Page 10 - VETcpd - Vol 1 - July 2014

Clinical anatomy and physiology of the rabbit gut

Molly Varga BVetMed DZooMed, MRCVS

Molly graduated from the Royal Veterinary College in 1992. She gained her RCVS Certificate in Zoological Medicine in 2001 and her Diploma (Mammalian) in 2007. Her special interests are rabbit medicine and surgery, however she is ready and willing to see anything you can get through the door (safely that is!).

Molly has contributed to several books including the BSAVA Manual of Reptiles, Nursing Exotic Pets and Wildlife, Rabbit Medicine and Rabbit Surgery and upcoming international volumes on rodent and rabbit dentistry and rodent/rabbit medicine and surgery.Her most recent project, an update on the Textbook of Rabbit Medicine is due to be published in October 2014. Molly also reviews articles for other authors and journals, and has been involved in reviewing the WildPro volume on lagomorphs as well as the new Ferret volume. She is an examiner for the RCVS Diploma in Zoological Medicine.

E: molly@cheshirepet.co.uk

IntroductionAlthough rabbits are the third most popular pet in the UK, many owners as well as veterinary professionals struggle to understand the

differences in the rabbit gut, as compared with more familiar species. By discussing the unique anatomy and physiology of the rabbit gastro-intestinal tract it is pos-sible to appreciate more fully why rabbits have such specific dietary and medical requirements.

Mouth and teeth: The rabbit is an herbivore with a very fibrous diet. It has open rooted, continuously growing teeth. Its dental formula is I2/1 C0/0 PM 3/2 M3/3. The presence of peg teeth is a defining feature of lagomorphs. The incisors function in tearing and prehension of food, whilst the cheek teeth are used to grind the fibrous diet, disrupting the cellulose cell walls and initiating mechanical digestion of the diet. Rabbit saliva contains a small amount of amylase that initiates starch digestion (see table 1). Congenital and acquired dental disease result in inability to prehend and adequately chew food and may result in excess salivation, dropping food and manifestation of pain.

Oesophagus: the rabbit oesophagus is similar in both structure and function to those of other more familiar species. It specifically carries food from the oral cav-ity to the stomach. No additional mixing of digesta or digestion is carried out dur-ing transit. The food enters the stomach through the muscular cardiac sphincter. On occasion food or other objects may become lodged in the oesophagus leading to excess salivation, the appearance of food being regurgitated and discomfort.

The aim of this article is to equip veterinary professionals with an understanding of the rabbit gastro-intestinal tract and how it differs from those of more familiar species. It describes the applied anatomy and physiology of the rabbit gut. Form and function with relevance to clinical situations are discussed. The importance of diet and preserving the integrity of the gut microflora is emphasised.

Key words: Rabbit, gut physiology, gut microflora, caecal digestion, caecotrophy.

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Stomach: the rabbit stomach is similar in structure and function to other species. The muscular contractions promote the mixing and disruption of the ingesta whilst the secretion of acid and enzymes promotes the biochemical digestion. The cardiac sphincter is very muscular and does not allow the rabbit to vomit or regurgitate stomach contents. Compared to other mammalian species, the muscular portion of the rabbit stomach is relatively weaker, and the significance of muscular contractions compared to biochemical digestion is therefore reduced. It is normal to find a loose mat of hair and digesta within the stomach. Sugars and starches, important energy sources, are digested and absorbed in the stomach and small intestine (see table 2).

Figure 2: Relative jaw positions affect occlusion

Figure 1: Rabbit skull (Image courtesy: www.skullsunlimited.com)

Incisors

Cheek teeth

Figure 2A: When the jaw is at its caudal position then the cheek teeth should be in contact (occlude)

Figure 2B: When the jaw is in its rostral position the incisors should occlude

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• First palpate the head externally.

• Feel along the ventral mandibles and check for erupting tooth root apices.

• Feel along the nasal bone and determine its angle with respect to the ventral mandible: these should meet at a point in front of the rabbit’s nose: if they do not, and in fact feel parallel, then the cheek teeth are overlong.

• Gently manipulate the mandible through its range of motion, paying particular attention to whether the mobility is adequate, symmetrical and whether there are points at which the teeth ‘catch’.

• Manipulate the lower law in an anterior: posterior plane as well: • When the jaw is at its caudal

position then the cheek teeth should be in contact (occlude) – Figure 2A.

• When the jaw is in its rostral position the incisors should occlude (Figure 2B).

• Examine the occlusion of the incisors: note whether they are present and appear normal – are there ridges or discolouration? Is the occlusal plane horizontal? If not this is an indication that the rabbit has been favouring one side of the mouth. Remember that the rabbit uses its incisors to prehend food.

• Use an otoscope to examine the cheek teeth. The easiest way to accomplish this is to insert the otoscope cone into the diastema on the right hand side of the mouth. From this position one can view the buccal side of the right hand side upper and lower arcades. By manoeuvring the cone over the top of the tongue, a view of the lingual side of the left hand side upper and lower arcades can be obtained. This can then be repeated by inserting the otoscope cone from the left hand side diastema to complete the examination.

• It is important to recognise the limitations of this type of examination, between 30 and 50% of problems will be missed because the rabbit is moving, the soft tissue hampers visualisation, or saliva, pus or blood restricts the view.

• Any abnormalities detected should be further evaluated using radiography, examination using endoscopy under anaesthesia or advanced imaging such as CT.

Table 1: Key points in examining the oral cavity

• Palpation is the mainstay of gut examination during the clinical examination.

• Pay particular attention to the position of the guts, the size (dilated or empty), whether there is pain associated with palpation and what the guts feel like they contain (i.e. fluid, gas, normal digesta, dehydrated or ‘doughy’ digesta)

• Remember to auscultate the guts in order to assess motility: • In normal circumstances (although

stress, the point in the digestive cycle and pathology will alter this), the guts will move 2-3 times a minute. This should sound as though there are small bubbles of gas moving through liquid.

• Loud tinkling fluidy sounds or borborygmi are not normal.

• In very stressed animals, sounds may be diminished or absent and it is often necessary to be prepared to auscultate each side of the abdomen for more than a minute in order to hear gut sounds.

• If abnormalities are detected then fur-ther evaluation should be undertaken: • Radiography is useful to determine

the relative size and position of the guts as well as to rule in/out obstruction (Figures 3-6).

• Ultrasonography can be valuable in considering the structure of the liver and pancreas as well as mass lesions. Many ultrasound machines do not have good enough discrimination to detect subtle gut thickening in rabbits. Ultrasonographic examination can be hampered by the presence of excess gas in the intestines.

• Laparoscopic examination or exploratory surgery maybe indicated for definitive diagnosis.

Table 2: Key points in examining the hind gut

Figure 3: Gas cap in the stomach DV view

Figure 4: Gas in the caecum and small/large intestine secondary to gut stasis.

Figure 6: Normal abdominal radiograph.

Figure 5: Caecal tympany