Overview of Tasmanian Devils
The Tasmanian devil is the largest marsupial carnivore in existence, currently restricted to the island-state of Tasmania, Australia. Devils have black fur, and white flashes on the chest and rump may be present. They are sexually dimorphic, with males having a thicker neck and broader head than females. Males typically weigh 9–12 kg (up to 14 kg), and females 6–8 kg (up to 9 kg). Devils are mainly nocturnal and hide during the day in rock dens, log cavities, or underground burrows made by other animals. They live up to 6 yr in the wild and 9 yr in captivity. Devils are nonterritorial and generally live within an area of 10 km2. They can occupy a wide range of habitats, from dry sclerophyll forest, open eucalypt environment, and coastal woodland, to pasture and agricultural areas where carrion (from domestic livestock and macropod populations) is abundant. Devils are specialist carrion feeders but will hunt prey, particularly those weakened by disease, injury, or old age. Wallabies, wombats, and sheep are the usual source of carrion, but other dead domestic livestock, roadkill, and 1080-poisoned wildlife are also consumed. The proportion of hunting to scavenging is unknown. Female devils are facultative polyestrus with up to three estrous cycles within a breeding season, each cycle ~60 days apart. They are polyovular (up to 114 oocytes per ovulation) and may give birth to up to 40 embryos. Mating peaks over late February to the end of March and, as is typical for marsupials, females give birth to highly undeveloped young ~3 wk after mating. Females have a rear-facing pouch, with four teats in the pouch cavity, limiting the maximum total offspring raised per year to four. The young are carried in the pouch until they are 4–5 mo old, weaned at 5–8 mo, and become independent at 10–12 mo. Sexual maturity is reached at 2 yr of age for both females and males, although females have been confirmed to reproduce as young as 1 yr old.
Wild Tasmanian devils can be restrained and examined in a hessian sack by an experienced handler. Devils have an exceptionally strong bite for their body size, and handling conscious animals must only be done by or attempted in the presence of experienced personnel.
The handler sits on the ground and locates and holds the devil’s muzzle closed from outside the sack. The devil is then positioned between the handler’s legs, with the devil facing outward and head uppermost. The head and face are gently revealed with one side of the sack left covering the devil’s eyes. The handler then moves the free hand directly onto the muzzle, continuing to hold it closed, and the teeth/mouth can be examined to estimate age of the devil by lifting the upper lip. The mouth can be opened for intraoral examination. It is important to visualize all aspects of the oral cavity, including under the tongue, for signs of tumors. Normal lymph nodes are not palpable in devils; therefore, subcutaneous masses should be considered suspicious. If a blood sample is required, the devil is positioned as described, ie, facing away from the handler. An assistant can collect up to 12 mL of blood from the jugular vein. The jugular vein is not visible or palpable; to locate it, the index finger of one hand palpates the groove immediately cranial to the clavicle and just lateral to the sternohyoideus muscle. The finger pulls the skin distally and, with the other hand, a 21-gauge needle is inserted 0.2–0.5 cm into the groove while applying negative pressure to the syringe. Gentle digital pressure is applied to the venipuncture site after blood collection is complete. The devil can then be placed back into the sack, with the head facing up and away from the handler, so the tail, cloaca, and pouch can be examined.
Devils bred in captivity or brought to captivity before 2 yr of age are generally not amenable to handling, so general anesthesia is required for examination. These animals can be trapped in their enclosure and transferred to a hessian sack or picked up by the tail base and transferred to the sack. Anesthesia can be induced by using a mask on the devil through the sack. Isofluorane has been used successfully. Ideally, the devil’s muzzle is positioned in a corner of the sack to facilitate mask placement. The sack should be secured around the devil to prevent excessive movement and escape from the mask. Devils are induced in 2–3 min and then removed from the sack. A pulse oximeter can be attached to the ear to monitor heart rate and peripheral oxygen saturation. For quick procedures such as physical examination and blood collection, a mask is sufficient to maintain anesthesia. For longer procedures, an endotracheal tube (5–5.5 Fr) is indicated. The devil’s larynx is narrow, and a laryngoscope is essential. Local anesthetic must be applied to prevent laryngospasm before intubation. As the devil starts to recover from anesthesia, it is placed back in the sack and observed until it is fully awake and ready to be returned to its enclosure.
Tasmanian devils are prone to developing proliferative lesions, the most common being devil facial tumor disease (DFTD). Cell proliferations (benign and malignant) affecting skin and adnexa, and endocrine and lymphoreticular tissues (eg, squamous cell carcinomas, lymphosarcomas, adenocarcinomas) are commonly seen at necropsy in adult and senile captive devils. In older captive devils (≥5 yr old), mammary and anal gland tumors are the most common.
Devil facial tumor disease (DFTD) is a malignant infectious tumor first observed in 1996. As a result of >70% of the population having succumbed to DFTD, devils have been listed as an endangered species. The prevalent characteristic of DFTD, as the name suggests, is the presence of tumor(s) on the facial area. Tumors are also found inside the mouth (gingival mucosa, hard palate, lips) and on the head and neck. Rarely, DFTD tumors may be found on the rump or other areas of the body, but these are thought to be metastatic tumors rather than primary. More than one tumor is often seen, and tumors on the same devil often vary considerably in size and appearance. The tumors often ulcerate and display epithelium break up, necrosis, exudation, and bacterial contamination. Most devils affected with DFTD show signs of metastases (65% of cases in a study of 91 affected devils), principally to the regional lymph nodes but also to the lungs and spleen.
The most remarkable characteristic of DFTD is its transmissible nature, a concept supported by several independent lines of research. The tumor is of Schwann cell origin and is transmitted by biting. Devils with tumors in the oral cavity have free tumor cells “coating” the canine teeth. The tumor cells are transplanted when a bite wound penetrates the subcutaneous tissue or mucosa of the next devil. The most common bite site in devils is the head, which, therefore, is where almost all DFTD tumors are present. The incubation period is unknown but is estimated to be 3–15 mo. Most devils die within 6 mo of a tumor becoming grossly visible. Death results from starvation, depending on the size and location of the tumor, or from metastatic disease. No evidence of this disease in other species has been found.
No preclinical diagnostic test is available. The tumor cells have a characteristic and consistent karyotype, showing loss of three autosomes, loss of both sex chromosomes, and addition of four marker chromosomes. Histologically, DFTD tumors share the same morphologic features as a high-grade malignant tumor composed of diffuse sheets of small, rounded to polygonal shaped cells with ovoid hyperchromatic nuclei. Mitotic figures are frequent, and eccentrically located nuclei result in a plasmacytoid appearance. Tumor cells might appear cohesive and epithelioid in appearance and form elongated trabecular and solid alveolar structures. The gold standard diagnostic test for DFTD tumors is immune-staining with periaxin. It is common for devils to develop other cell proliferations, which must be differentiated from DFTD.
There is currently no known treatment for DFTD; vincristine and doxorubicin have been tried without tumor reduction, and to date surgery has not been successful. Radiation therapy has not been assessed. Research into a vaccine is underway.
The only other tumor transmitted by direct cell implantation is the canine transmissible venereal tumor (CTVT, see Canine Transmissible Venereal Tumor) of dogs. Although CTVT is generally a benign tumor, it shares the same immune-escape mechanism as DFTD; the tumor cells "down regulate" major histocompatibility complex (MHC) molecules on the cell surface, so that antigen presenting cells do not recognize the tumor cells. In dogs, the immune system might overcome this by producing cytokines and “forcing” the tumor cells to express MHC and become targets for cytotoxic T cells.
This lymphoid skin tumor has been recently seen in both captive and wild caught devils, affecting predominantly older females. No etiologic agent has been implicated. It can present as a localized erythematous lesion, mainly in the limbs and ventral part of the body, or as a generalized alopecic and exudative form. This condition progresses over several months. There is no known treatment.
Tasmanian devils are susceptible to both Demodex sp infection and the sarcoptiform Diabolicoptes sarcophilus. The latter is common and can be treated with ivermectin 200 mcg/kg, SC, every 4 wk for 3–4 mo. Wild Tasmanian devils often harbor the dasyurid-specific flea Uropsylla tasmanica. This unusual flea has all stages of its life cycle on the host. The larva is commonly found embedded in the subcutis of the tail, limbs, or ventral area. Adult fleas are less common.
The two most common GI parasites are the nematode Baylisascaris tasmaniensis and the cestode Anoplotaenia dasyuri. If the cestode burden appears excessive in captive devils (visible proglottids on feces), they can be treated with praziquantel (5 mg/kg, SC).
A common condition in aged (≥5 yr old), captive Tasmanian devils has been described as a degenerative leukoencephalopathy and myelopathy. It manifests by a progressive swaying of the hindlegs, eventually worsening to paresis and paralysis in several months. Histologically, it is characterized by axonopathy and vacuolation of the spinal cord. Intervertebral disc disease is a common finding. There is no known treatment.