Taxonomy

There are six orders within the subclass Marsupialia: Dasyuromorphia (carnivorous marsupials), Peramelemorphia (bandicoots and bilbies), Diprotodontia (koala, wombats, possums, and macro-pods), Notoryctemorphia (marsupial mole), Didelphimorphia (opossums), and the Pauci-tuberculata (shrew opossums). Marsupial species number approximately 260 and are mainly located in Australasia with some species in North and South America.

Anatomy, Reproduction, and Physiology

Marsupials (metatherians) are a unique group of animals that are characterized by a very short gestation, the birth of a relatively undeveloped baby (blind, hairless, well-developed forelimbs, partially formed hind limbs), and an extended period of lactational development that commonly occurs in a pouch (often supported by the epi-pubic bones). Marsupials have a lower basal metabolic rate when compared to placental mammals (eutherians). Basal metabolic rates of marsupials are generally 65-74% of the value expected from an equivalent body mass in eutherians (basic marsupial metabolic rate (kcal/d) = 70.5 x body weight.^0.49)This reduction in basal metabolic rate results in their core body temperatures also being lower.For example, the North American opossum (Didelphis virginiana) has a mean body temperature of 35.2°C.

Diets for marsupials are as varied as the species, but knowledge of the gastrointestinal tract can aid in making recommendations. Kangaroos, wallabies, and rat-kangaroos are all foregut fermenters. The forestomach is divided into a blind sac (the sacciform forestomach) and a tubular region (the tubi-form stomach). Microbial fermentation occurs throughout both regions of the forestomach. The browsers have a relatively larger sacciform region and the grazers have a relatively larger tubi-form region. Wombats are hindgut colon fermenters (the cecum is vestigial and the proximal colon is capacious. Arboreal folivores (e.g. ringtail possum, brush-tailed possum, and koala) are hindgut cecum ± colon fermenters. Eucalyptus foliage eaters detoxify volatile oils in the liver and excrete the by-products in the urine conjugated with glucuronic acid.

The female reproductive tract has two ovaries, two oviducts, and two completely separate uteri and cervices. The uteri do not fuse because the ureters course medial to them. Each uterus opens into a vaginal cul-de-sac, separated by a median septum, from which spring two separate lateral vaginas. The lateral vaginas empty into the urogenital sinus. This sinus then connects to the cloaca, which also receives the urinary and gastrointestinal tracts. The lateral vaginas are separated by the urogenital strand (a mass of fibrous connective tissue that forms a pseudovaginal canal at parturition) connecting the vaginal cul-de-sac to the urogenital sinus. In most macro-pods, opossums, and the honey possum (Tarsipes rostratus), this becomes permanent and epithelialized after the first parturition. In most other marsupials, the pseudovaginal canal disappears and then reforms at the next parturition. The prostate (proportionally very large in marsupials) and the bulbourethral glands are the only accessory glands in males.

Marsupial milk composition changes throughout lactation. For hand-rearing joeys, feed 10-20% of the body weight in milk daily; feed every 2 hours until fine-furred; every 3-4 hours until they first leave the pouch; and once every 6 hours thereafter. Maintain the Joey at 32-34°C. Joeys require stimulation to defecate and urinate after every feeding. Diarrhea is the most common problem in hand-reared joeys. Check that the milk composition and volume is correct. Give oral electrolytes initially and then slowly reintroduce milk. Candidiasis is also common cause of diarrhea and can be treated with oral nystatin. Salmonella and Campylobacter can also cause diarrhea in joeys. Coccidia and nematodiasis may be problematic when joeys are weaned onto contaminated pasture.

Manual and Chemical Restraint

All marsupials like to bite and scratch. Manual restraint of sugar gliders is not difficult. Hand-raised gliders will allow themselves to be picked up in cupped hands, but if startled will leap and run at amazing speeds. Gliders can bite and are best restrained by holding the head between the thumb and middle finger and placing the index finger on top of the head. The body of the animal is palmed. Alternatively, place a small cloth bag over the restrainer's hand (seams inside). The animal is palmed and the bag pulled off the hand and over the animal (seams outside to avoid entanglement of frayed edges with the animal).The animal can be weighed in the bag, body parts can be exposed for examination, or the head exposed for anesthesia induction using a facemask.

An experienced handler can manually catch and restrain macro-pods of up to 36 kg (80 lb). One or two hands are used to catch the base of the tail. After the tail is controlled, one arm is swung round the animal's chest just below the forelimbs. The macropod is lifted off the ground and the tail is angled forward between the legs. Care is taken to prevent the hind limbs or the ventral tail coming into contact with any surfaces or the handler. A macropod held in this manner cannot get leverage for escape. If needed, the animal can be tipped head first into a large denim or hessian sack for transport, weighing, or to allow blood collection from the lateral tail vein.

Isoflurane using a facemask as an induction chamber can be used for many small marsupials (e.g. sugar glider). Intubation is difficult in small species and possibly not warranted due to the high risk of blockage of the endotracheal tube with respiratory secretions. For larger marsupials, if safely restrained, masking with isoflurane has been used, but injection with medetomidine and ketamine IM followed by intubation is a better practice. For surgical or painful procedures, additional multi-modal pre-emptive analgesia is recommended.

Macro-pods are prone to trauma-related injuries and capture myopathy during restraint. Therefore a rapid and smooth induction and recovery are advantageous. Medetomidine (0.04-0.07 mg/kg) mixed with ketamine (4-7 mg/kg) administered IM has been used successfully in a range of macro-pods. Reverse with atipamezole IM at a dose 5 times the mg of medetomidine administered.

Intubation of macro-pods is difficult due to the limited access caused by a narrow mouth that does not open widely. Intubation can be attempted with the macropod in dorsal recumbency and the head fully extended over the edge of a table or with the macropod sitting up with the head and neck fully extended. Both methods align the glottis with the angle of the mouth making the passage of a straight semi-rigid endotracheal tube easier. A long-bladed laryngoscope (15 cm, 6 inches) is used to visualize the glottis. Visualization of the glottis can be difficult once the tube is in the pharyngeal area, but if the head is well aligned with the glottis, this loss of final visualization does not cause much of a problem. If difficulties are encountered, then a smaller diameter tube (e.g. 6FG dog urinary polypropylene catheter) can be passed with visualization into the trachea first and then the endotracheal tube blindly threaded over the top of this. Isoflurane is used to maintain anesthesia.

Regurgitation under anesthesia is rare in marsupials (occasionally seen in macro-pods), so preanesthesia fasting is typically not necessary other than to ensure the mouth is free of food for intubation.

Diagnostics

Blood can be collected from the lateral tail vein (especially macro-pods), the cephalic vein, femoral vein, jugular vein, the ventral coccygeal vein, and the medial metatarsal vein. Lymphocyte percentages commonly exceed neutrophil percentages in healthy marsupials.

Surgery

The most common surgeries in marsupials are neuters and trauma-related wounds. Castration of sugar gliders is best achieved in young animals using methods that "cut and coagulate" (laser, electrocautery, or radiosurgery). Remove the testes and scrotum by transecting the scrotal stalk approximately 2 mm from the body wall. Use of traditional surgical methods, leaving the scrotum, and performing the surgery in older animals appears to result in a higher incidence of post-surgical wound self-mutilation. Ovariohysterectomy can be performed in sugar gliders, but magnification and familiarity with the anatomy and microsurgical techniques are recommended and there is a risk of post-surgical wound self-mutilation. Intact hand-raised male kangaroos are notorious for harming humans after puberty is reached. Castration of these animals pre-puberty may reduce the development of aggressive tendencies, but anecdotally early castration may be linked to an increased incidence of long bone fractures.

Common Diseases of Sugar Gliders and Opossums

The most common diseases of sugar gliders include nutritional osteodystrophy (metabolic bone disease and hind limb paresis / paralysis), obesity, social / stress-related behavioral disorders, dental disease, trauma, and bacterial diseases. Correction of the diet and supplementation with calcium to correct calcium / phosphorus deficiencies / imbalances are the recommended treatments for nutritional osteodystrophy. Obesity is corrected with exercise and dietary modifications. Periodontal disease and tooth root abscesses are typically associated with the feeding of soft, sticky, sugary diets (often high in fruit). Increasing crunchy, teeth-cleaning foods like insects and raw vegetables can help prevent dental disease. Sugar gliders are normally social animals living in groups of 5-12 animals in the wild with close to equal numbers of males and females. Solitary sugar gliders make poor pets, as their social needs are rarely met, resulting in abnormal behaviors including self-mutilation, aggression, eating disorders, and stereotypic behaviors. Salmonellosis has been reported in sugar gliders with some gliders acting as carriers and therefore creating a zoonotic risk. Clients should be warned to practice good hygiene when handling their pets.

Gliders and possums are also susceptible to Tyzzer's disease (Clostridium piliforme), which can present as sudden death or diarrhea, depression, and anorexia. Necrotizing hepatitis and myocarditis are seen. Possums are susceptible to yersiniosis (Yersinia pseudotuberculosis) causing depression, dehydration, and diarrhea with enteritis, septicemia, and multifocal hepatic, splenic, and renal abscessation. Prevent by restricting wild bird and rodent access. Chaga's disease (Trypanosoma cruzi) causes no clinical signs in opossums but the organisms can be found in their blood, skeletal and heart muscle, kidney, intestine, urine, and skin. Besnoitiosis (Besnoitia darlingi) is seen in the southern opossum and is caused by the ingestion of cat feces. There are no clinical signs, but tissue cysts are found in the skin, skeletal muscle, tongue, heart, lungs, and adrenal glands. Opossums are the definitive host for Sarcocystis neurona and as such develop no clinical signs, but the organisms can cause disease in other animals (e.g. horses, equine protozoal myeloencephalitis). The spirurida, Turgida turgida, is seen in opossums (intermediate host). Heavy burdens can lead to emaciation, weakness, and anemia with gastric ulcers.

Sugar gliders can be infested with a variety of endoparasites and ectoparasites. A self-mutilation syndrome has been associated with a nematode. Fenbendazole, ivermectin, selamectin, and carbaryl powder can be used.

Common Diseases of Macro-pods

There are a number of viral diseases of macro-pods that have mainly been reported in Australia. They include: macropod herpesvirus type I and II, viral chorioretinitis (orbiviruses of Wallal and Warrego serogroup), and Tammar sudden death syndrome (orbiviruses of the Eubenangee serogroup), encephalomyocarditis virus (cardiovirus), and poxvirus.

Of the bacterial diseases, lumpy jaw or necrobacillosis is the most common and important one. A variety of organisms may be isolated from these cases, including Fusobacterium necrophorum, Bacteroides nodosus, Actinomyces spp., and Corynebacterium spp. Trauma to the gingiva (from stalky hay), soft, sticky foods, overcrowding, and muddy environments predispose macro-pods to the development of this disease. Clinical signs include purulent nasal discharge, facial swelling, and proptosis, increased salivation, halitosis, difficulty prehending food, and weight loss. Lesions include gingivitis, cellulitis, abscesses of the mandible and maxilla, osteomyelitis, tooth loss, bone resorption, and pathologic jaw fractures. Systemic disease can develop and lesions include abscesses in the liver, lungs, spleen, stomach, tail tip, and hind toes. Treatment includes flushing the lesions and administering systemic clindamycin, penicillin, or oxytetracycline. Prevention methods include reducing pasture contamination by decreasing the fecal load (removing shedding animals and decreasing stocking rates), reducing the access to feces / mud by feeding off the ground, and by not feeding bread.

Tetanus has been commonly reported in macro-pods. Clinical signs include convulsions, muscle stiffness, dilated nostrils, protrusions of the nictitating membrane, and death. Treatment includes penicillin IM, tetanus antitoxin, and diazepam or midazolam. Prevent with tetanus toxoid (human or equine products appear safe in macro-pods). Mycobacteriosis has been commonly reported in tree kangaroos and is typically caused by M. avium. Transmission is by inhalation and clinical signs include weight loss, dyspnea, lameness, abscesses, skin ulcers, and neurological signs. Lesions include pyogranulomatous dermatitis and abscesses in multiple organs including bone. Treatment with antimycobacterial drugs will limit, but unlikely cure lesions.

Toxoplasmosis is caused by Toxoplasma gondii and can affect all marsupials. It is not uncommon in macro-pods, but generally is inapparent. Transmission is from cat feces, eating the flesh of another animal that contains the parasite encysted in its muscle, and eating infected earthworms, flies, and cockroaches. Clinical symptoms may include sudden death, blindness, neurological signs, depression, anorexia, fever, and dyspnea. Pathology may include no lesions or myocardial hemorrhage, splenomegaly, gastric ulceration, and multiple foci of necrosis in the lung, myocardium, skeletal muscle, lymphoid tissue, CNS, adrenal gland, pancreas, and liver. Tissue cysts are most commonly seen in the brain, muscle, and adrenal gland. Subclinical infections in macro-pods may become fulminant in animals subjected to stress leading to fatal myocarditis and encephalitis Treatment is often unsuccessful, but clindamycin, TMS, and atovaquone have been used. Prevention is best. Prevent access to cat feces and cook or freeze meat.

Ringworm is a common fungal problem. Trichophyton and Microsporum spp. have been cultured from lesions with alopecia, erythema, and hyperkeratotic scales. Topical antifungals, ketoconazole or itraconazole can be used. Pseudomonas pyocyanea and other bacteria can cause pouch infections. Clinical signs include hairs around the pouch being moist and the pouch contains a greasy foul-smelling liquid. Treatment includes cleaning twice daily with chlorhexidine and then drying. Ceftazidime IM has been used in severe cases.

Coccidiosis is particularly common in weaning joeys. Eimeria wilcanniensis, E. Arundel, Isospora boughtoni, and others are reported. Transmission is by ingestion and can cause sudden death and dysentery (especially eastern grey kangaroo), hemorrhagic enteritis, hepatic coccidiosis (tammar wallabies), and large schizonts and hemorrhage in the pyloric antrum (western grey kangaroos). Toltrazuril ± sulfonamides and fluids have been used to treat clinical cases. Adult plasma IV can be used prophylactically to prevent problems in weaning joeys.

A variety of nematodes can cause clinical disease in macro-pods. Angiostrongylus cantonensis can cause ataxia, paresis, depression, seizures, and death with an eosinophilic meningoencephalitis and malacia. Eosinophilia is seen in the CSF, but rarely is a systemic eosinophilia seen. Strongyle, trichostrongyle, and strongyloides nematodes can cause granulomatous or hypertrophic gastritis and hemorrhagic enteritis, particularly in recently weaned joeys on overcrowded or heavily contaminated pasture. Eggs are typically seen in the feces and ivermectin and fenbendazole have been used successfully. Macro-pods are the intermediate hosts for Echinococcus granulosus (feral dogs and dingoes are definitive hosts). Clinical signs include emaciation, weakness, and death with large fluid-filled cysts found in the lungs or throughout the thoracic and abdominal cavities. Fasciola hepatica can cause lethargy, anemia, and death in macro-pods. Hepatic fibrosis and thickened bile ducts containing large numbers of flukes are seen with an eosinophilic cholangiohepatitis and anemia.

Hypovitaminosis E has been reported in macro-pods and signs include paresis with atrophy and pallor of the hind limb muscles, muscle necrosis and fibrosis (signs consistent with capture myopathy). Prevention can be achieved with vitamin E 60-200 mg/kg/day and large enclosures.

Preventive Medicine

Preventive medicine recommendations are an annual physical examination with identification confirmation (e.g. transponder), weight, CBC, biochemical profile, and fecal examination. Vaccinations are not routinely administered unless risk assessment indicates (e.g. likely exposure to rabies or a history of clostridial disease in a collection).

Suggested Resources

1.  Holz P. 2003. Marsupialia (marsupials). In: Fowler M. E. and R. E. Miller. Eds. Zoo and Wild Animal Medicine. 5^th edition. Saunders, St. Louis, Missouri. Pp. 288-303.

2.  Ness R. D. and R. Booth. 2004. Sugar gliders. In: Quesenberry K. E. and J. W. Carpenter (eds.). Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Pp. 330-338.

3.  Pye G. W. 2005. Sugar gliders. In: Carpenter J. W. (ed.) Exotic Animal Formulary. 3^rd edition. Elsevier Saunders. St. Louis, Missouri. Pp. 347-358.

4.  Veterinary Information Network (https://www.vin.com)

5.  Vogelnest, L. and R. Woods (eds.). 2008. Medicine of Australian Mammals. CSIRO Publishing. Victoria, Australia.