Sympathy for the devil

Scientists working on trying to control the facial tumour disease which threaten to wipe out the Tasmanian devil (Sarcophilus harrisii) have increased their understanding the disease. The Tasmanian devil is the largest carnivorous marsupial remaining and is now found only on the island of Tasmania, having been exterminated from the Australian mainland. The disease that is threatening this endangered species is a transmissible neoplasm that grows on the face leading to death from starvation. These creatures, immortalized by the Looney Tunes cartoon  character ‘Taz, are about the size of a small dog and have a predisposition to aggressive behaviour, as well as unearthly screeching and raucous communal feeding that has earned them their name. Their aggressive behaviour is probably the most likely way in which the disease is transmitted, through biting as they fight over food and mates.

Devil facial tumour disease was first seen in 1995 and has caused between 20-50% reduction in the devil population. More than half of the State of Tasmania is affected. Two ‘insurance’ populations of disease-free devils are being established at an urban facility in the Hobart suburb of Taroona and on Maria Island off the east coast of Tasmania. The decline in devil numbers is an ecological problem, since its presence in the Tasmanian forest ecosystem is believed to have prevented the establishment of the Red Fox, illegally introduced to Tasmania in 2001. Foxes are a problematic invasive species in all other Australian States, and the establishment of foxes in Tasmania may hinder the recovery of the Tasmanian Devil.

The scientists from the University of Sydney School of Veterinary Science working on the disease are saying that the tumour probably arose from a single individual and has spread on to other devils. The immune system of the original animal probably did not recognize the tumours as foreign, and because Tasmanian devil population is now so genetically similar, their bodies do not recognize that the tumours are foreign cells and so do not attack them. Tasmanian devils have lost genetic diversity in the most important gene region for the immune system, the Major Histocompatibility Complex (MHC). The devils all had a similar MHC type to the tumour. There appears to be some similarities with a transmissible cancer of dogs (canine transmissible venereal tumour).

There is a pilot program on the Tasman Peninsular where all the diseased devils captured are killed. There is currently no test for infected animals that do not yet show the neoplasm, so it is only those showing visible signs of the disease that are removed. There seems to be some evidence that this is working to protect the overall population, because, over time, the average age of the animals they capture has become older, suggesting that more animals are avoiding the cancer and surviving longer. But the program is expensive, and it is not clear how effective the programme will be in stopping the disease.

The problem seems, then, to be a result of the lack of genetic diversity in a small population. There are many other endangered species of wildlife that have populations as small or smaller and could suffer the same fate, falling to new or emerging diseases. Looking through the CAB Abstracts Database to see if there was any other information on biodiversity threatened by disease, I came across a paper by Maillard and Gonzalez entitled Biodiversity. The problems caused by lack of genetic diversity in making the population open to infectious organisms are described. The disease in the Tasmanian devil shows that certain neoplasms can also exploit the restricted gene pool.

Maillard, J. C.; Gonzalez, J. P. 2006. Biodiversity and emerging diseases. Annals of the New York Academy of Sciences, 2006, Vol. 1081, pp. 1-16 Record No: 20073114695

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