UNIQUENESS OF RABBITS

Rabbits, guinea pigs, and chinchillas are all classified as hindgut fermenters, depending on primarily cecal micro flora for nutrient composition. The rabbit has some unique anatomical features including the sacculus rotundus and the vermiform appendix. Gastrointestinal disorders in these animals can be a challenge to clinicians as not only the motility of the hindgut must be maintained, but the micro flora as well. Dysbiosis, or changes in the micro flora can release toxins and further alter the pH, micro flora and motility. The clinician must also be aware of gastrointestinal pain and hydration status accompanying most gastrointestinal disease.

The Rabbit Gastrointestinal System

Although dental health and thorough examination of the teeth should always be included in the physical examination of a rabbit presented with suspected digestive system disease, this discussion will concentrate on the gastrointestinal system. Nutrition plays an important role in the functioning of the rabbit digestive system and will be discussed as it pertains to the gastrointestinal anatomy, physiology, and major disease syndromes. Rabbits are true non-ruminant herbivores. Their digestive reservoir permits and increases the efficiency of utilization of fibrous diets. They have a large stomach and well-developed cecum relative to other non-ruminant herbivores such as the horse.

Stomach

The stomach of the rabbit holds approximately 15% of the volume of the entire gastrointestinal tract.  It is thin- walled, J-shaped, and lies to the left of the midline. The well-developed cardiac sphincter is lined with non- glandular stratified squamous epithelium and prevents vomiting. The fundus contains parietal cells that secrete acid and intrinsic factor as well as chief cells that secrete pepsinogen. The pylorus has a well-developed, muscled sphincter. The adult rabbit stomach has a pH of 1 – 2. The rabbit feeds frequently – up to 30 times per day of 2 – 8g of food over 4 - 6 minute periods. The stomach normally will contain a mixture of food, hair, and fluid even after 24 hours of fasting. The stomach pH of rabbits up until the time of weaning falls into the range of 5.0 – 6.5. Bacteria is kept in check during the first 3 weeks of life by the production of milk oil containing octanoic and decanoic fatty acids produced by the enzymatic reaction of the suckling rabbit’s own digestive enzymes on the doe’s milk. Young rabbits acquire gut flora by consumption of the doe’s cecotrophs beginning at 2 weeks of age. Milk oil production ceases at 4 – 6 weeks of age. By this time, some ingested organisms have colonized the cecum and hindgut fermentation can begin as the bunny weans. Gastric transit time is approximately 3 – 6 hours. The high voluntary feed intake (VFI) is at least 4 times higher pro rata than a 250kg steer. It is also associated with a low gut retention time of 17.1 hours in the rabbit compared with 68.8 hours in the bovine. High VFI together with re-utilization of gut content by reingestion of Association of Avian Veterinarians 10 cecal material supports the rabbit’s high nutrient requirement per unit of body weight and improves feed utilization for the rabbit The bovine’s main volatile fatty acid (VFA) produced by rumen fermentation is propionic acid while the rabbit’s main VFA is acetic acid with cecal fermentation. The primary microflora of the rabbit is Bacteroides species while Lactobacillus species is the primary microflora of the bovid.

Small Intestine

The small intestine is approximately 12% of the gastrointestinal volume in the rabbit. The bile duct enters into the proximal duodenum. The right lobe of the pancreas is situated in the mesoduodenum of the duodenal loop. The left lobe lies between the stomach and transverse colon. There is a single pancreatic duct that opens at the junction of the transverse and ascending loops of the duodenum. The duct drains both pancreatic lobes. Technically this is the accessory pancreatic duct as the main pancreatic duct connection to the duodenum disappears during embryonic development. The jejunum is the longest section of small bowel and appears convoluted. Aggregates of lymphoid tissue (Peyers patches) are present in the lamina propria with increasing prominence distally. The distal end of the ileum has a spherical thick-walled enlargement known as the sacculus rotundus. This marks the junction between the ileum, cecum, and colon. The sacculus rotundus is often called the “cecal tonsil” because of its lymphoid tissue and macrophage composition. This organ is unique to rabbits. An ileocolic valve controls movement of ingesta from the ileum into the sacculus and prevents reverse movement of ingesta back up into the ileum. The ileocolic valve opens into the ampulla coli at the junction of the ileum, colon, and cecum. There is a weak ileocecal valve that allows chyme to pass into the cecum. Gastrointestinal smooth muscle is stimulated by motilin, a polypeptide hormone that is secreted by enterochromaffin cells of the duodenum and jejunum. Motilin is released in response to fat while carbohydrates inhibit release. Motilin activity is not present in the cecum, but is present and stimulates smooth muscle in the colon and rectum.

The stomach and small intestine in the rabbit function similarly to other monogastric animals. Cecotroph digestion and some fermentation takes place during the 6–8 hours they remain in the gastric fundus. Cecotrophs contain microorganisms and products of microbial fermentation including amino acids, volatile fatty acids, and vitamins. A gelatinous mucous coating protects them from some of the stomach acid. As the cecotrophs passed through the colon, lysozyme was incorporated. The lysozyme has bacteriolytic activity that degrades microbial proteins for absorption in the small intestine. Bacteria within the cecotroph produce amylase that converts glucose to carbon dioxide and lactic acid. These products along with amino acids and vitamins are absorbed primarily in the small intestine. Digestion in the stomach begins with hydrochloric acid and pepsin and continues into the proximal small intestine. Amylase from the pancreas is added, although amylase is also present from saliva and cecotrophs. The pancreas also contributes proteolytic enzymes and chymotrypsin through the accessory duct as well as most likely through small ducts connecting directly to the duodenum. Bicarbonate is secreted by the proximal duodenum to neutralize the acidity of ingesta leaving the stomach. The bicarbonate is absorbed in the jejunum. Transit time through the jejunum is 10–20 minutes and 30–60 minutes through the ileum.

Hindgut

The hindgut consists of the cecum and colon. The cecum of the rabbit is large and may contain 40% of intestinal content. It has 10 times the capacity of the stomach. The cecum is thin-walled and coiled in 3 gyral folds. It ends in a blind-ended tube called the vermiform appendix. This appendix contains lymphoid tissue and secretes bicarbonate that buffers the cecal acids, and water to form the cecal paste. In addition to Bacteroides species, there may also be ciliated protozoa, yeasts, and small numbers of E coli and clostridia species in the cecal flora. The fermentation process in the cecum results in volatile fatty acids that are absorbed across the cecal epithelium. Cecal contents have an alkaline pH in the morning and an acid pH in the mid afternoon, termed a “transfaunation” as types of microorganisms fluctuate. In addition the predominant VFA of acetate, butyrate, and propionate are also produced. The ascending colon is divided into 4 sections. The ampulla coli opens into the first section, approximately 10 cm long and having 3 longitudinal flat bands of muscular tissue (taeniae) that separate rows of haustra or sacculations. The mucosa of this section has small protrusions approximately 0.5mm in diameter that is termed “warzen” or warts. These are unique to lagomorphs and greatly increase the surface are of the colon for absorption. The warts may also aid in mechanical separation of ingesta. The taeniae are innervated with autonomic fibers from the myenteric plexus. The second section of colon has a single taenia and fewer, smaller haustra. There are segmental and haustral contractions that mechanically separates the ingesta into indigestible particles and liquid contents. As the large pellets pass down the middle of the lumen, water is re-absorbed and they are excreted as hard dry pellets. The third section is the fusus coli. It is a muscular area about 4cm long, highly innervated, and vascular. Its mucosal surface has prominent longitudinal folds and goblet cells. It opens into the fourth section of ascending colony that is indistinguishable histologically from the transverse and descending colon. The distal colon (sections distal to the fusus coli) ends at the rectum. Its mucosa has short crypts with abundant goblet cells. It is thin-walled and usually contains hard fecal pellets.

Cecotrophy, not Coprophagy

Cecotrophs are formed in the proximal colon and cecum. Rabbits begin consuming them between 2 and 3 weeks of age as they begin to eat solid food. Fiber material greater than 0.5 mm does not enter the cecum but transits to be formed and passed as hard fecal pellets. The smaller particles and fluid remain in the cecum or are returned to the cecum via antiperistalsis to form high nutrient particles that become coated with mucus as they pass through the colon. They are usually passed 8 hours or so after feeding, which coincides usually to nighttime. This mechanism requires high fiber diets to function properly. Low fiber diets increase cecal retention time and promote hypomotility of the entire gut, which further reduces the cecotrophs produced. Fiber in the diet should be indigestible and at least 15%. A low protein diet increases a rabbit’s cecotroph ingestion. A high protein diet and low in fiber reduces consumption. In crude fiber terms, diets that are less than 150 g/kg of feed will almost always result in digestive upset while diets with greater than 200 g/kg crude fiber result in increased incidence of cecal impaction and mucoid enteritis. A diet devoid of fiber has a coefficient of apparent digestibility of organic matter of 0.90. This declines in a linear fashion to 0.40 when the diet contains 350 g crude fiber per kilogram of feed. Increased crude fiber of the diet increases the crude fiber of the cecal contents. This decreases the protein content. Compounded, pelleted diets require the addition of hay in order to supply a complete diet. In general, the recommendation that hay be supplied on a free-choice basis as a rule of good husbandry of the pet rabbit should be emphasized.
High carbohydrate diets cause several problems. Excessive glucose allows Clostridium spiroforme and E coli to colonize. Excess VFAs produced drop the cecal pH, that inhibits normal flora and allows pathogens to proliferate and colonize. Gas and toxins can be produced by pathogenic bacteria, and motility and nutrient production and absorption are interrupted. Fats such as full-fat soybeans, oilseeds can be used as a source of energy without causing cecal hyper-fermentation. However, feeding of vegetable fats and seeds decrease the fiber content of the diet, and lead to motility and functional depression.

References

 Deeb B. Digestive system and disorders. In: Flecknell PA, ed. Manual of Rabbit Medicine and Surgery. Quedgeley, UK: British Small Animal Veterinary Association; 2000:39–46.

Harkness JE, Wagner JE. The Biology and Medicine of Rabbits and Rodents. 4th ed. Media, PA: Williams and Wilkins; 1995.Flecknell PA. Guinea pigs. In: Meredith A,

Redrobe S, eds. BSAVA Manual of Exotic Pets 4th ed. Quedgeley, UK: British Small Animal Medical Association; 2002:52–64.

Hoefer HL, Crossley DA. Chinchillas. In: Meredith A, Redrobe S, eds. BSAVA Manual of Exotic Pets. 4^th ed. Quedgeley, UK: British Small Animal Medical Association; 2002:65–75.

Donnelly TM. Disease problems of chinchillas. In: Quesenberry KE, Carpenter JW, eds. Ferrets, Rabbits and Rodents, Clinical Medicine & Surgery. 2nd ed. St. Louis, MO: Saunders; 2004:255–265.