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Part I
Small Mammals
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Chapter 1
Small Mammal Handling and Chemical Restraint
Classification of small mammals
The commonly seen species of small mammals in veterinary practice are classified in Table 1.1.
Table 1.1 Classification of commonly seen small mammals.
RABBIT
Biological average values for the domestic rabbit
Table 1.2 gives the biological parameters for domestic rabbits.
Table 1.2 Biological parameters for the domestic rabbit.
| Weight (kg) | 1.5 (Netherland dwarf) to 10 (New Zealand whites and Belgian hares) |
| Rectal body temperature (°C) | 38.5–40 |
| Respiratory rate at rest (breaths per minute) | 30–60 |
| Heart rate at rest (beats per minute) | 130 (New Zealand whites) to 325 (Netherland dwarf) |
| Gestation length (days) | 29–35 (average 31) |
| Litter size | 4–10 |
| Age at sexual maturity (months) | |
| Male | 5–8 |
| Female | 4–7 |
| Lifespan (years) | 6–10 |
Musculoskeletal system
The skeletal system of rabbits is light. As a percentage of body weight, the rabbit's skeleton is 7–8%, whereas the domestic cat's skeleton is 12–13%. This makes rabbits prone to fractures, especially of the spine and the hindlimbs.
Skull
The mandible is narrower than the maxilla, and the temporomandibular joint has a wide surface area, allowing lateral movement of the mandible in relation to the maxilla.
Axial skeleton
The cervical vertebrae are box-like and small and give mobility. The thoracic vertebrae possess attachments to the 12 paired ribs, which are flattened in comparison to cat's ribs. The pelvis is narrow and positioned vertically. The iliac wings meet the ischium and pubis at the acetabulum, where an accessory bone unique to rabbits, called the os acetabuli, lies. The pubis forms the floor of the pelvis and borders the obturator foramen which is oval in rabbits.
Appendicular skeleton
The scapula is slender and distally has a hooked suprahamate process projecting caudally from the hamate process. The scapula articulates with the humerus which in turn articulates with the radius and ulna. In rabbits, the ulna fuses to the radius in older animals and the two bones are deeply bowed. The radius and ulna articulate with the carpal bones which in turn articulate with the metacarpals and the five digits.
The femur is flatter than a cat ventrodorsally, and the tibia and fibula are fused in the rabbit. The tibia articulates distally with the tarsal bones where there is a prominent calcaneus bone. The tarsals articulate with the metatarsals which articulate with the four hindlimb digits.
The hindlimbs are well muscled and powerful.
Respiratory anatomy
Upper respiratory tract
Rabbits, like horses, are nasal breathers, with the nasopharynx permanently locked around the epiglottis; hence, upper respiratory disease or evidence of mouth breathing is problematic. The nasolacrimal ducts open onto the rostral floor of the nasal passage. The epiglottis is not visible easily from the oral cavity, making direct intubation difficult. It is narrow and elongated and leads into the larynx which has limited vocal fold development. The larynx leads into the trachea which has incomplete C-shaped cartilage rings for support.
Lower respiratory tract
The trachea bifurcates into two primary bronchi. There are two lungs, which are relatively small in proportion to the overall rabbit's body size. This means that even minor lung disease may cause serious problems. Each lung has three lobes, with the cranial ones being the smallest (see Figure 1.1).
Respiratory physiology
The impetus for inspiration derives from the muscular contraction and flattening of the diaphragm. The lung parenchyma possesses a cellular population that is well supplied with anaphylactic mediating chemicals. These are strong enough to cause fluid extravasation and blood pooling as well as spasms within the walls of the main pulmonary arterial supply, leading to rapid right-sided heart failure.
Digestive system
Oral cavity
The dental formula is
I 2/1 C 0/0 Pm 3/2 M 3/3.
The teeth are elodont (‘open rooted'), allowing continual growth throughout the rabbit's life. The molar enamel is folded providing an uneven occlusal surface with the ipsilateral jaw which allows interlocking. Wear is kept even by the lateral movement of the mandible, allowing independent left and right arcades to engage in mastication. The incisors differentiate Lagomorpha from Rodentia as rabbits and hares have two smaller incisors, or ‘peg teeth', behind the upper two, whereas rodents have only two upper incisors. The larger incisors only have enamel on the labial surface, whereas the smaller maxillary peg teeth have enamel on the labial and lingual sides. This creates a wedge-shaped bite-plane where the lower incisors close immediately behind the upper large incisors and fit into a groove made by the peg teeth. The permanent incisors are present at birth, although the peg teeth are replaced by permanent peg teeth at around the second week of life. The deciduous premolars present at birth are replaced and joined by permanent molars by the fourth week of life. There are no canines; instead, there is a gap, or diastema, between the incisors and premolars (Figure 1.2).
Stomach
The stomach is a large, simple structure, with a strong cardiac sphincter (see Figure 1.1). This makes vomiting in the rabbit virtually impossible. There is a main body, or fundus, and a pyloric section with a well-formed pyloric sphincter. The lining of the stomach wall contains acid-secreting and separate pepsinogen-secreting cells. The pH of the rabbit's stomach contents is surprisingly lower than a cat's or dog's at 1.5–1.8. In addition, a healthy rabbit's stomach never truly empties.
Small intestine
The total length of the small intestine in the average rabbit may be some 2–3 feet! It is difficult to determine the divisions between duodenum, jejunum and ileum as they all have a similar diameter.
Caecum and large intestinal anatomy
At the junction of the ileum and caecum lies the sacculus rotundus. This is a swelling of the gut, infiltrated with lymphoid tissue and a common site for foreign body impactions. The caecum is large, sacculated and spiral-shaped, finishing in a blind-ended, thickened, finger-like projection known as the vermiform appendix, which also contains lymphoid tissue. The bulk of the caecum is thin walled and possesses a semi-fluid digestive content.
The start of the large intestine is the ampulla coli which sits near to the sacculus rotundus and caecum. It is a smooth-walled portion of the gut with some lymphoid infiltration of its walls, unlike the rest of the larg...
