared lumps on its face and head. Lumps may partially or completely cover an eye and may also appear inside the mouth. Animals with the disease live for only six to twelve months once the tumors appear. They often die due to starvation because the tumors around their mouth prevent them from eating.
The disease is transmitted by living cancer cells being passed from one animal to another. When cells from another individual's body enter a recipient, the recipient's immune system normally recognizes that an invader is present and attacks the cells. For some reason, this doesn't happen in DFTD. The devil's immune system remains quiescent and the cancer cells are able to multiply.
No treatment or effective vaccine for DTFD exists at the moment. Researchers are studying the disease in an attempt to help Tasmanian devils. Some interesting discoveries have been made, but more research is necessary.
Researchers think the devil's immune system isn't activated because the cancer cells don't produce MHC molecules. "MHC" stands for major histocompatibilty complex. MHC Class 1 molecules are glycoproteins (proteins with carbohydrate attached) found on the surface membranes of cells with a nucleus. The molecules help the immune system to distinguish self from nonself. MHC molecules are involved in the fight against pathogens and in the rejection of a tissue transplant from a genetically-different individual.
Function of MHC Molecules
MHC molecules display a small piece of protein known as a peptide, which is obtained from inside the cell. This peptide may be a normal component of the cell or it may be abnormal, such as a peptide obtained from a virus or bacterium that has infected the cell. Certain T cells in the immune system have already been primed to recognize problematic peptides that have entered the body. An appropriate T cell "finds" a dangerous peptide on an MHC molecule by binding to it. Another type of T cell then destroys the cell displaying the peptide.
Transmission of DFTD cells from one devil to another is a type of allograft (a tissue transplant from one member of a species to another member who is genetically different). We would expect the recipient's body to recognize that the tissue doesn't belong in the body because the MHC molecules on its cells display the wrong peptides. Since the devil cancer cells have no MHC molecules on their surface, however, there is nothing for the T cells to bind to and they don't recognize that the tissue is harmful.
The devil's immune system does recognize allogeneic skin grafts and at least some types of cancer cells originating outside their body. There seems to be a specific problem with DFTD cells that prevents the animal's immune system from attacking them.
Researchers have found that DFTD cells contain the genes needed to make MHC molecules, but the genes are inactive. Activating the genes in the cancer cells might be helpful in preventing or treating the disease.
Immunotherapy is the modification of the immune system's action in order to treat a disease. The immune system may be enhanced in some way or it may be inhibited. In 2017, a team consisting of multiple researchers reported the use of immunotherapy in Tasmanian devils. Due to the endangered status of the population, the researchers couldn’t use many animals in their project. The results of the experiment could be significant, however.
The research involved nine healthy animals, some of which were at an "advanced" age. This may have affected the results of the experiment. The experiment lasted for five years. The immunization consisted of the administration of modified DFTD cells that had been triggered to develop MHC molecules. Encouraging effects of the immunotherapy appeared in some of the animals.
- One of the immunized animals didn't develop tumors after exposure to unmodified DFTD cells.
- Six of the animals developed tumors when exposed to unmodified DFTD cells before immunization. When they were later immunized by modified DFTD cells, the tumors regressed in three of the animals. The regression was accompanied by the formation of antibodies to the cancer cells.
Two animals in the experiment were never immunized. One was given an adjuvant (a substance that was used to boost immunity) while the other received no special treatment. These animals were used as controls. Controls are used in experiments in order to prove that a factor that is being tested—in this case, modified cancer cells—is the cause of any benefit that is observed.
Problems With the Experiment
Though the results of the immunotherapy experiment are intriguing, the sample size was small and the age of some of the animals wasn't ideal. The animals were aged five to seven at their entry into the experiment, which meant that at least some were near the end of their natural lifespan. The fact that there was some success as a result of the experiment is a hopeful sign, though.
One issue that might be a problem for some people is that healthy animals were exposed to the devil facial tumor disease during the experiment. A discussion about the ethics and value of deliberately producing a disease in a healthy animal would be a long one. I think it's an important topic to consider, however.
Efforts to manage the devils, such as the development of an immunotherapy, are ongoing, but remain in a research-and-development phase.— David Pemberton, Save the Tasmanian Devil Program
Tasmanian Devil Joeys
An Uncertain Future
The fact that healthy Tasmanian devils are present and reproducing in captivity gives us a safety net for the species. It also allows people to see the animals up close, which may promote public concern. Life in captivity isn't ideal for the animal, but captive animals are creating a population that could be released into the wild. The strategy won't be helpful if the released animals develop cancer, however.
A population analysis published in early 2018 reported that the population of Tasmanian devils is apparently still decreasing. Exact data isn't available, but some researchers say that the population has decreased by around 70% overall since DFTD first appeared and by around 90% in some areas. They say that the animal may be extinct in the wild in twenty to thirty years unless it's helped.
In certain areas the number of animals is currently not as bad as might have been expected due to an interesting phenomenon. The animals in the area are rarely living beyond the age of two due to the disease. The age of reproduction has decreased, however. Now females as young as one are having babies, which replace the older animals who have died.
Tasmanian devils have to survive road traffic and habitat loss. For small devils, predation by eagles or owls or by a quoll (another type of carnivorous marsupial) is a danger. Surviving devil facial tumor disease is the biggest problem for the devils, however. We need to find a way to overcome the disease. The Tassie devil may be a strange animal by human standards, but I think it's worth saving this unique member of the animal kingdom.
Tasmanian devil facts from the San Diego Zoo
Information about Tasmanian devils from the Save the Tasmanian Devil website (which is run by the Tasmanian government)
FAQs about Tasmanian devils from Parks and Wildlife Service Tasmania
The major histocompatibility complex from Garland Science and the National Institutes of Health (NIH)
Cell-mediated immunity from John W. Kimball (a retired Biology professor and textbook creator)
Tovar, C. et al. Regression of devil facial tumour disease following immunotherapy in immunised Tasmanian devils. Sci. Rep. 7, 43827; doi: 10.1038/srep43827 (2017).
DFTD information from the Transmissible Cancer group at the University of Cambridge
San Diego Zoo Global. (2018, February 12). Tasmanian devil populations continue to decline: Devil facial tumor disease poses ongoing risk for wild populations. ScienceDaily. Retrieved February 13, 2018 from www.sciencedaily.com/releases/2018/02/180212170015.htm
© 2018 Linda Crampton