Devil Facial Tumour Disease: The disease decimating Tasmania’s largest marsupial.

INTRODUCTION

Tasmanian devils (Sarcophilus harrissii) are the largest extant marsupial carnivore and are endemic to the island of Tasmania off the coast of Australia. Found throughout the island range, they inhabit open eucalypt environments, coastal scrub and pasture mixed with native sclerophyll forests (Jones and Barmuta, 2000; Pukk, 2005). This sexually dimorphic species (male 7.5-13.0 kg; female 4.5-9.0 kg) are primarily nocturnal, lead hunter-scavenger lives, and usually live between 5-6 years in the wild (Guiler, 1978). In the last two decades the population of devils in Tasmania has drastically declined since the emergence of the fatal transmissible cancer; Devil Facial Tumour Disease (DFTD). Since 1996, the wild devil population has declined by 80% (Lazenby et al, 2018), and in areas where the disease has been present longest, local populations have declined by up to 95% (Lazenby et al, 2018; McCallum et al, 2007). Over the last twenty years, extensive research has been conducted to better understand the nature of DFTD and create conservation solutions in order to protect this ecologically important species.

BACKGROUND

DFTD appears to have evolved fairly recently, first photographed in 1996, with the first pathologically documented case occurring in 1997. No record of the disease appeared in any of the 2000 Tasmanian devil individuals captured between 1964-1995, by researchers conducting capture-mark-recapture studies (McCallum and Jones, 2006).

Since its emergence in the north-eastern peninsula, DFTD is now present in populations across Tasmania. The spread of the disease supports that DFTD has single origin, rather than multiple ones (McCallum et al, 2007). Figure 1. shows how the disease gradually spread south and southwest at a mean rate of 7km/yr^-1 (McCallum et al, 2007). As of 2018, DFTD is prevalent across 80% of Tasmania (Woods et al, 2018).

Transmission of the disease between individuals most likely occurs through biting (Pearse, and Swift, 2006), associated with sexual behaviour and mating, though transmission through sharing food and cannibalism of infected carcasses is possible (Hawkins et al, 2006). Studies of smears from the canine teeth of devils with orally erupting DFTD tumours reported presence of DFTD cells (Obendorf, and McGlashan, 2008). The tumour cells appear to be easily dislodged (Pearse, and Swift, 2006), and with bites most frequently occurring around the mouth and neck matching locations where tumours tend to occur, there is strong evidence for DFTD passing between individuals in this way.

(Woods et al, 2018)

Figure 1. Spread of DFTD. DFT1 was first recorded in the north-east Tasmania (green) in 1996. The first recorded case of DFT2 was separated by distance and time to DFT1 as the first case of DFT2 (red) was in 2014 in south east Tasmania. DFT1 has spread over most of the island, whereas DFT2 is contained to a small area.

PATHOLOGY

DFTD comprises of two independent transmissible cancers, DFT1 (first observed in 1996) has caused the drastic decline of the species since its emergence. DFT2 was first observed as histologically distinct to DFT1 in 2014 (Pye et al, 2016a) though its observed range has been restricted to the Channel Peninsula. DFTD tumours can be caused by two genetically distinct transmissible cancers, that are grossly indistinguishable from one another, but the tumours they cause are histologically distinct (Pye et al, 2016a).

DFT1 is of neuroendocrine origin, the tumour originating in a Schwann cell (Murchison et al. 2010) of a female devil, evidenced by DFT1 having DNA from two homologous X chromosomes (Pye et al, 2016a). The genetic expression of tumours in all DFT1 cases are identical. Its cytogenetic profile is characterised by the absence of both sex chromosomes, both chromosomes 2, one chromosome 6 and a deletion on the long arm of chromosome 1, plus four unidentifiable marker chromosomes [Figure 2.] (Pearse, and Swift, 2006). The tumours only contain 13 chromosomes, unlike the devils 14 (Pearse, and Swift, 2006). DFT1 tumours are aggressive, poorly differentiated, malignant neuroendocrine round cell neoplasms, that primarily affect the face, neck, with frequently occurring metastatic spread to the regional lymph nodes and visceral organs (Loh et al, 2006).