Managing an emerging disease in a threatened species: Tasmanian devil facial tumour disease
The emergence of new infectious diseases is increasingly being recognised as a significant threatening process in conservation biology. We present a decision tree, based on Tasmanian devil facial tumour disease, for handling novel diseases in wildlife, prioritising steps that should be taken with imperfect knowledge and that do not rely on potentially time-consuming steps of defining the aetiology of the disease or development of a vaccine or treatment. The Tasmanian devil, formerly common and as the largest surviving marsupial carnivore an iconic species, has recently been placed on threatened species lists. A disfiguring and invariably fatal facial cancer, first reported in 1996, has now spread across most of the range of the devil, leading to population declines of up to 90% and a prognosis of likely extinction in 20-25 years. Transmission experiments have confirmed that the cancer is infectious and genetic evidence strongly suggests that it is a transmissible cell line. Limited management options include establishing isolated "insurance" populations, barriers to disease spread, and culling. Captive “insurance” populations have been established and establishing free-ranging disease-free populations on offshore islands is now being investigated. A disease suppression trial, in which all infected animals captured are removed, is underway on a large peninsula connected to Tasmania by a bridge. Results show that the removal program has influenced the progression of the tumour epidemic. The stage structure of the tumour population has changed, with fewer large tumours now being found. In diseased populations without disease suppression, very few individuals older than 2 years are captured, whereas in this manipulated population, the age structure is similar to that of undiseased populations. The rate of geographic spread of the disease appears to have been slowed. Whether eradication can be achieved remains uncertain. We present a model structure to evaluate the effectiveness of this removal program.