Practical anatomy and physical examination: Ferrets, rabbits, rodents, and other selected species (Proceedings)

Although the principles of examination and care are universal in all species, familiarity with normal anatomy is essential in recognizing abnormal conditions. While it would be impossible to discuss all the variations for different species of small mammals, this discussion will concentrate on the basic variations from more traditional species for the more commonly seen pets.

Physical exam

Even in an unfamiliar species, a thorough systematic examination can detect abnormalities regardless of anatomic variation. Proper restraint is also essential for the safety of the patient as well as the practitioner. In fractious or nervous patients, or those difficult to restrain, sedation may be required to perform a thorough physical examination. Inhalant anesthesia is usually preferred for brief evaluation.

Rabbits

Integument

The skin of rabbits is quite thin in comparison to the dog and cat. When clipping, rabbit skin is exceptionally prone to tearing, with the exception of the intact buck, whose skin is comparable to that of a tomcat. Intact does will develop a prominent dewlap. The dewlap usually decreases in size in response to neutering.

Scent glands in rabbits are located between the mandibles, but are rarely identified. The more commonly noticed scent glands are the paired inguinal glands, appearing as a fold adjacent to the genitalia and anus. Brown secretions may accumulate in this area. Anal glands are also present in rabbits, although odor is generally not detected. A sensory pad is located at the mucocutaneous junction of each nostril, covered by fur.

Digestive system

Dental Formula: 2(I 2/1 C 0/0 P 3/2 M 3/3). Caudal to the upper incisors is a second set of maxillary incisors, the 'peg teeth', This is one of the characteristics that differentiate rabbits from rodents. All teeth are open rooted and continuously erupt. The incisor growth rate of the upper arcade is 2.0 mm/week; the lower incisors grow at 2.4 mm/week.

As herbivores, rabbits have an extremely long digestive tract. The stomach has a cardia and a pylorus, but there is a limiting ridge at the junction of the esophagus and stomach which prohibits vomiting. The small intestine of rabbits has an extremely small lumen throughout (approximately the size of a pencil). The ileum ends at a T-shaped junction with the cecum and large intestine, in a section called the sacculus rotundus or ileocecal tonsil. This is a potential site for intestinal impaction.

The cecum of rabbits is thin-walled but extremely large and distensible. It coils upon itself three times within the abdominal cavity, and contains bands and saccules. Following meals, rabbits produce moist, mucous-covered feces called cecotrophs which are re-ingested to provide bacteria and nutrients for the rabbit. The cecum of rabbits holds 57% of the dry matter of the large intestine.

Urinary system

Rabbit kidneys are relatively mobile and generally palpable deep within the abdominal cavity. There is a single papilla entering the ureter from each kidney. The urethra of the female rabbit empties in to the proximal end of a deep vaginal vestibule. The urine of rabbits may be orange or brownish red in color. The cause for this is unknown but has been attributed to dietary compounds, plant pigment, or stress. The color production is usually intermittent, but may be mistaken for hematuria. The calcium excreted in the urine may lead to a chalky or cloudy appearance to the urine, and calcium carbonate or calcium oxalate crystals may routinely be present in normal urine.

Respiratory system

Rabbits are obligate nasal breathers. Inadvertent occlusion of the nasal passages during any procedure, including oral exam, can lead to respiratory compromise due to the ineffectiveness of mouth breathing. This can be a concern even when using a small facemask for oxygen or anesthesia administration if the nares are forced against the wall of the mask.

The nasal passages are in close proximity with the maxillary dental arcade, and changes in either the nasal passages or molar tooth roots may affect each other adversely. Diseases invading the nasal passages may alter bone structure, and may ultimately lead to molar tooth movement and malocclusion; conversely, molar abnormalities and root elongation may impinge on nasal passages and compromise respiration.

The rabbit trachea is deeply recessed within the oral cavity behind the torus of the tongue. The trachea itself is narrow relative to body size. The thoracic cavity is small in comparison with the large abdominal cavity. Because of the small thoracic cavity, rabbits have more referred upper airway and bronchial sounds and may sound somewhat harsh. Significant respiratory compromise may occur if a rabbit is placed in dorsal recumbency for surgery when the stomach or cecum is greatly distended. Positioning the patient on a tilt table or elevating the thorax with towels or pillows can decrease the risk of respiratory compromise at surgery. The thymus persists through adult life in rabbits, and may be visible radiographically.

Cardiovascular system

The cardiovascular system of rabbits is unique. Both the right and left atrioventricular valves are bicuspid in rabbits. The heart is small relative to total body size, comprising only 0.3% of the total body weight. Rabbits have the most muscular pulmonary artery of any species, which contributes to their predisposition for pulmonary hypertension. Other vessels in rabbits are thin-walled, and prone to collapse and hematoma formation with venipuncture. The external jugular vein provides the main route for venous drainage from the head, as compared to the internal jugular vein in most mammals. There is a lack of anastomoses between the external and internal jugular veins. This is clinically significant because ligation or thrombosis of the external jugular vein can lead to temporary exophthalmos. Ligation of the external carotid artery will cause ocular necrosis on that side.

Musculoskeletal system

The bones of rabbits are much lighter than most other species, comprising only 8% of the body weight, as compared to 12 to 13% in cats. The bones have thin cortices and are easily shattered. The forelimbs have five digits but the hind limbs only have four. The nails are long and narrow for digging and burrowing, but are not retractable, and rabbits should not be declawed. There are no footpads; instead the feet are thickly furred to protect the plantar surfaces. The powerful hind limb musculature and light skeleton enable powerful jumping over long distances; however, the longer spinal column is more prone to luxation with a powerful kick or struggle if the hind end is not well supported during restraint.

Reproductive system

The testes of male rabbits are located within hairless scrotal sacs which are located cranial to the penis. The inguinal canals remain open throughout the life of the rabbit. Male rabbits lack nipples. Differentiating males from females can be difficult, as the anogenital distance is the same in males and females. Eversion of the cranial orifice will reveal either a circular entrance to the penis or a slit entrance to the vagina. Female rabbits have two ovaries, two uterine bodies, and two cervices. The females have 4 to 5 pairs of mammary glands and nipples.

Ferrets

Integument

Ferrets have a double coat of fur; the thick outer coat and a lighter undercoat. Some ferrets may become lighter in color with the summer molt, darkening again in the fall or winter. This is more prominent in ferrets kept outdoors. The skin of ferrets is thick, even when neutered.

Digestive system

Dental formula: Adult dentition erupts at 50-74 days, beginning with the canine teeth. Dental formula is 2 (I3/3 C1/1 PM 3/3 M3/3). Ferrets have typical carnivore dentition, closed rooted, with powerful long-rooted canine teeth. The stomach is in the left cranial abdomen and can greatly expand. Ferrets can vomit. The small intestine is about 180-200 cm; there is no demarcation between the jejunum and ileum. Ferrets lack an ileocolic valve. The scent or musk glands (anal sacs) are located at either side of the anal canal. Gastrointestinal transit time in ferrets is approximately 3 hours.

Urogenital system

The kidneys are located retroperitoneally, and can easily be palpated in most ferrets. They are fairly mobile within the abdominal cavity. The bladder normally holds about 10 ml of fluid at low pressure. In females, the ovaries are caudal to the kidneys. The uterus is comprised of two long horns, a short uterine body, and a single cervix. Most ferrets are spayed or castrated prior to sale. The vulva is small. In males, the penis contains a J-shaped os penis. There is prostatic glandular tissue at the base of the bladder which may surround the urethra.

Respiratory system

The ferret trachea can be easily visualized for intubation. The left lung is comprised of cranial and caudal lobes. The right lung has 3 lobes, the cranial, middle, and caudal. There is a 6th accessory lobe. Anatomy is similar to that of most mammals. The respiratory system is elongated in comparison to most mammals.

Cardiovascular system

The heart is located further caudally than most mammals, between the 6th and 8th ribs. There may be periapical fat, creating the radiographic appearance of elevation from the sternum on a lateral view.

The liver has two crura and is divided into 6 lobes. The spleen lies predominantly on the left side of the abdomen, running along the greater curvature of the stomach, and can vary greatly in size depending on age and state of health. The spleen will enlarge greatly in the anesthetized ferret.

Musculoskeletal System

The ferret has short legs and an elongated body. A healthy ferret will have an arched back which may become more prominent with ambulation. Loss of this arch may represent weakness or illness. The ferret spine is extremely flexible, making spinal or disk injuries extremely rare. Unlike most mammals, the ferret has 15 thoracic vertebrae, 5 lumbar, and 3 sacral vertebrae. There are 5 toes on each of the four feet, also with non-retractable claws. Nails can be easily trimmed but should not be declawed.

Endocrine system

The pancreas is bilobed and similar to dogs. The pancreatic ducts extend from the central area of the pancreas to the duodenum. The adrenal glands lie cranial and medial to each kidney. The left adrenal gland is supplied by the easily identified adrenolumbar artery; the adrenolumbar vein crosses the gland on the ventral surface. The right adrenal gland is almost always adherent ventrally to the caudal vena cava. Its blood supply comes from 3-5 vessels that arise from the right renal artery, the right adrenolumbar artery, and the aorta.

Rodents

Guinea pigs, chinchillas

These two species are anatomically similar, so they will be discussed together. Differences will be identified.

Integument

Guinea pigs have sebaceous glands at the dorsal rump and tail base, which may become quite prominent and may appear oily.

Chinchillas are well known for their full coats, which can contain up to 60 hairs from a single follicle. If scruffed or grabbed by the coat, chinchillas will shed a large patch of fur as an escape mechanism. This is known as "fur-slip" and may take months for regrowth. Bathing is accomplished by dust baths, which may occur as often as daily. Although many owners believe that chinchilla fur cannot become wet, the process of dust bathing easily restores the softness of the fur even after becoming wet.

Digestive

The dental formula for both species is 2 (I 1/1 C 0/0 PM 1/1 M3/3). All teeth are open rooted. The incisors are yellow in chinchillas, but not in guinea pigs. The stomachs have a glandular epithelial lining. The intestinal tracts of both species are long, with a prominent cecum. The cecum contains taenia coli, longitudinal bands which form lateral sacculations. The cecum of guinea pigs holds 44% of the dry matter content of the large intestine. Gastric emptying time of guinea pigs is 2 hours, with GI transit time of approximately 20 hours. Coprophagy can prolong transit time. Some older male guinea pigs can develop fecal retention or impaction in the anal region, which may represent a loss of muscle tone, and may require periodic cleaning. The cecum of chinchillas is smaller than guinea pigs, holding only 23% of dry matter content of the large intestine. Both species practice coprophagy.

Urogenital

Male guinea pigs have an os penis. The inguinal canals are open. There are large paired vesicular glands that extend for up to 10 cm in the abdominal cavity and can be mistaken for uterine horns. Female guinea pigs have a cartilagenous pelvic symphysis, which fuses with age and can cause difficulty in delivery if not bred prior to 6 months of age. Female guinea pigs have paired uterine horns and a single cervix.

Male chinchillas lack a true scrotum, and the testes are freely mobile. There is no os penis but the penis is easily exteriorized. Female chinchillas have paired uterine horns and two cervices. Females have a large urogenital papilla which is frequently mistaken for a penis. The most reliable method for sexing chinchillas is the anogenital distance, which is greater in males.

Urine of both species is alkaline and may have crystals.

Respiratory

Lungs are functionally similar to most mammals. In both species, the thoracic cavity is small in comparison to the abdominal cavity.

Cardiovascular

The right a-v valve is tricuspid, and the left is bicuspid. Mild heart murmurs may be present in chinchillas without significant cardiac disease.

Musculoskeletal

Guinea pigs have four digits on the front limbs, and three digits on the rear limbs. Chinchillas have four toes on all feet.

Mice, rats, hamsters, gerbils

These species are similar and will also be discussed together, with differences addressed.

Integument

Although not truly integumentary, these rodents possess harderian glands behind the eyes, which produce ocular secretions that are rich in lipid and porphyrin. The porphyrins may be mistaken for blood, and can be especially apparent if the animal is ill and not grooming regularly. Rodents do not possess sweat glands and should not be exposed to high temperatures, as they also are unable to pant. Gerbils have a ventral area of alopecia known as the ventral marking gland. It is a sebaceous gland which is influenced by reproductive hormones and may produce secretions in males. Hamsters have brown colored hip or flank glands. Yellowing of the coat of older rats is normal.

The tails of rats and mice are long and hairless. It is safe to restrain these species by the base of the tail. Hamster tails are short and furred. Gerbil tails are long and furred with a tuft at the end. The skin and fur of a gerbil tail is easily pulled off if these animals are grabbed by the tail, leaving only muscle and bone exposed. If this occurs, amputation is required.

Digestive

Dental Formula: 2(I1/1 C0/0 PM0/0 M3/3). The incisors of these species are used for gnawing, and the lower incisors are approximately three times the length of the upper incisors. The incisor teeth are yellow and are open rooted. The molars, however, are closed rooted and should never be trimmed. There is a limiting ridge at the junction of the esophagus and stomach which prohibits vomiting. These rodents are mostly herbivorous or omnivorous. Hamsters have extremely large cheek pouches.

Urogenital

Females are prolific breeders. They have paired ovaries, and a single cervix. Most females will form a copulatory plug following mating, which is a combination of mucous and secretions from the accessory sex glands. Inguinal canals in males remain open. Males also possess an os penis. Sexual maturity may occur as early as 4-5 weeks. Nipples are only present on females. The number of nipples present in each species is as follows: Gerbils, 4; hamsters 6-7; rats 6; mice 5. However, both sexes have vast amounts of mammary tissue which can extend cranially to the cervical region and caudally to the pelvis.

Musculoskeletal

Gerbils have five front toes and four rear toes. Hamsters, mice, and rats have four toes in the front and five on the rear feet.

Other species

Sugar gliders

Sugar gliders have an extremely unique dentition. 2(I 2 or 3/1 or 3 C 1/0 P 1 or 3/1 or 3 M 3 or 4/3 or 4). Their incisors are specialized for gouging the bark of trees, and are closed rooted. These teeth should never be trimmed. They are nocturnal and as such have extremely large eyes. They have four toes on the hind feet; an opposable first digit; and the first and second digits of hind feet are fused (syndactyl). Scent glands (bald patches) are present on the forehead and upper chest in intact males. Newborns possess a continuous cartilaginous arc in the shoulder girdle, which provides support for the newborn to climb into the pouch. These bones break down immediately after birth. Females possess a prominent pouch; males have testes which are contained within a scrotal sac that is located cranial to the penis. Gestation is 15-17 days, but the young don't emerge from pouch until 60-70d post-parturition. This is a cause for confusion when discussing the age of a juvenile sugar glider.

Marsupials are naturally hypoglycemic, with normal resting blood glucose in wild animals ~50mg/dL.They are relatively glucose intolerant. Metabolism is about ⅔ that of other mammals, and heart rate is about ½ the rate of comparably sized placental mammals. Average body temperature around 90 F.

Hedgehogs

Hedgehogs have dorsal spines, but ventral fur. The spines of hedgehogs, unlike porcupines, are not easily removed from the skin. In fact, in a healthy hedgehog, the spines are so firmly adherent that the entire animal can be picked up by the spines. (NOT recommended!) Hedgehogs also have unique dentition: the dental formula is 2(I3/3 C1/1 PM 3-4/3-4 M3/3). The teeth lie in an irregular pattern within the oral cavity and are used for biting through the exoskeleton of insects. They are insectivores and omnivores, but insects represent an important part of their diet. They are good swimmers, climbers, and burrowers. Hedgehogs have an unusual behavior in which they 'anoint' themselves with frothy saliva when introduced to a new substance. They reach sexual maturity at 2 months, and gestation is 34-37 days. Male hedgehogs have prominent seminal vesicles which lie dorsal to the bladder and are responsible for production of secretions. They also have paired Cowper's glands which lie adjacent to the penis.

References

Fox, JG. Biology and Diseases of the Ferret. Lea & Febiger, Philadelphia, 1988.

Harcourt-Brown, F. Textbook of Rabbit Medicine. Elsevier Science Limited, Kent, UK, 2002.

Harkness, JE. Essentials of Pet Rodents. AAHA Press, Lakewood, CO, 1997.

Hillyer EV and Quesenberry KE: Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. WB Saunders, Philadelphia, PA, 1997.

Johnson-Delaney CA. Exotic Companion Medicine Handbook for Veterinarians. Lake Worth, FL:Wingers, 1997.

Johnson-Delaney CA. The marsupial pet: sugar gliders, exotic possums, and wallabies. Proc Annu Conf Assoc Avian Vet. 1998:329-339.

Lewington, JH. Ferret Husbandry, Medicine and Surgery. Elsevier Science Limited, Edinburgh, 2002.

Reeve, N. Hedgehogs. T&AD Poyser Ltd, London, 1994.