Kathryn C. Gamble, DVM, MS, DACZM
Marsupials in Private Practice
The general assumption is that the external pouch (marsupium) is the defining anatomic feature of the marsupial taxon; however, this and other anatomic features - cloaca, dentary and epipubic bones, syndactylism, or lack of patella and corpus callosum - are present variably by species or even shared with other taxa.4,5 It is actually the placement of the tubular reproductive tract lateral to that of the urinary tract that uniquely distinguishes the order.3-5 The focus of this presentation will be the two primary marsupials other than macropods (kangaroos and wallabies), which could present commonly to the US veterinary practitioner: sugar gliders (Petaurus breviceps) and Virginia opossum (Didelphis virginiana).3
Sugar gliders [SG] are small (90–150 g), nocturnal, arboreal Australian marsupials.1,7,3 They have plush, grey-brown pelage with a dark dorsal stripe.11 These animals glide by a bilateral skin fold (patagium) that stretches from manual digit 5 to the hallux.7,11 Syndactyl pedal digits 2 and 3 are used for grooming and an elongated manual digit 4 is used for foraging.1,7,11 Retina in SG are avascular and no tapetum lucidum is present.14 Dental formula (3/1 0-1/0-1 1-2/1-2 4/4) is diprotodont and feeding strategy is omnivorous with cecal fermentation.3,5,7,11 Although usually spring breeders, polyestrous cycling occurs and female SG frequently twin.7 Neonates (joeys) are raised within a pouch with 2–6 teats and no epipubic support.3,11 Males have a pre-penile scrotum, two accessory sex glands - prostate and bulbourethral, and a bifid phallus.1,7,11 Both genders possess paracloacal scent glands and the males, forehead and sternal glands, which are used for marking territory.1,3,5,7,11 Social animals, SG usually are housed as male-female pairs or polygynous as one male-dominant groups.1,3,7,11 Typical life span for SG is 4–7 years but they may reach 12 years in captivity.1
Virginia opossums [VO] are medium-sized (3–5 kg), nocturnal, arboreal marsupials from North America, and the only native US marsupials.3 Agile climbers, VO also have a fully prehensile tail.3,10 While they do not have syndactyl grooming claws, VO fastidiously maintain a coarse, grayish pelage.3,10 Retinas in VO are vascular and have a tapetum lucidum.14 Dental formula (5/4, 1/1, 3/3, 4/4) of the VO allows it to exploit many foodstuffs for its simple stomached, non-fermenting gastrointestinal tract.3,10 These opossums may breed year-round, although young typically emerge in late spring with litters up to 10 joeys not uncommon. A deep pouch in the female is supported by epipubic bones, which are observed in both genders. The VO resemble SG in the male reproductive tract and for paracloacal glands in both sexes.3-5 Generally solitary in the wild, VO are best housed singly in captivity unless they are introduced as juveniles.3,10 As a pet, VO typically live 3–4 years but have reached 8–10 years.10,11
For both SG and VO, it is important to remember their arboreal - or even flighted - lifestyle. Enclosures must be sized to permit normal mobility while props and walls are provided to protect the animals from inadvertent toxicity or injury. Nests should be selected for security and warmth and bedding free from entanglements. Especially for SG, enclosure carrying capacity should be considered as crowding effects can induce immunosuppression.4,7
As a rule, marsupials have a BMR 30% that of a comparably sized placental mammal (eutherian) and normal body temperature (91–96°F/33–36°C) is lower.3-6,10,11 Seasonal variability in fat storage has been observed in SG, and lower ambient temperatures can induce torpor.5,6,11 Although SG have been determined with daily energy needs nearly equal to their eutherian model, obesity remains a problem due to reduced exercise and dietary challenges;1,4,7 excess fat is observed within the patagium.8 However, malnutrition can occur4,8 and be concurrent with bloat or diarrhea. Obesity in captive VO is nearly uniform and often fat is observed as an enlarged tail base or periorbitally.10 Both species are omnivorous although SG and VO are opposite in their extremes of selectivity. As VO will consume essentially anything in the wild or captivity, their nutritional management is more straightforward with complete domestic carnivore kibble that is supplemented with varied produce and supplemental protein sources.4,10 In contrast, wild SG have very specific, yet variable selections of plant products and insects on a seasonal basis. Often in captive diets, too much sugar or fat is present and attempts to provide the animal a wide variety of foods generally promotes unbalanced self-selection.7 Although several diets for SG are documented, literature evaluation supports that no proven diet specifically exists.1,3,7,8,11
While both species have no absolute need for Vitamin D3 in their diets, hypocalcemia and nutritional metabolic bone disease can occur. Typically, SG present with rear limb paresis or paralysis without fractures, although tetanic seizures can occur.1,7,8 In VO, caudal body paresis also occurs although fractures are more common and may be accompanied by fibrous osteodystrophy.3,4,10 Excessive sugars in the SG diet can produce dental disease.1,7,11
Both SG and VO can bite when agitated or disturbed, so should be handled confidently with control of the head paramount.1,3,6,11 Use of towels for SG and gloves for VO can be utilized for calmer animals. Sedation for fractious animals or more invasive procedures than supportive care or physical examination may be needed.13,15 Usually isoflurane via facemask alone is sufficient to provide a manageable patient and longer term maintenance for VO is provided by routine intubation.
Blood can be obtained from several peripheral veins for each species and standard volume guides should be followed. The jugular and subclavian/caval vein, or medial tibial arterial sampling is routine for SG, while the simplest blood collection point for VO is the ventral tail vein at the hair and skin interface.1,3,4,7,8,11 Standard blood results for each species are published8,11 and small domestic carnivore ranges can be used in urgent care4. For fecal sample or rectal culture, it is important to evert the external cloaca to access the dorsal rectum.3,4 Urine collection may be managed more routinely for females by cystocentesis, while the bifid phallus of the male provides for easy urethral access at the point of the fork.4,11 During urinalysis, spermatorrhea is not uncommon, and in VO in vitro pairing of spermatozoa can be observed.4,5 Primary urinary tract disease is not particularly common in SG or VO, although occasional struvite uroliths have been reported from SG (L. Ulrich, personal communication, Minnesota Urolith Center, St. Paul, MN 55108, USA). Routine radiographs and ultrasound are possible as only anatomic taxon differences need to be remembered for accurate interpretation.4
Especially problematic for the obese marsupial, heat intolerance is common and evidenced by ptyalism or heavily wetted forelimbs.3,4 Juvenile cataracts have been observed with debatable contribution of galactosemia from hand-rearing formulae.7,14 For SG, lymphoma is not an uncommon differential for skin masses.1 Alopecic dermatitis can be present in SG due to crowding or overgrooming, while obese female VO have presented with a Cushing's-like syndrome.10 Inappropriately housed SG can self-mutilate and males can be focused especially on perineal structures, producing penile trauma to an extent requiring amputation.8,11 Predatory trauma for both species and vehicular trauma for VO can present.8,10
Following cat bite trauma, SG have the concern of Pasteurella multocida.7 In SG, one also should consider mycobacteriosis as a differential for skin masses.1,5 Parasites for SG tend to present more issue for the host, such as systemic toxoplasmosis and fatal coccidial diarrhea,7 while VO more often are unaffected hosts for parasites of the lung (Didelphostrongylus), skin (Besnoitia),2 blood (Trypanosoma cruzi), and gastrointestinal tract (Physaloptera)5. Of more importance is the role that VO serve as definitive host for Sarcocystis that cause illness for psittacines, equines, and marine mammals.12
For both species, literature citations are available for routine medication needs.8,11 However, on an urgent basis, pharmaceutical guidance for small, young domestic cats would be a good approximation.6 When administering fluids to SG, it is important to minimize dependent edema to the patagium and for both species, it is encouraged to avoid dextrose in fluids due to a general taxon intolerance to this sugar.6 Furthermore, due to predisposition to hypocalcemia and its effect on skeletal quality, placement of intraosseous catheters should be completed with caution.6,8
Due to the unusual internal position and anatomy of the female's reproductive tract, standard ovariohysterectomy is not performed routinely.3,7 By contrast, surgical sterilization of the male marsupial can be accomplished by routine castration or vasectomy along the prominent scrotal stalk with aesthetic partial scrotal ablation.3
Neonatal Marsupial Management3,4,9,10
Parturition of essentially mobile feti (joeys) occurs after a gestation that is uniformly shorter duration than the estrous cycle. True dystocia is non-existent. The remainder of the gestation is external, so neonatal joeys are developed disproportionately. The pectoral girdle is robust to facilitate the arduous climb from the vulva to the nipple within the pouch, but pelvic limbs are rudimentary. The stomach and duodenum are well developed so that the joey can thrive upon ingested milk, although the kidneys are unable to concentrate urine. The joey has sealed eyes and ears until midway through gestation. Although the joey is able to breathe and use external oxygen, it has heavily vascularized naked integument that aids oxygen absorption. Endocrine development is limited and no thyroid function for temperature control or true immune function is present until mid-pouch life. At birth, and indeed through much of pouch life, the neonatal marsupial is highly at risk from exposure and infection, more so than the neonatal eutherian.
Periparturient preparation occurs at the mammae usually within a pouch. The interior of the healthy pouch is sparsely haired, and well vascularized. The skin surface is coated lightly to moderately by a yellow- to-black-brown sebaceous secretion with lanolin consistency but no odor.6 Mammae are variable in number by species, and gland development is variable in a given individual. The exterior pouch sphincter (panniculus carnosus) generally is closed, but it can be dilated easily by gentle manipulation. The pouch compensates the joey with heat and protection from dehydration, and contributes to immunologic protection. Regular cleaning of the pouch interior by the dam is needed for hygiene. Even more complex than the environment, lactation varies widely throughout pouch life to provide optimally for joey growth. Once entered into the pouch, the joey identifies a mammary gland with a short teat and anchors by a strong oral cavity with minimal gape. Teat swelling is induced to secure this attachment and gradual elongation occurs to provide increased security. Initial lactation is concentrated minimally to support the poor water conservation mechanisms of the joey. Protein and fat content in early lactation is low but doubles or quadruples over pouch life, respectively. Carbohydrate content is not a marked energy source in marsupial milk and has only moderate concentration. Furthermore, the primary carbohydrate is not lactose, but an oligosaccharide. Immunoglobulins are concentrated within milk. Mineral variability is present as higher concentrations of copper and iron early, while calcium and phosphorus predominate at weaning. Sodium and potassium are contrastingly higher and lower at the extremes of lactation reflecting the joey's development of competent renal function. The typical progression of lactation presents quite a challenge to mimic during hand-rearing of joeys.
While absolute duration of pouch life varies by species, with larger species generally a longer duration, pouch life can be staged between birth and emergence as pre-hairing; fine hairing; dense hairing; weaning; pouch emergence; and independence. It is important to stage joeys accurately as non-haired joeys at 40% of pouch life have an extremely guarded prognosis for survival even in the hands of very experienced caretakers. Older joeys that are covered in hair at 70% of pouch life still present substantial time commitment. Not until nearly weaning do joeys present only the risks more typical of a eutherian neonate. In addition to the presence of hair by type and location and ear position, fecal quality can be used to stage joeys. For macropod staging, measurements of tail and pedal length have been documented (accessed 30 August 2012, www.wombaroo.com).
Presentation of orphaned joeys often occurs due to the death of its dam.10 Those young found still attached to the nipple likely should be euthanized due to poor survival prospects. Older joeys found within the pouch, but not yet independent, can be assessed for injuries based on the dam's presentation and hand-rearing initiated. The other source of orphaned joeys is dam abandonment, or 'thrown joeys.' More typically associated with macropods, the dam forcibly may dislodge the neonate, or an emergent, but uncoordinated joey could be dropped from the pouch. After address of injury and stabilization, the joey is returned to the pouch under manual restraint or sedation of the dam. In cases of repeated throwing, joeys may be secured within the pouch by partial closure of the orifice with tape. While this latter approach secures the joey and prevents eviction, the dam remains able to clean the pouch and adequate air circulates to the pouch interior.
For all joeys, artificial pouches are essential housing for hand-rearing. Soft cloth bags with disposable or washable liners provide seclusion, reduced stimuli, and comfort for the joey. The necessary warmth is provided by hanging the pouch within an incubator, which has a graduated decline in temperature to mimic the joey's maturing self-thermoregulation. Pouch humidity is simulated by topical application of occlusive ointment, especially frequent for those non-haired joeys, until the joey has a full coating of dense hair. Pouch hygiene is maintained by lining rotation, regular piddling of the neonate at feedings, and careful feeding technique. During pouch cleaning each day, a joey should be weighed and cloacal temperature measured. Extra caution to personal hygiene and consideration for gloves will minimize contamination risk to the joey. Milk quantity is targeted to 10–15% of body weight each day provided that steady, gradual growth is maintained. Actual suckling of the milk can be fraught with aspiration risk due to the small size of the neonate, oral contour, nipple shape, and intermittent, rather than constant, nursing. While naso- or oro-gastric tubes can be placed, even mid-stage joeys can be accommodated to long specialty nipples available commercially. Transfer of older stage neonates to lapping from spoons can minimize further the risk of aspiration.
Mimicry of milk composition for the marsupial neonate usually is accomplished by blending commercially available rations in different concentrations throughout the hand-rearing process. Some commercial product lines (accessed 30 August 2012; www.petag.com/products/exotics/exotics-milk-replacers/; www.perfectpet.net/wombaroo/ have developed marsupial specific formulations, which greatly simplify this process. However, blended lactose-based milk substitutes may be used once incubated with Lactaid (McNeill Nutritionals, Fort Washington, PA 19034, USA) to reduce lactose intolerance issues. Addition of canola oil later in joey development often is necessary to address increased fat content needed at this age. With the more lactose-rich diets, concern for bloat and osmotic diarrhea remains high so close monitoring of the joey is essential.
The neonate should be positioned dorsally recumbent within its pouch for nursing. Although some species live in groups within their pouches, separate hand-rearing pouches facilitate individual care and minimize the risk of injury from littermates. In the dam's pouch, neonatal emergence is induced by space fill of pouch - either by joey size or number - and inability for the joey to thermoregulate the excessive heat within the pouch. Emergence is encouraged physically in hand-rearing situations and by housing the pouch at room temperature. Overnight pouch housing is encouraged, however. Additionally, as neonates approach weaning, they should be encouraged to sample adult foods, which have been handled or inoculated by species-compatible feces from healthy adults.
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