Mapping the disease
Devil Facial Tumour Disease (DFTD) is one of only three recorded cancers that can spread like a contagious disease. The cancer is passed from devil to devil through biting. The live tumour cells aren’t rejected by the animal’s immune system because of a lack of genetic diversity among Tasmanian devils.
For this reason, cases continue to occur in areas where the disease had not previously been recorded. As at February 2011, there has been an 84% decline in average sightings of devils across Tasmania during the annual spotlight surveys. In the north-east region, where signs of DFTD were first reported, there has been a 96% decline of average sightings.
While the majority of diseased populations studied have shown a standard response to DFTD (loss of abundance, loss of older age classes, females breeding earlier), there is a population that has been studied for the past 5 years by a PhD student that is showing a different pattern of responses to the disease.
Although DFTD has spread through this population at the same rate that it is spreading elsewhere, abundance throughout the population has not plummeted, the full age structure is still intact and tumours appear to grow slower, regress and take longer to kill the animal.
The STDP, in collaboration with UTas, will begin trapping a neighbouring and similar population to determine whether this atypical response occurs in more than one population and whether it is the disease or the devil population that is different.
Disease front and DFTD distribution as at June 2012
The STDP continues to monitor devil populations across Tasmania to determine how far DFTD has spread and the impact that it is having.
The disease now affects 75% of the state.
In November 2011 STDP staff and volunteers spent 10 days in the forestry coupes around Takone for this annual activity.
65 animals were caught during the trip, including 63 devils and 2 spotted tailed quolls. There were 38 new devils and 25 recaptured devils.
The 63 devils were made up of 37 females and 26 males.
More than half of the females were lactating.
Three devils were caught with tumours consistent with DFTD, and were given a DFTD score of 5. All 3 animals tested positive.
It seems that the disease is moving further into the population but prevalence (0.05%) is still very low, suggesting the fairly recent arrival of the disease into the area.
The disease front trapping work supports a range of activities including planning for saving populations, managing diseased populations and monitoring any changes in the disease.
Disease front and DFTD distribution - 2010
Following the success of the ‘snapshot' surveys conducted in 2009 to find the westerly extent of DFTD (see below), further surveys were conducted in October and November 2010 to update our knowledge on where the disease front currently lies.
Some lessons were learnt about the efficacy of snapshot-type surveys in areas of varying devil abundance, so the surveys this year were concentrated in an area to the west of the Murchison Highway, between Wynyard in the north and just below Waratah in the south where devil abundance was high.
Areas of low devil abundance will be targeted with the use of remote-sensing cameras.
Four teams conducted 7-day trapping trips, following the same protocol used in 2009.
All animals caught with a possible DFTD tumour had the disease confirmed using histology.
A total of 234 individual animals were caught and inspected for DFTD, with 8 animals confirmed with the disease. A single animal with a high probability of carrying DFTD was located 5km due-west of the most westerly confirmed disease from 2009, but this could not be confirmed. Unfortunately this animal was located at the most westerly extent of the front, suggesting that the disease may have moved as much as 7km in the past year.
The Program hopes to set sentinel cameras on the disease front in the coming months so that disease spread on the front can be monitored throughout the year.
Locating the western extent of Devil Facial Tumour disease - 2009
During September and October 2009, surveys were conducted in western Tasmania with the objective of locating the western extent of Tasmanian devil facial tumour disease (DFTD). Knowing the location of the disease ‘front' is essential to inform other management projects aimed at preserving the species in the long term, including the potential fencing off of disease free areas and the development of free range captive enclosures. Prior to these focused trips, there had been no intensive, focused trapping trips to find the location of the disease front.
Six teams surveyed sites between Burnie in the north and Kelly basin in the south. A roving veterinary team responded to reports of potential disease in captured animals, and collected biopsies and blood samples for confirmation of the disease. As the principle aim of these trips was to catch as many unique animals as possible, it was decided to use a ‘snapshot' technique where traps would be moved to a new site each day to maximise the number of new devils caught and reduce the number of re-captures. In sites of high devil abundance two nights trapping maximised the chances of catching any possible diseased animals. Likewise, in sites of very low devil abundance trapping was extended to two nights to maximise chances of catching any devils in the area. Prior to the beginning of the disease front trips, two 5-day reconnaissance trips were conducted to determine the best areas to locate transects. Map 1 indicates the locations of traps deployed by each team.
A total of 231 individual Tasmanian devils were captured and inspected for presence of devil facial tumour disease. Of these at least thirteen devils have been confirmed as diseased by histology. Three devils were captured by remote cameras clearly showing signs of what appeared to be DFTD - however these cannot be confirmed by histology. The most western extent of DFTD recorded in this survey was approximately 15km west of the last known location of DFTD (Surrey Hills, recorded in 2008). However, the high prevalence of DFTD at Surrey Hills in 2008 suggests that DFTD will have been present further to the west at that time, and the distance travelled from 2008 to 2009 would not have been as great as 15km. The current most westerly point of confirmed disease is just to the east of the Murchison Highway. To date no confirmed cases of DFTD have been recorded west of the Murchison Hwy.
There was an obvious gradient of devil abundance from the north to the south, with 50 devils caught by one team in the north and 11 by another team over the same period of time in the south. The very different habitat and the difficulty of access in the southern regions makes it difficult to catch devils and even harder to locate the disease. Our ‘snapshot' method of trapping for these trips will need to be revised for these areas of lower devil abundance before we re-trap in any of these areas.