Virginia Opossum (Didelphis virginiana)
When Europeans began to settle in North America, the range of this species extended as far north as Ohio and West Virginia. Over time, and with the introduction of the opossum into California in 1910, populations now extend into British Columbia and throughout Central America. The Virginia opossum’s range extends throughout the United States, except in extremely mountainous or arid regions. 2,4,5,11,18,22,24,29
Virginia opossums are often seen as pests and in many states, it is illegal to keep this species as a pet. The opossum is not sold in the pet trade, and animals kept as companion animals or zoological specimens were often orphaned during infancy. 11,21,25,29
According to the International Union for Conservation of Nature Red List, the Virginia opossum is a species of least concern with growing and stable populations. 25
Virginia opossums are medium-sized marsupials with a long and largely hairless, prehensile tail; large, hairless ears; and short, stumpy legs with five toes that include an opposable thumb on both the front and hind feet. Opossums possess a white face, wool-type fur with a whitish undercoat, and guard hairs of varied color. 6,10,11,29
The Virginia opossum is an omnivore. Free-ranging animals consume a varied diet that includes plant material, grasses and leaves, grains, fruits, seeds and bulbs, carrion, snails, slugs, worms, insects, rats, mice, snakes, amphibians, eggs, crayfish, and fish. Opossums are highly opportunistic and a large amount of their diet consists of scavenged food items, like carrion. 5,11,16,21,29
Offer captive opossums a balanced diet consisting of small amounts of a formulated diet (omnivore diet or low fat/low calorie cat food) supplemented with vegetables, small amounts of fruit (10-20% of the diet), and occasional (once or twice weekly) protein sources. 6,11,22 These sources of protein can include: invertebrates, such as calcium gut-loaded crickets, superworms, or other dusted adult insects, 1 small or medium whole mice, canned salmon, 1 sardine, 1/2 hard-boiled egg or scrambled egg with the shell, 1 tablespoon yogurt, or cooked chicken liver (Kathy Duncan, written communication to author, July 5, 2018).
Note: Owners may refer to “Peter’s Food”, an adult opossum diet consisting of 1-part cat food plus 1-part blended vegetables and 0.25%-part non-fat yogurt. 14 “Peter’s Food” was developed for the National Opossum Society by Dr. Anita Henness.
In a rehabilitation setting or home with multiple animals, a diet that can be made up in large quantities may be preferable. Pacific Wildlife Care has successfully used this diet long term, and it is based on a regimen described by Gode and Ruth (Kathy Duncan, written communication to author, July 5, 2018). 7
As a general rule, metabolism in a marsupial is approximately one-third that of a similarly sized placental mammal. Pairing this slower metabolic rate with the opossum’s voracious appetite, means it is easy to overfeed pet opossums. Owners should carefully monitor and control food intake while encouraging activity to minimize the risk of obesity. 6,14,31
|Cage design||A modified rabbit or ferret cage is often used for captive opossums. If housed outdoors, know that opossums can dig beneath fences. 11, 21|
|Cage size||Young opossums, whether housed individually or together, require a cage at least 35 cm (1.1 ft.) high and 45 cm (1.5 ft.) wide by 75 cm (2.5 ft.) long.
Juveniles over 350-400g should ideally be housed in large, outdoor cages, either 4’x4’x 8’ or 4’x 8’x 8’ with branches for climbing (Kathy Duncan, written communication to author, July 5, 2018).
House adult opossums in “gang yards” at least 30 x 50 m. 11
|Cage furniture||Virginia opossums are agile climbers and the enclosure should ideally provide climbing structures. 6,11,21,31
Also provide some form of visual security, such as a cardboard or wooden box filled with newspaper pellets, shredded paper, or wood shavings. The nest box should be at least 30 x 45 x 40 cm large. 11,31
|Supervision||Although owners often allow pet opossums to freely roam the home, constant supervision is recommended as with other exotic pets. Fortunately, opossums do not tend to be destructive although they will attempt to nibble on houseplants. 11,14,21|
|Environmental temperature||The ideal temperature for housing an opossum is 22°C (71.6°F), with an appropriate range of 10-30°C (50-86°F). 11,31|
|Humidity||> 58% 11,31|
Virginia opossums are agile climbers. Juveniles use their prehensile tail to swing from branches. Adult opossums are too heavy to support their body weight with the tail, however adults still use their tails to grasp bundles of leaves or bedding material. Opossums are also able to swim, dig, and even run. 6,11,21,29
|Nocturnal||Free-ranging opossums are nocturnal, however pet animals can adjust to their owner’s diurnal lifestyle. 6,11,21,29|
|Social behavior||Free-ranging opossums tend to be solitary animals and are best housed singly in captivity unless the animals were reared together as juveniles. Depending on the wildlife rehabilitation facility, single juveniles weighing less than 100-150 grams are often combined (Kathy Duncan, written communication to author, July 5, 2018).
Adult opossums can display aggression or even cannibalism if housed under crowded conditions. Also separate wounded animals, even from littermates, because of the species’ tendency towards cannibalism (Kathy Duncan, written communication to author, July 5, 2018).
Mating behavior is one of the few social behaviors displayed in the Virginia opossum.
|Grooming behavior||Opossums are fastidious groomers. 6,21|
|Elimination behavior||Opossums tend to select a latrine area and can be trained to use a litter box or newspapers. 6,21|
|Defensive behavior||If something startles an opossum, its first reaction is to flee to a place of safety, like its cage or nest box. It is also possible for a threatened adult opossum to bare teeth and stand its ground. Normal defensive behavior that can be expected in a recently trapped opossum can include lowering the lip, drooling, growling and hissing, as well as striking and attempting to bite. The animal may empty its paracloacal glands, releasing a noxious odor that resembles death and decay. In addition to hissing, other vocalizations associated with aggression include clicking, growling, and screeching. 11,21,29
“Playing possum” is a rare defensive catatonic state that occurs in response to a threat most frequently displayed in young opossums. The animal falls over and lies motionless to feign death for as little as 1 minute or up to 6 hours. 11,29 This collapse is often accompanied by a small release of odiferous glandular material from the paracloacal glands.
|Reproductive behavior||The breeding season varies with the population range, lasting from February to September in Northern climates and between January to August in Southern climates. Free-ranging males roam, searching and vying for reproductive females. Intact female opossums tend to be extremely restless. They will pace and drool on household objects while making a clicking sound. Mating behavior is one of the few social behaviors displayed by the Virginia opossums. During the breeding season, mates may communicate with a series of metallic clicking sounds. After mating, females resume their aggressive, solitary behavior. 11,14,29|
|Parental care||Male opossums provide no parental care. Females with pouch young becomes very protective, frequently licking at the pouch and their offspring. Females maintain auditory contact with their young through a series of clicks, lip smacking, and bird-like sounds. Females show little interest once young leave the pouch. 29|
|Pet quality||It is illegal to keep opossums as pets in some jurisdictions, including the state of California. Hand-raised opossums can be comfortable in captivity and bond with their caregiver, however they are relatively high-maintenance pets requiring a great deal of exercise, a special diet, and a significant time commitment from the owner. Opossums in captivity tend to be docile and can respond to their name and other verbal cues. 11,14,21 Pet opossums may seek human companionship, climbing into the owner’s lap or onto a shoulder. They may enjoy being stroked or groomed with a small, soft brush.|
Normal physiologic data
|Lifespan||Wild opossums have a relatively short lifespan of approximately 1.5-2 years. Captive opossums normally live between 3-4 years, however up to 8-10 years has been described. 6,11,21,29|
|Body weight||Adult female||1.9-2.1 kg 11, 29|
|Adult male||2.1-5 kg 11, 29|
|Cloacal body temperature||32.2-35°C (90-95°F) 11, 31
Cloacal temperature is lower than true body temperature, therefore tympanic temperatures readings are more likely to accurately measure core body temperature.14
|Heart rate||Usually between 180–240 beats per minute (bpm) with a range of 70-300 bpm reported. 8,11|
|Respiratory rate||25-40 breaths per minute 11|
|Dental eruption||Deciduous teeth eruption is quite unique in the opossum. Incisors and canines as well as most cheek teeth begin to erupt and then are reabsorbed and replaced by adult teeth. 1,31|
|Dental formula, adult||I 5/4 C 1/1 P 3/3 M 4/4 6,11,21|
|GI tract||Simple stomach, non-fermenting gastrointestinal tract. 6|
|Integumentary||Opossums possess a long, mostly hairless, prehensile tail and large, hairless ears. Male opossums also possess sternal scent glands (See Sexual dimorphism below). 10,29|
|Musculoskeletal||Like most marsupials, the opossum lacks an ossified patella. Opossums possess epipubic bones, which rest on and articulate with pelvic and pubic bones and serve as attachment surfaces for several abdominal muscles. 11,14,31|
|Puberty||Females ~6 months
Males ~8 months
Breeding typically begins at ~10 months. 11,13,29
|Sexual dimorphism||Males possess sternal scent glands. The secretions create a musky odor and stain the fur, particularly before the onset of the breeding season (see Reproduction below). 29
Males also possess a prominent scrotal sac on the mid-ventral abdomen. Females possess a pouch.
|Female reproductive tract||Opossums are polyovular and polyestrous. Each lateral uterus opens into its own vaginal canal through its own cervix. The paired lateral vaginae open separately into the urogenital sinus at the level of the urethral opening and are separated along their length by a soft tissue median septum. 11,13,31|
|Estrous cycle||29.5 days 29|
|Pouch||Like many, but not all, marsupials the female opossum has a deep pouch or marsupium. The interior of the healthy pouch is sparsely haired and vascular and the surface is lightly to moderately coated with a yellow to black-brown, odorless, sebaceous secretion. The pouch contains a variable number (4-13) of teats.6,11,31 Thirteen teats are most commonly observed (Johnson-Delaney, written communication to author, April 25, 2015).|
|Males||The male opossum possesses a forked glans or bifid penis. The scrotum is prominent, pendulous, pedunculated, and pre-penile. Male opossums also possess sternal scent glands (see Sexual dimorphism above). 13,29,31
The urogenital tract of marsupials is significantly different from that of eutherian mammals. In all marsupials the urinary ducts pass mesially to the genital ducts, and in placental mammals they pass laterally. This results in male eutherians having the vas deferens loops around the inside of the ureter to reach the testes, and in male marsupials this loop is absent. 13,14
|Breeding season||Opossums can breed year-round, but young typically emerge in the late spring. 6|
|Internal gestation||12-13 days 13,29|
|Parturition||During parturition, the fetus tunnels through the connective tissue between the median vagina and the urogenital sinus to form a central vaginal canal. Unlike most marsupials in which the birth canal is transient, in opossums this central vaginal canal becomes epithelialized and remains patent as a permanent median vagina. 6,14,31|
|Litter size||4-25, with an average of 15, however litter survival is dependent on the number of teats. 29|
|Litters per year||Average one litter per year in northern regions, up to three annually in warmer climates. 13,29|
|Birth weight||0.13-0.20 grams 29|
|External gestation||Jellybean-like embryos possess a well-developed pectoral girdle that allows the embryo to make the grueling trek from the birth canal to the pouch. Many joeys do not survive this trip. Once young reach the pouch, the ordeal may not be over. There are usually only 13 teats and some may not be functional. The nipple swells up in the neonate’s mouth to secure attachment. The nipple also gradually elongates to provide increased security.
Neonates possess a well-developed stomach and duodenum for digesting milk, however there are only rudimentary pelvic limbs and the kidneys are unable to concentrate urine. Young remain attached to nipples for approximately 50-70 days. Females average 8 pouch young per litter, although the number of teats determines the number that will survive. 6,14,29,31
|Lactation||To provide optimal growth, lactation varies widely throughout pouch life. Initially, marsupial milk is dilute, to support the joey’s weak renal function, and protein and fat levels are low. Over pouch life, protein content doubles and fat levels quadruple. Mineral levels also vary in marsupial milk. High levels of copper and iron predominate early on, while calcium and phosphorus predominate at weaning. Marsupial milk contains only moderate levels of carbohydrates. The primary carbohydrate is not lactose, but an oligosaccharide. 6,31
When handfeeding juvenile opossums, select a canine formula short-term 22, then transition the joey to a commercial marsupial milk replacer (Wombaroo via Perfect Pets) (Johnson-Delaney, written communication to author, April 25, 2015).
Young are usually independent after weaning, although some juveniles remain in the weaning den with their mother until they are ~120 days old. Approximately 60% of weaned free-ranging young do not survive once fully independent. 29
|Urinary system||The path of the ureters is unique in marsupials as they pass between the median and lateral vaginae on each side. 31|
|Cloaca||In females, the urogenital sinus and rectum open into the common vestibule or cloaca. The penis sits within the cloaca in males.|
|Special senses||The retina is vascular with a tapetum lucidum. This nocturnal species has a rod to cone ratio of 50:1 (cats have 10:1). While Virginia opossums likely have keen eyesight, their ability to recognize color is limited. Opossums also possess sensitive vibrissae that assist their movement in the dark. 6,29|
|Endocrinology||There is no thyroid function until about halfway through pouch life (see Thermoregulation below). 6,14
The adrenal glands of normal females are twice the size of male glands. Cortisol is the most abundant corticosteroid. 14
|Immunity||Opossums lack true immune function until mid-pouch life, but fortunately immunoglobulin levels are concentrated in marsupial milk. 6|
|Thermoregulation||If the juvenile is removed from the pouch during the first half of pouch life, body temperature will closely reflect environmental temperature.
The ability of young marsupials to regulate body temperature coincides with the start of thyroid function about halfway through pouch life. Thermoregulation occurs through evaporative mechanisms, such as panting, sweating, and licking. 14,31
Brown adipose tissue, which is used by eutherians to generate heat, is not found in marsupials. 31
|Metabolism||The metabolic rate of marsupials is lower than that of placental mammals. Opossums can enter a shallow torporous state for up to 11 hours in which body temperature drops between 11°C to 28°C. 14,31|
|Resistance to envenomation||A remarkable resistance to envenomation by venomous snakes has been reported in the Virginia opossum. 4|
The 50 teeth of the Virginia opossum can be intimidating, however opossums lunge and bite only on rare occasions (see defensive behavior above). Instead most individuals will hiss and sit still during transfer or sedation.
Most opossums can be restrained by wrapping a towel around the body or placing the animal on a flat surface with the towel draped over the head while grasping the tail base. Keep in mind that females can eject pouch young during capture and restraint.
If the opossum is aggressive, wear leather gloves. Sedation or inhalant anesthesia like isoflurane or sevoflurane, is particularly useful for fractious animals or more invasive procedures. 6,11,17,21,31
Juvenile opossums can be kept in a cloth pouch or pocket. 11
Veterinarians are permitted to provide humane veterinary care to any injured or orphaned wild animal. Further rehabilitation and eventual release of wild animals must be performed by licensed individuals. 17 Opossums that have been hand-reared, are too habituated to humans, or have permanent disabilities can be kept as pets in some cases. In many jurisdictions, this requires a permit and veterinarians should be aware of the wildlife regulations concerning the opossum in their area.
- Proper client education is crucial. The owner should carefully manage dietary intake and encourage exercise to minimize the risk of obesity. Also design a regular program of weighing and body condition scoring. 14,31
- Perform routine fecal parasite exams. Screen every presenting opossum for nematodes. Pet opossums should also be regularly screened for Sarcocystis spp. 11,14,15,31
- Spaying and neutering is recommended for companion opossums.13 Intact females display restlessness, pacing, and drooling and they are also at significantly increased risk for chronic urogenital tract infections, endometritis, and a Cushing’s-like syndrome. 6,11,13 Ovariohysterectomy of the opossum may include removal of the lateral vaginas, with care being taken to separate the ureters from the loop between the central and lateral vaginas. 6,11,13 See Therapeutics below for discussion of the midline incision.Neutering significantly reduces the incidence of scent marking. Castration of the opossum involves orchiectomy and scrotal ablation. 11,14,21
- Virginia opossums can shed the protozoal parasite, Sarcocystis spp. in their feces, including S. neurona, S. falcatula, and S. speeri. Based on molecular characterization of sporocysts from intestinal scrapings or feces, the prevalence of S. neurona infection ranged from 26% for opossums in Mississippi to 5.9% in central California. Fecal shedding of sporocysts was higher during the spring and early summer in the California opossums. 6,11,27
- The opossum is also an important natural reservoir of the spirochete, Leptospira spp. 4
- Opossums can also carry the cat flea (Ctenocephalides felis), which can transmit murine typhus, Rickettsia typhi. 4,21
- Opossums carrying Trypanosoma cruzi (Chagas disease) have been found in Texas and the southeastern United States. 4,6,8
- Ringworm and histoplasmosis also infect opossums. Trichophyton mentagrophytes has been described as a cause of sparse, scaly lesions. 4,14
- Virginia opossums are also known to harbor Toxoplasma, Besnoitia, Coccidia, and Trichomonas. 4
- Public health and veterinary professionals frequently have the misconception that marsupials, like the Virginia opossum, cannot contract rabies virus. Although more common rabies reservoirs, like the raccoon, skunk, fox, and bat, pose a higher risk in the United States, there can be spillover in enzootic areas into other species, including the opossum. For instance, opossums can contract rabies virus by eating infected bat carcasses. Clinical signs of rabies virus infection described in the opossum have included banging the head against a wall, growling, aggression, and other abnormal behaviors. In one case, a rabid opossum was found dead. 3,4,21
Most reports of cardiovascular disease in the opossum are limited to experimental models, wildlife necropsies, and anecdotal cases in obese pet animals. Both dilated and hypertrophic cardiomyopathies have been described in opossums over 2 years of age. In research settings, the opossum is used as a model for endocarditis and systemic hypertension. Heart rate as well as electrocardiogram wave configurations, intervals and amplitudes are not affected by “playing dead” in the opossum. 8,14
METABOLIC BONE DISEASE
Nutritional secondary hyperparathyroidism, due to a lack or improper balance of calcium and phosphorus, is common in pet opossums offered lean meat exclusive diets. Fractures, accompanied by fibrous osteodystrophy are commonly observed however caudal body paresis can also occur. 6,11,14,21,22
Obesity is one of the most common medical conditions seen in captive opossums. The marsupial’s basal metabolic rate (BMR) is approximately 30% that of a comparably sized placental mammal. This lower BMR creates lower maintenance requirements for energy, protein, water, and other nutrients, however captive opossums are often fed concentrated diets with a low fiber content and opossums also have voracious appetites. Obesity can occur if the animal is fed too much or able to access another animal’s food. Clinical signs of obesity in the opossum include a bulging tail base and fat protruding periorbitally. Potential sequelae for obese opossums fed concentrated, low-fiber diets include dental disease and gastrointestinal disease. 6,11,14,21,26,31
Ectoparasites, like fleas, ticks and mites, can be found in opossums. Manually remove ectoparasites to reduce the risk of zoonotic disease transmission. Mitchell (2oo9) has used both Fipronil (Frontline®, Merial) and Imidacloprid (Advantage®, Bayer) in opossums without adverse effects. 11,15,22,31
Free-ranging Virginia opossums tend to be heavily parasitized because of their foraging and scavenging behavior. Three nematodes have been reported to cause significant morbidity and mortality in Virginia opossums when present in large numbers. 14,15,22,23,31
- Physaloptera (Turgida) turgida is a common parasite of the stomach.
- Cruzia americana attaches to the wall of the cecum and large intestine. Diarrhea is the only overt clinical sign reported in animals infected with C. americana. The organism can also cause blood loss and anemia. 15
- Verminous pneumonia can be caused by the lungworm, Didelphostrongylus hayesi, however asymptomatic infections are more common in opossums. 6
Perform repeated fecal exams in all free-ranging opossums. Fecal centrifugation and Baermann analysis techniques are also recommended for endoparasite diagnosis and to monitor a patient’s response to treatment. Mitchell (2oo9) has recommended metronidazole for protozoa, praziquantel for cestodes, and fenbendazole or ivermectin for nematodes. 14,15,22
The long, hairless tail of the opossum is a common victim of frost bite. 29
“Crispy or crusty ear”, also called dermal septic necrosis, is a syndrome involving septicemia and necrosis of the edges of the ears and tail tips. Streptococcal endocarditis and associated vasculitis should be considered in young heterothermic animals with these clinical signs. Mange has also been considered a possible underlying cause. Management includes aggressive antibiotic therapy, debridement, non-steroidal anti-inflammatory drugs (NSAIDs), and aloe vera. Laser therapy have been more successful recently in resolving the lesions, however this should be coupled with systemic antibiotics and NSAIDs (Johnson-Delaney, written communication to author, April 25, 2015). The prognosis is guarded to poor. 8,11,14
Trauma, both from predators and vehicles, is one of the most important causes of morbidity and mortality in free-ranging marsupials. 6,17,31 Separate wounded animals, even from littermates, because of the species’ tendency towards cannibalism (Kathy Duncan, email communication to author, July 5, 2018).
Orphaned joeys often present due to the death of the dam during vehicular trauma. Joeys still attached to the nipple have an extremely guarded prognosis for survival even in the hands of very experienced caretakers. House neonates in a plastic container with holes to maintain sufficient humidity (Kathy Duncan, email communication to author, July 5, 2018). Older, furred joeys covered in hair may survive but still present a substantial time commitment. Initiate hand-rearing once immediate supportive care for dehydration and hypothermia has been provided. Marsupials cannot thermoregulate and are not immunocompetent until about. halfway through pouch life and the neonatal marsupial is at a higher risk for septicemia when compared to the neonatal eutherian. 6,11,31
In a short-term hospital setting, select a canine formula to hand feed juvenile opossums. For optimal results, Pacific Wildlife Care uses Fox Valley Opossum Formula Day One 32/40 for neonates under 45 grams, and Day One 25/30 for juveniles over 45 grams. Tube feeding is recommended for joeys less than 25 grams (Kathy Duncan, email communication to author, July 5, 2018).
|P wave duration (second)||0.025–0.035|
|P wave amplitude (mV)||0.005–0.011|
|P–R interval (second)||0.06–0.08|
|QRS duration (second)||0.030–0.060|
|R wave amplitude (mV)||0.040–0.080|
|QT interval (second)||0.14–0.18|
|T wave amplitude (mV)||0.010–0.020|
|Mean electrical axis, (degrees)||negative|
FECAL SAMPLE OR RECTAL CULTURE
Evert the external cloaca to access the dorsal rectum. 6
The urethral opening in female marsupials is inaccessible for catheterization. Urine collection is routinely managed in females by cystocentesis, preferably using ultrasound guidance.
- Ventral caudal (tail) vein at the hair-skin interface
- Jugular vein
- Medial or lateral saphenous vein, typically only for small volumes as this site tends to collapse or form hematomas
- Brachial or cephalic vein (small volumes)
- Lateral caudal vein is small, difficult to locate, and tends to collapse
- Brachiocephalic vein should only be used by experienced personnel
- Pouch veins can be accessed in sedated or anesthetized adult females
INJECTION SITES 11
- Intramuscular: thigh or upper arm muscle mass
- Subcutaneous: intrascapular or flank area
- Intravenous: cephalic or lateral tail veins
- Intraosseous catheters: femur or tibia
The surgical approach must consider the presence of epipubic bones and the pouch. To access the caudal abdomen, tilt the body head down by approximately 30° when a ventral midline approach is used. To access the cranial abdomen, make a midline incision inside the pouch midway between the teats and the cranial border of the pouch, if the pouch is empty or if the pouch young are temporarily removed. It is useful to apply stay sutures to hold the pouch open. If pouch young cannot be removed, create an incision lateral to the pouch opening at the cranial end of the epipubic bone. Then reflect the skin toward midline to expose the linea alba. 31
A tube feeding diet designed for carnivores or omnivores can be offered to critical patients. For debilitated patients that are able to self-feed, the standard diet can be supplemented with live insects, cat or dog food, and other protein sources (see Diet above). 31
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