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

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Aug 01, 2009

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.