John M. Sykes IV, DVM, DACZM
Wildlife Conservation Society, Bronx, NY, USA
Marsupials can be generally divided into American marsupials and Australian marsupials. Most American marsupials live in Central or South America. With the exception of the Virginia opossum (Didelphis virginiana), few American species are regularly held in captivity. The Australian marsupials include 4 extant orders which encompass carnivorous marsupials (e.g., Tasmanian devil; Dasyuromorphia); bandicoots (Peramelemorphia); marsupial moles (Notoryctemorphia); and the possums, gliders (e.g., sugar glider), koalas, and the macropods (Diprotodontia). The term "macropods" refers to animals within the superfamily Macropodoidea. There are two families within this group: the Potoridae, which contains rat-kangaroos, bettongs, and potoroo; and the Macropodidae, which contains most of the wallabies and the kangaroos.2 This lecture and these notes focus on the Macropodidae family.
Unique Anatomy/Unifying Features
In general, macropods have strong hind limbs with an elongated fourth toe and smaller forelimbs. They tend to hop or walk using "pentapedal" locomotion. They have a flexible atlanto-occipital joint, which is a common site of trauma in some species. The epiphyses of limb bones remain unfused for life, the mandibular symphysis is unfused in kangaroos and wallabies, and they lack a patella.4,5
The female reproductive tract consists of 2 ovaries which lead to 2 separate uteri and cervixes. The 2 cervices join at the vaginal cul-de-sac which has a median septum. From the vaginal cul-de-sac, there are 2 lateral vaginas which lead to the urogenital sinus. Additionally, between the 2 lateral vaginas, there is connective tissue called the urogenital strand, which also connects the vaginal cul-de-sac with the urogenital sinus. A passage opens within this strand during parturition to allow the fetus to travel from the vaginal cul-de-sac to the urogenital sinus. This passageway may or may not remain as a permanent structure between births called the median vagina, depending on the species. Sperm travels through the lateral vaginae. After parturition, the young travels from the cloaca to the pouch and attaches to a teat. The composition of the milk changes as the pouch young (PY) grows. The size of the teat changes as well when a PY is attached, or has recently been attached. Macropods generally have only one offspring of a given age at one time, but may have up to three offspring of different ages and they exhibit embryonic diapause. Typically, the oldest offspring leaves the pouch (but may still be nursing) (offspring #1), birth occurs a few days later (offspring #2), post-partum estrus occurs with conception of a new fetus (offspring #3) and brief development of this fetus before diapauses begins. The embryo development recommences when the PY is 4 weeks away from leaving the pouch.
Kangaroos and wallabies are herbivorous foregut fermenters. In general, the smaller species may be browsers or mixed browsers/grazers, while the larger species are primarily grazers. These differences in diet can also be seen in the relative size of the different portions of the stomach, where grazers have a larger sacciform region compared to browsers. Many of the grazers also demonstrate molar progression similar to that seen in elephants, where the rostral molars are shed as they are used, the remaining molars progress forward, and new molars are grown out caudally for a total of four, or in some species five, molars.2,5 Some animals may be seen regurgitating forcibly and then re-swallowing their food. This is called mercyism and may be important in transferring gut flora to young. Rapid changes in diet for macropods are discouraged as endotoxemia can result with such changes.2
Handling and Restraint
In order to keep stress to a minimum, macropods should be provided with an enclosure appropriate to their natural history and social needs. In general, large enclosures with few obstacles are recommended. It is prudent to put fence posts on the outside of enclosures as these species tend to move along fence lines when nervous or pressed. Sharp corners are discouraged as animals tend to run into fences and cause trauma. All species should have areas to hide or block line of sight by public or other animals in the group. Rock wallabies should have rocks or stumps to sit on. Larger species should be provided with dust baths. Enclosures should be designed to exclude predators (foxes, raccoons) and especially cats, as macropods are particularly sensitive to toxoplasmosis (see below).2
In order to avoid exertional myopathy and undue stress, capture of macropods should be well thought out. Procedures should occur during cool environmental temperatures. Avoid chasing or startling the animals as they are prone to running into fences or other objects causing major trauma. Smaller and medium-sized animals can be caught with hoop nets and transferred into strong cloth sacks for restraint. Alternatively, they can be restrained by holding the tail near the base and lifting the hind feet off the ground, or by holding the animal around the thorax and pulling its back into the holder's chest. Attempting to restrain the hind legs can be very difficult, cause more struggling, and may result in trauma to the animal or handler. Note that some animals will bite when restrained. Macropods will lick their forearms and pant when overheated - if these signs are seen during capture attempts, the procedure should be aborted.2,5 Larger animals (< 40 kg) should be chemically restrained.
Macropods are prone to developing capture myopathy (exertional rhabdomyolysis). Classically, this condition has been subdivided into capture shock, ataxic myoglobinuric, ruptured muscle, and delayed-peracute syndromes. These are all likely different stages of the same process. The pathogenesis is thought to be related to an overstimulation of the sympathetic nervous system leading to peripheral and organ ischemia (shock). This shock then leads to muscle and other organ damage.3 In macropods, the condition occurs often during stressful events, particularly capture and restraint. Clinical signs include dyspnea, tachycardia, hyperthermia, tremors, weakness, collapse, and/or death. Animals may develop chronic sequella in the muscles resulting in chronic lameness. Death may occur sometime after the stressful event (delayed-peracute syndrome). Diagnosis is based on clinical signs and compatible history, though elevated CK and myoglobinuria are supportive. Treatment can be attempted in affected animals and can include IV fluid support, dantrolene sodium, prednisolone, or flunixin meglumine. As treatment is often unrewarding, attention should be placed on prevention. Planning capture events in cool weather, minimizing stress, and minimizing chasing are all important components to prevention. Some authors recommend using diazepam parenterally immediately upon capture of macropods to decrease struggling and stress of restraint.5
Macropods are particularly sensitive to developing disease from toxoplasmosis. This is thought to be related to their evolution on an island devoid of felids. Toxoplasma gondii is an intracellular coccidian parasite. The definitive hosts are only felines. Oocysts are shed in feline feces. These are initially uninfective. After 1–5 days in the environment, the oocysts sporulate (now contain sporozoites) and become infective. An intermediate host ingests the oocysts, wherein the sporozoites divide to become tachyzoites which then continue to multiply. These tachyzoites then become encysted in tissues (GI or others) and develop into bradyzoites. The feline eats the intermediate host containing the encysted bradyzoites and the life cycle is completed.1 Macropods are generally exposed to Toxoplasma oocysts via feline feces (as they do not eat the intermediate hosts). It is critical to prevent cats from accessing macropod enclosures or barns where they can contaminate the hay or browse fed to macropods.
Clinical signs of toxoplasmosis may be nonspecific depending on which organs become most affected by the disease in an individual animal. They include dyspnea, tachypnea, depression, anorexia, diarrhea, and neurologic signs (blindness, ataxia). Exam findings may include ocular lesions (keratitis, uveitis, retinitis). Serology may be helpful in making a diagnosis, though can be difficult to interpret. Modified agglutination test (MAT) may offer both IgM and IgG results. Comparison to banked samples or comparison over time in an individual may be needed to help interpret results. An elevated IgM is suggestive of an active infection, whereas a low IgM with elevated IgG may indicate exposure. Using these titers in clinically normal animals has not been rewarding in the author's practice as many nonclinical macropods will have measureable IgG to Toxoplasma. Treatment is often unsuccessful. Infections may become latent and recrudesce at a later point. Atovaquone, clindamycin, and sulfadiazine and pyrimethamine combinations have been tried.5 In our collection, we have administered ponazuril at 20 mg/kg both to attempt to treat affected individuals (with little success) and prophylactically (with some potential success). Cats tend to like to defecate in sandy/dry soil rather than grass. In our experience, keeping macropod exhibits covered with healthy grass (possibly needing to re-sod yearly) can help prevent the infection.
Oral Necrobacillosis (aka Lumpy Jaw)
This problem presents as swelling of the mandible or maxilla, sometimes with a draining tract. Animals may be inappetant, drool, or have difficultly eating. The infection can shower other organs and lead to systemic signs as well. Examination of the oral cavity will often reveal infected or lost teeth and damage to the gingiva. CBC and serum chemistries may be suggestive of inflammation (degenerate neutrophils, neutrophilia, elevated fibrinogen), or reflect organ damage due to spread of the infection. Radiographs help evaluate the extent of the lesion as affected tooth roots and degree of osteomyelitis or bone lysis can be determined. Treatment requires removing affected tissue and getting the infection under control. Affected teeth should be extracted and any other dental abnormalities addressed. Areas of osteomyelitis of the bone need to be surgically debrided. The wounds can be flushed with antimicrobial solutions (2% povidone-iodine, metronidazole) and drainage should be established. Local antibiotic therapy in the form of antibiotic-impregnated beads may be indicated. Long-term systemic antibiotic treatment is often required, which can be challenging. Oral clindamycin or metronidazole can be tried, or repeated injections with oxytetracycline may be used. Prevention of this condition is more effective than treatment. Attention to proper diet (no bread) and avoidance of hay with sharp awns is important. The addition of browse to the diet, even for grazers, may help improve overall oral health. Keep feeding stations clean and free of fecal contamination - elevated feeders may help prevent contamination of oral lesions with fecal bacteria.5
Case Study in Osteomyelitis
A 6 yo female intact tammar wallaby (Macropus eugenii) presented with an acute-onset swelling of the L mandible. Examination revealed a soft-tissue swelling of the mandible with periosteal reaction along the ventral mandible and periapical lysis at the base of the L incisor. The soft-tissue abscess was incised, flushed, and allowed to drain. Prevotella sp. was cultured from the lesion and oral treatment with clindamycin was initiated. As the incisors of these animals are very large, removal of the incisor may have led to instability of the mandible and a high risk of fracture, so saving the tooth was an important goal in this case. After 1 month of oral antibiotics, the soft-tissue swelling was improved, but apical lucency remained. Local treatment using an osmotic pump was initiated. An Alzet osmotic pump model 2004 (pumps at 0.25 µl/h for 4 weeks) was loaded with chloramphenicol (333 mg/ml) and placed SQ behind the L ear. The mandible around the affected tooth root was debrided and a small hole created from the mandible to the root. A polypropylene catheter was tunneled under the skin between the pump placement site and the tooth root. One month later an apicoectomy and apex restoration was performed and the osmotic pump replaced to deliver antibiotics for another month. Nine weeks after the initial pump placement, the pump was removed and oral antibiotic treatment was stopped. At this time, the infection appeared resolved. Follow-up radiographs 2 and 11 months after pump removal indicated complete healing of the apicoectomy site and no evidence of recurrent osteomyelitis.
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2. Jackson S. Macropods. In: Australian Mammals: Biology and Captive Management. Collingwood, VIC, Australia: CSIRO Publishing; 2003:245–296.
3. Spraker TR. Stress and capture myopathy in artiodactylids. In: Fowler ME, ed. Zoo and Wild Animal Medicine, Current Therapy 3. Philadelphia, PA: Saunders; 1993:480–488.
4. Vogelnest L. Marsupialia (Marsupials). In: Miller RE, Fowler ME, eds. Fowler's Zoo and Wild Animal Medicine. Volume 8. St. Louis, MO: Elsevier; 2014:255–274.
5. Vogelnest L, Portas T. Macropods. In: Vogelnest L, Woods R, eds. Medicine of Australian Mammals. Collingwood, VIC, Australia: CSIRO Publishing; 2008:133–226.