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Chapter 12: Short-tailed opossums.

OBJECTIVES

After completing this chapter, the student should be able to

* properly house a short-tailed opossum.

* provide appropriate client education to new short-tailed opossum owners.

* provide the correct diet for a short-tailed opossum.

* provide basic nursing care to a short-tailed opossum.

* assist in the anesthesia of a short-tailed opossum.

* demonstrate appropriate restraint techniques.

Key Terms

eutherian

marsupalia

prehensile

hygrometer

enrichment

neoplasia

polyuria

polydipsia

hematuria

pyuria

melanoma

sarcoma

arthrosclerosis

Introduction

Marsupials are distinct from the eutherian mammals. Eutherian mammals have a placenta, a temporary organ that forms and joins the developing offspring to the dam with an umbilical cord. At birth, the young have completed development during a gestation period within the placenta. Marsupials do not form placentas, and the young are born in an immature, embryonic state where they continue to develop outside of the dam's body.

While most people think pouched when defining the characteristics of marsupials, not all marsupials possess a pouch, especially the smaller species. The pouch is a specially developed fold of abdominal skin in which to carry and nourish the developing young. Some marsupials, considered more primitive, do not have a pouch, but the skin around each nipple forms a donut-shaped cushion which keeps the neonates warm and secure in their attachment.

All marsupials are born in an immature state, only partially developed at birth. The newly delivered young are able to crawl or wriggle up the mother's abdomen and locate a teat. The teat swells, completely filling the mouth, and firmly attaching the neonate. Marsupials are so named because they have distinctive, modified bones that are absent in other mammals. The marsupial bones, os marsupalia, form part of the pelvic attachments for abdominal muscles.

There are approximately 260 species of marsupials. Many of them are native to Australia, but many of the smaller species are found in South America. The only representative in North America is the Virginia opossum. There are no species native to Europe.

Short-tailed Opossums

The short-tailed opossum, commonly called the Brazilian short-tailed opossum, or simply referred to as an STO, is related to several small opossums with approximately 17 subspecies. All are found in South America. They are native to fairly humid, forested areas of Brazil and Bolivia. They are ground dwellers, foraging on the forest floor for insects and small rodents. They are welcomed into countryside homes and it is considered good luck to have a resident possum. They are very curious and unlikely to carry or transmit diseases, and will readily consume more unwelcome house guests such as large spiders and scorpions. While never considered pets in their native lands, the taxonomic name, (Monodelphis domestica), reflects their long and amiable domestic relationship with humans.

Short-tailed opossums are recent arrivals to the pet market, although they have been used extensively as laboratory animals for many years. They are considered not only social and appealing, but are relatively easy to care for, making them an undemanding yet exotic pet (Figure 12-1). All STOs currently used as laboratory animals are descendants of previously held research animals. They are now banned from being exported from their native habitats.

[FIGURE 12-1 OMITTED]

Short-tailed opossums are one of the smallest marsupials, with a body length of only four to five inches. The tail is slightly shorter than the body, hairless and without scales. They often use their tails, which are only slightly prehensile, to carry small twigs and grasses back to the nest, which is usually in a hollow log. The coat is thick and very soft. Their color varies slightly from silvery gray to a more smokey-gray color. Some animals may have white feet or white markings on the chest. The underbelly is lighter in color. Each foot has five toes. The first toes on the hind feet are clawless and opposable (Figure 12-2). Although primarily nocturnal, short-tailed opossums are easily roused and become active during the day, seeking food items or just exploring their environment.

In the wild, STOs stalk and pounce on many smaller prey items. They eat a variety of insects, arachnids, small vertebrates, ground nesting birds' eggs, and even the occasional bird. They also eat small amounts of fruits and some vegetables, however, there is no report or evidence that they eat plant material, per se.

Behavior

Short-tailed opossums are solitary by nature and will not tolerate the company of others. Attempting to place two or more together will result in severe injuries or death. In the wild, the only time they come together is when a female is ready to breed and tolerate the presence of a male.

[FIGURE 12-2 OMITTED]

Short-tailed opossums may breed year round. Estrus, the period of receptivity, varies from three to twelve days. Once mating has occurred, gestation is approximately 14 days. The tiny embryos are barely visible as they crawl up the mother's abdomen to locate and attach to a nipple.

Short-tailed opossums have no pouch but instead have 13 teats arranged in a circle on their abdomen. There may be more fetal pups delivered, but only those that are able to attach to a teat will survive. The attachment occurs because the nipple swells and completely fills the mouth of the pup. Many may be lost, as the average litter size is five to ten. They remain attached to the nipple for approximately 30 days and are weaned completely at 50 days.

Under no circumstances should a pup ever be physically removed from a teat. The pup, once removed, will not be able to reattach and it would be very difficult to save it. Prior to being completely weaned, pups climb up and ride on the dam's back as they begin to learn to search for food. A good time to start socializing the individual pup is when the little hitch-hikers begin to scamper off the dam and become fairly independent. Most pet short-tailed opossums are acquired around one or two months of age, by which time they are fully weaned and would normally leave the family group.

Prior to weaning, short-tailed opossums may nest together. Four to six-week-old pups may still be in the maternal nest, but as soon as the young are independent of the dam, they will leave the company of their litter mates and begin their solitary lives.

Male anatomy is typical of other marsupials. The testicles are fur-covered and cranial to the penis, located on the lower abdomen (Figure 12-3). The female has a very small genital opening that may be difficult to visualize because of the fur (Figure 12-4).

Sexual maturity is normally around four and a half months. STOs have an average life span of four years but are reproductively successful for approximately three years. Regardless of the size of habitat provided, it is not recommended to try to accommodate more than one animal per enclosure. Captive breeding can be difficult. Many times, even the introduction of a pair to each other will result in the death of one and serious injuries to the survivor. As it is for many exotic species, a USDA license is required to breed short-tailed opossums, even if only one pair is kept and the occasional litter produced.

[FIGURE 12-3 OMITTED]

[FIGURE 12-4 OMITTED]

Housing

Although the short-tailed opossum probably has a fairly large range of territory in the wild, they are easily kept in captivity without many special considerations. A 20 gallon (long) terrarium tank with a screen lid works well. Basic set-up tanks are readily available in a variety of sizes, but the most important thing in choosing one is that the lid fits securely and can be latched. Even though this little opossum is a ground dweller, it can and will climb readily.

As with all exotic animals, natural environmental ranges of humidity and temperature should be provided. The humidity of South American forests is much higher than the average North American household. The advantage of a glass aquarium is that it helps to maintain a slightly more humid environment. Recommended humidity is 50 to 60 percent. A small humidity gauge, or hygrometer, attached to the inside of the tank will help determine if the humidity is high enough. It may be necessary to add a small waterfall to the habitat. There are many self-contained waterfalls designed for use in reptile habitats that make an easy addition to the tank. The electrical cord should be checked regularly, but short-tailed opossums are not known to chew or gnaw at items. An additional source for raising the humidity is especially important during the winter months when ambient room humidity is far lower.

The ideal temperature range for a short-tailed opossum is between 72 and 74[degrees]F. In a larger habitat, an additional source of heat is needed. Under-tank heating pads used for reptiles are safe and designed to provide small areas of greater warmth. Although live plants are attractive, lighting required for their benefit is not healthy for the short-tailed opossum.

All short-tailed opossums should be provided with a nest box and nesting material. Small rodent houses work well, as do half-round logs (Figure 12-5). Both sexes make sleeping nests and material such as grass hay, shredded paper, or leaves should be provided so they can be gathered up by the opossum and put into the nest box. Bedding material should be nonabrasive, nontoxic, and at least two inches deep. Aspen shavings, shredded forest floor bark, or recycled paper bedding are all suitable. Cedar shavings should never be used as they contain oils known to be toxic and will cause severe respiratory problems.

[FIGURE 12-5 OMITTED]

Short-tailed opossums are fastidiously clean. They groom with their tongues, licking the fur clean. They have been observed washing their hands in the same manner as rats and mice and use their forepaws to clean their face and ears. They use a litter tray regularly. Small triangular plastic litter trays are available, but plastic food container lids work just as well as long as they are deep enough to contain the litter and are easy to clean and disinfect (Figure 12-6). Short-tailed opossums do not cover or bury their droppings. Plain (nonclumping), unscented cat litter is recommended.

[FIGURE 12-6 OMITTED]

The addition of enrichment items not only makes the habitat visually appealing, but provides the opossum with extra areas to explore. Many items available for reptile habitats can be incorporated into the opossum habitat, providing interest for this curious little marsupial. Artificial plants, as well as artificial ledges and natural branches, may be hung from the sides of the tank. Short-tailed opossums will use a rodent wheel for exercise, both during the day and at night. For safety, solid or wire mesh wheels are safer than those with horizontal bars (Figure 12-7).

[FIGURE 12-7 OMITTED]

Access to fresh water is easily provided with a small rodent sipper bottle. The water bottle should be rinsed out and changed daily. The sipper tube needs to be checked regularly to be sure that the water droplets flow with the lightest touch. Never add any vitamin or mineral supplements to the water source. If supplements are needed, the best solution is to correct the diet. It may be inadequate, either in the amount fed or by not meeting nutritional needs.

Diet

The best captive diet is still being researched. Omnivorous by nature, short-tailed opposums have been kept successfully with a great assortment of food items. Most of the knowledge regarding captive diets has been derived from research colonies, where they are maintained for general and reproductive health. A mix of commercial carnivore and insectivore diets is always recommended, along with fresh fruits, meal worms, wax worms, and crickets. Food should be freshly prepared and fed in the evening.

Short-tailed opossums require a diet that is high in protein and low in fat. Many owners feed a high quality kitten chow supplemented with low-fat cottage cheese, boiled egg (shell removed), live insects, and a weekly pinky or fuzzy mouse. Short-tailed opossums can be astonishingly quick when attacking live prey, and care needs to be taken when offering a mouse to an eager short-tailed opossum. Small rodents are quickly grabbed and dispatched with a bite to the head (Figure 12-8).

[FIGURE 12-8 OMITTED]

There is a specifically formulated diet available for the short-tailed opossum. It is a semi-soft crumble that doesn't need refrigeration, but it should be kept in a sealed container to preserve its moisture content. Commercial fox food diets have also been used successfully.

Handling and Restraint

The pet short-tailed opossum objects little to being handled. Many of them will scamper over and around an owner's lap, exploring pockets and clothing. They are rarely aggressive when being handled, but like any animal, will bite if provoked. Fingers may be nipped if human hands smell like food or treats offered in the past.

Short-tailed opossums have little in the way of defense but will gape and hiss in alarm if they are frightened. Pups should be handled gently and allowed to explore without restraint. It is best to be seated on the floor to avoid any danger of a fall. However short the drop, they are easily injured.

Restraint for a physical exam should be as minimal as possible. As they become very trusting of people, a scruff may well alarm them. They can be held gently with one hand yet still be examined, heart rate taken, and respirations observed. Should an exam require closer evaluation, it would be better to use an inhalant anesthesia.

Medical Concerns

Short-tailed opossums are generally hardy. The problems that do occur are usually the result of poor husbandry and diet in captivity. There is limited information available on health and diseases. As they become more popular and are seen more frequently by veterinarians, greater knowledge will be gained. Often, there is reluctance on the part of the owner to seek medical advice when necessary as short-tailed opossums are illegal in many states.

Gastrointestinal Disorders

The cause of death most often reported is rectal prolapse due to aging, an incorrect diet, or the combination of both. It can also be secondary to bacterial enteritis. Rectal prolapse is seen most commonly in older females. If the initial cause of the prolapse can be identified and resolved, it may help to prevent a prolapse from recurring. It is critical that the opossum be seen by an exotic animal veterinarian as soon as possible so that the enlarged and inflamed rectal tissue can be reduced and replaced before tissue necrosis occurs. Any sign of straining to defecate is cause for concern.

Bacterial enteritis, implicated in some prolapses, can also be caused by poor diet and husbandry. The opossum becomes lethargic and has a distended abdomen due to a build up of gas in the gastrointestinal tract. Additionally, diarrhea and anorexia compound the problem. The patient can quickly become critically dehydrated because of the diarrhea and accompanying loss of appetite. Antibiotics and fluid therapy may be successful.

Nondigestive Disorders

Hepatic and pancreatic neoplasia are most frequently seen in short-tailed opossums two years old or older. Hepatic neoplasia produces lesions on the surface of the liver that can be readily seen on necropsy. If hepatic neoplasia is suspected, a definitive diagnosis can be determined with a chemistry panel. Both aspartate aminotransferase (AST) and alanine aminotransferase (ALT) will be elevated. The patient will most likely present with abdominal enlargement and weight loss. Signs may be slowly progressive or appear in a very short period of time.

Pancreatic neoplasia may be primary or secondary to hepatic neoplasia. It is most often diagnosed postmortem. There may be signs similar to hepatic neoplasia, or the owner may find the short-tailed opossum dead in the cage with no prior sign of illness. Both types of neoplasia have a very poor prognosis.

Older opossums frequently develop nephritis in association with a bacterial infection. Signs may include polyuria/polydipsia (PU/PD), evident when the patient drinks more water than normal and urinates frequently. Hematuria and pyuria, with blood and/or pus in the urine may also be present. A routine urinalysis should be performed, along with a bacterial culture of the urine to identify the causative bacteria. Radiographs may show kidney enlargement. Treatment usually involves antibiotics and fluid therapy to ensure hydration and support for kidney filtration.

Pituitary adenoma can occur in the short-tailed opossum. It is usually first evaluated as a skin disorder because the primary sign is a rather large area of alopecia on the rump which may progress up the lower back (Figure 12-9). The hair loss is not associated with inflammation or irritation and many owners report that the bald patch appeared suddenly. With a pituitary adenoma, blood serum levels of both estradiol and prolactin (luteotrophic hormone LTH) will be elevated. Prior to a blood draw, decide which exotic specialty laboratory will be used and the specifics of the samples required. All labs will provide information regarding the type of samples that should be submitted for specific tests; serum or blood volume required, preferred preservative, and if the sample should be frozen or cooled.

[FIGURE 12-9 OMITTED]

Dermatitis is another condition that may be caused by an inadequate diet and poor nutrition. Hair loss is common with flea or mite infestations. The alopecia may be isolated or appear patchy anywhere on the body. There may be other causes, such as allergies, skin infection, or neoplasia. A skin scraping will help to determine the exact cause.

Benign skin lipomas have also been reported and, as with any small mammal, these should be surgically removed. Excessive ultraviolet light can induce melanoma and sarcoma. The history should include whether or not UV lighting is used and the number of hours of exposure. Habitats are often set up with live plants and, although beneficial to the plants, short-tailed opossums should not be exposed to this type of light.

Long-term dietary deficiencies and age can cause heart and coronary artery disease in short-tailed opossums. They can develop high levels of cholesterol leading to arthrosclerosis (hardening of the arteries). Signs are typical of heart and circulatory problems seen with other small animals, that of exercise intolerance, cold extremities and ascities.

Clinical Procedures

The veins are small and fragile. It will be difficult to safely obtain a blood sample without the use of general anesthesia. Either isoflurane or sevoflurane is suitable. The patient can be placed in an induction chamber or contained in a dog mask placed flat against the table. Induction is rapid and they are easily monitored. General anesthesia will reduce stress on the patient and help to prevent a hematoma or laceration of a vessel in a struggling and very small patient. For recovery, a clear plastic container (sold as a small habitat) provides a secure container and lid with good ventilation. A soft cloth should cover the bottom.

A 25 gauge needle with a tuberculin syringe is adequate for the small amount of blood required, which should not be greater than 1% of body weight by volume. Blood may be collected from the ventral coccygeal vein, femoral, or saphenous veins. Attempts with a direct cardiac puncture or accessing the vena cava often result in disaster and death for the patient.

Parasites

Incidental reports of parasites in short-tailed opossums are rare. They could potentially be exposed to and infested with fleas or mites. If suspected, intestinal parasites could be diagnosed with standard fecal floatation methods or by direct smear. To date, data cannot be found regarding the use of specific anthelmintics in the short-tailed opossum.

Review Questions

(1) Why are short-tailed opossums classified as marsupials if they don't have a pouch?

(2) How do short-tailed opossums use their tails?

(3) What is the typical diet of the short-tailed opossum in captivity?

(4) How long do neonates stay attached to the nipple before exploring independently?

(5) What happens when the tiny embryos are born?

(6) What is the most common cause of illness or disease in a short-tailed opossum?

(7) What can cause rectal prolapse in a short-tailed opossum?

(8) What is the best method of anesthetizing a short-tailed opossum?

(9) List the veins that may be used for blood collection.

(10) What types of bedding should be used in the habitat?
fast FACTS

Short-Tailed Opossums

* Brazilian Short-Tailed Opossum (Monodelphis
  domestica) STO
* Diurnal/nocturnal ground-dwelling
  marsupial
* Solitary, aggressive with others
* Omnivore/insectivore diet
* Five toes on each foot; 1st digit of each hind foot is
  claw-less and opposable

WEIGHT
* Average male: 100 to 125 g
* Average female: 75 to 95 g

LIFE SPAN
* Approximately four years (48 months)

REPRODUCTION
* Sexual maturity: 16 to 20 weeks (4 to 5 months)
* Gestation: 14 days
* Litter size: 5 to 10
* Weaning: 48 to 50 days

VITAL STATISTICS
* Temperature: 89.6[degrees]F, 31.6[degrees]C
* Marsupial body temperature is normally much lower
  than placental mammals
* Heart rate: 239/300 bpm

DENTAL
* Respiratory rate: 52 to 65 breaths/min
* Dental formula: 2(4/3, 1/1, 3/3, 4/4) = 48 teeth total


Further Reading

Holz, Peter (2003). Marsupialia (Marsupials). In Fowler, Murray E. & Miller, R. Eric. Zoo and Wild Animal Medicine, 5th edition.

Delaney, Cathy A. Johnson (2005). The Marsupial Pet: Sugar Sliders, Opossums, and Wallabies. Presented at Dallas Veterinary Medical Association.
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Title Annotation:UNIT II
Publication:Exotic Animal Care and Management
Geographic Code:1USA
Date:Jan 1, 2008
Words:3526
Previous Article:Chapter 11: Rats and mice.
Next Article:Chapter 13: Sugar gliders.
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