Rabbit GI surgery (Proceedings)

As with any surgical procedure in any species; prior to cutting it is important to familiarize oneself with the relevant anatomical and physiological details of the species. A very detailed discussion of these two topics is beyond the scope of this presentation but the reader is strongly advised to familiarize oneself with these topics by the references listed below.

The rabbit is a mono-gastric, hind-gut fermenting herbivore, very similar to a horse in this aspect. For the rabbit the indigestible fiber in the diet is critically important for normal GI motility. A chronic lack of fiber in the diet will eventually lead to ileus among other problems.

Just as in the carnivore, the rabbit's stomach is a simple structure, acting primarily as a storage vessel for ingested food, however as a true herbivore it is never empty. This makes endoscopic removal of ingested objects or just plain gastroscopy extremely difficult and often not a successful procedure. The anatomy of the stomach precludes the rabbit from being able to vomit. In the normal, adult rabbit, the pH of the stomach is around 1. The low pH in the stomach acts as a first line of defense for the whole GI tract. The duodenum leaves the pylorus in an acute angle, this is an area in which intestinal obstruction may easily occur and it is considered the most common site for a foreign body impaction.

The acute angle of the gastric outflow tract will easily cause an outflow obstruction in case of a gastric enlargement or bloat.

While the small intestine function is very similar to that of other mono-gastric mammals, its lumen is relatively narrow and the walls thickened in comparison to a dog or cat of the same size.

The second common location for foreign body impaction is the sacculus rotundus. This structure is located in the terminal aspect of the ileum has been modified to form a round, muscular area. There is a valve located at this ileo-cecal- colic junction and its main function is to preclude any retrograde movement of ingesta from the large to the small intestine. As in the horse, the cecum is a very large organ and it comprises approximately 40% of the total volume of the GI tract. The cecum is a delicate, thin walled structure which is easily torn when mishandled in surgery. An anaerobic environment in the cecum is required for the fermentation of the food. The primary microbes involved is the gram-negative bacillus, Bacteroides.

The ascending colon progresses through a series of flexures and has a number of sacculations or haustra. The transverse colon is a short structure ending in a muscular structure known as the fusi coli. This portion of the colon acts as a "pace maker" by managing the separation of the coarse fiber from the more digestible material. The coarse material is thereby eliminated as fecal material. The more digestible material is moved in a retrograde manner by coordinated movements of the sacculations and haustra into the cecum for fermentation. The distal aspect of the colon and rectum are more simple and more durable to the rigors of surgical manipulation.

The overall prognosis for surgery usually decreases the lower the surgery site is in the GI tract anatomy (i.e. pharygostomy better than gastrotomy better than small GI surgery than large GI surgery). In order to have a good prognosis a thorough evaluation of the patient is of utmost importance. Very often the anorexia of the animal has gone unnoticed by the owner especially if the patient is form a multiple rabbit household. One of the most common sequelae to ileus is hepatic lipidosis, represented on the plasma biochemistry profile as a moderate increase in ALP, with a minimal increase in ALT. Hepatic lipidosis in the rabbit will have a very sudden onset. Often anorexia of 24 hours can already start the pathological process. Most of the time the lactate value is extremly when compared to the levels of dogs and cats and the high lactate level does not appear to have prognostic value in these cases. Values as high as 23 mmol/L have been seen in rabbits with gastric stasis, and may be related to production of D lactate in the stomach (Paul-Murphy, J. 2007).

It is important to realize that anorexia is a very non-specific clinical sign in the rabbit. Stress will often lead to anorexia and this can initially be caused by pain, systemic disease, or even ''anxiety.' It is a common phenomenon that differential diagnosis and treatment is only geared towards pathology of the GI Tract. Keep in mind that no matter what the problem is, anorexia is usually the first clinical sign displayed by the affected animal despite the origin of the problem.

It is highly recommended not to rush any anorexia cases into surgery. In the authors experience renal disease is often presented with an acute onset of anorexia and on radiographs, a GI bloat is often seen, this example illustrated that a thorough clinical work is always indicated before rushing into a diagnosis caused primary by a GI tract problem.

Surgery should only attempted after the animal has been thoroughly assessed including blood work and imaging, stabilized and attempted to treat medically for at least 24 hours. If in deed a foreign body has been identified or another cause which supports an invasive surgical procedure, supportive care is often indicated prior to the surgical procedure.

While distentions of the GI tract can be severe, with supportive care and adequate pain medication these condition can often improve significantly within 24 hours without the need for surgery. The author has great success in avoiding surgery with aggressive fluid therapy (100–120 ml/kg/day) and pain medication (oxymorphone at 0.2 mg/kg q 4 h). If aggressive supportive care has been initiated it is a good idea to image the pateint just prior to the surgical procedure as the supportive care can often be curative within 12-18 hours and therefore eliminating the surgical intervention. However, sometimes surgery is truly indicated and can't be avoided.

Once it has been established that surgery is indeed needed to correct the situation routine surgical techniques for GI problems should be applied. However, a few peculiarities should be considered:

1. Enterotomy should be avoided unless absolutely necessary. In cases of an obstruction every attempt should be made to try to gently manipulate the material orad to move it into the stomach where a gastrotomy can be performed. Alternatively, it can be attempted to move the material into the large GI tract where it might not cause an obstruction and will be voided normally.

2. As mentioned above the lumen of the GI tract is relatively smaller when compared to the same size of bowel in dogs and cats. Care needs to be taken, not to cause a narrowing of the lumen after the enterotomy. Closure of the enterotomy site perpendicular to the incision site is a good way to avoid complications due to a narrowing of the GI lumen.

3. Rabbits are very prone to adhesion formation post-GI surgery. For this reason, manipulation of the GI tract should be kept at a minimum (i.e. no 'running' of the bowel during surgery), if needed, gentle tissue handling is of utmost importance. Additionally the use of verapamil (200μg/kg PO, SC q 8 hr for 9 doses) has been shown to reduce adhesion. In addition no blood clots should be left sitting on GI loops as they can readily form adhesions.

4. Without appropriate analgetic follow-up, the animal will go into ileus after the surgery and motility enhancers will not provide significant help. If ileus is present after surgery in most cases, the whole analgetic regime should be reviewed for potential improvement.

Against popular believe the rabbit GI tract does not appear to slow down when opioids are given over prolonged periods. The author has use oxymorphone for many days in different rabbit patients and we have not seen an unwanted side effect. Appropriate analgetic coverage should continue until the animal is eating and eliminating successfully after the procedure. While oxymorphone can't be given by the owner, tramadol appears to be a good alternative to provide for the owner to cover analgesia post the hospital discharge. We currently use 5-10 mg/kg PO q 12-24 hours. Tramadol can be combined with an NSAID to achieve multimodal analgesia. Our NSAID of choice is meloxicam given PO at 0.5-1 mg/kg SID x 5 days post surgery.

If the abdomen has been contaminated with GI content an antibiotic with a good gram positive spectrum should be used. While most oral betalactams are toxic to rabbits, we prefer to use Penicillin G Procain/benazathine mix at 60,000 IU/kg given SC q 48 h x 4.

Conclusion

With the appropriate preparation some knowledge of species specific anatomy and physiology and the adequate post surgical care, GI surgery in the rabbit can be a very rewarding procedure and does not need to be feared significantly.

Keywords

• GI surgery

• Rabbit

• Emergency

• Supportive care

• Anatomy

• Analgesia

References

Popesko P, Rajtova V, Horak J; A colour atlas of the anatomy of small laboratory animals: Rabbit, guinea pig (Vol 1) London: Wolfe Publishing, 1992.

Silverman S, Tell L. Radiology of rodents, rabbits and ferrets: an atlas of normal anatomy and positioning. Philadelphia: W. B. Saunders, 2005.

Meredith A., Flecknell P., BSAVA Manual of Rabbit Medicine and Surgery, 2nd Edition ISBN: 978-0-905214-96-2 Wiley, December 2007

Paul-Murphy, J. Critical Care of the Rabbit Vet Clin Exot Anim 10 (2007) 437–461

Carpenter J.W, Exotic Animal Formulary, 3rd Edition 2005 Saunders ISBN: 978-0-7216-0180-9