Join Dr. Micah Kohles of Oxbow Animal Health for an animated exploration of hindgut fermentation in rabbits.
Domestic rabbits (Oryctolagus cuniculus) are herbivores, concentrate selectors, and are classified as hindgut (cecum) fermenters. They are mostly crepuscular and nocturnal feeders. They are anatomically and physiologically adapted to handle significant amounts of low-energy density fibrous food and to effectively utilize the nutrients found in a high-fiber diet. However, due to their small body size, they are unable to store large amounts of food material and therefore utilize a unique process to rapidly eliminate fiber for the gastrointestinal system.
Get to Know the Rabbit's GI System
A rabbit's gastrointestinal system allows for the intake of large amounts of low-energy density, high-fiber food that is separated based on particle size. The easily fermentable component of the diet is retained, while the slowly fermentable components of the diet (preeminently cellulose based plant fiber) are rapidly eliminated. This colonic separation mechanism (CSM) allows for the production of two types of stool: a dry, hard, high-fiber fecal pellet and an enlarged, soft fecal pellet that is covered in mucous and referred to as a cecotroph (cecal, night stool, etc.). In rabbits it is commonly referred to as the “Wash back” CSM.
Rabbits, like all herbivores, have a symbiotic microbe relationship with a diversity of gut flora (primarily Bacteroides), as they produce limited mammalian enzymes to break down the cellulose components of their plant based diets. Some of the most common issues with domestic rabbits, both in companion and commercial animals, are related to the gastrointestinal system. The symbiotic relationship rabbits have with the micro flora in the cecum allows them to effectively ferment plant structural-carbohydrates which provide their main energy source.
The rabbit cecum is very large and has a capacity roughly ten times that of the stomach, making up around 40% of the total gastrointestinal tract. It is a thin-walled organ that folds on itself multiple times and has an internal surface made up of a long spiral fold (sometime referred to as spiral valve) that is continued into the beginning of the colon, an area known as the ampulla coli. The distal tip of the cecum is known as the vermiform appendix. This tube-like area is narrow and thickened compared to the cecum and contains significant amounts of lymphoid aggregates which secrete bicarbonate ions into the cecum. The bicarbonate ions act as a buffering agent for the volatile fatty acids formed during cecal fermentation.
Sections of the Colon Explained
The colon is divided into three sections, the ascending, transverse, and descending colon. Because the fusus coli (located anatomically at what is referred to as the transverse colon) forms such a natural division between the morphologically and functionally distinct ascending and descending sections of the rabbit colon, many physiology texts have abandoned the three-section description and simply use the terms proximal and distal colon. The colon is a very functional component of the hindgut and is characterized by sacculations (haustra) and bands (taeniae). The ascending colon has four sections that contain small protrusions, approximately 0.5 mm in diameter, that are commonly referred to as warzens. These wart-like protrusions are believed to be unique to rabbits. They potentially represent an increase in the surface area of the colon that would favor absorption and also may assist in mechanical separation of gut contents.
The Colonic Separation aka "Wash Back" Mechanism
The fusus coli has a mucosa that is four to five times thicker than the descending colon and contains ganglion aggregates. The fusus coli is commonly referred to as the “pacemaker” of the hindgut and controls the retrograde and normograde peristaltic activities that occur during the formation of soft and hard feces. This is the Colonic separation mechanism which in rabbits is known as the wash back CSM. The descending colon mucosa is smooth, and while it contains numerous goblet cells, does not contain obvious specialization and usually contains hard fecal pellets.
There is an almost continuous flux of material between the cecum and the proximal colon. The mixture of fluid and nutrients is moved through vigorous peristalsis in both normo- and retrograde directions. These contractions result in large indigestible fibrous particles accumulating in the center of the colon where they are rapidly transported along the colon to the rectum for defecation, usually within four hours of ingestion. Conversely, smaller, more fermentable fibrous particles accumulate at the periphery of the colon where through retrograde peristaltic contractions of the haustra, they are returned to the cecum for further fermentation.
Cecotroph Formation Explained
Periodically there is a dramatic change in the peristaltic activity of the cecum and colon as the retrograde movement of smaller fiber particles ceases and a large quantity of the cecal content is expelled into the colon. These contents are excreted, usually once to twice daily, as soft, mucous covered cecotrophs. This process is controlled by the fusus coli. Because rabbits do not completely ferment fiber the ability of the colon to rapidly eliminate large indigestible fiber particles and retain smaller, more digestible fiber for further fermentation, makes the rabbit an extremely efficient herbivore, capable of surviving on very low-quality forage.
The Role of Resident Bacteria
The cecotroph fermentation process depends heavily on an appropriate diet and the action of resident bacteria (especially Bacteroides and Clostridial spp) and protozoa which are vital to the gastrointestinal health of the rabbit. The diversity of bacterial flora and other organisms within the cecum is substantial and includes many bacterial species dependent on location within the cecum lumen or wall, unidentified anaerobic species, ciliated and flagellate protozoa, and a rabbit specific yeast. The effects of bacterial fermentation include the production of volatile fatty acids (VFA) including acetic, formic, propionic, and butyric acids. These are a major energy source for rabbits. A portion of these VFAs are contained in cecotrophs and utilized upon re-ingestion, while another portion is absorbed directly across the cecal mucosa.
There is still much to learn and understand about the microbial populations of the rabbit hindgut. Fermentation produces volatile fatty acids, vitamins B, C, and K, and proteins which are eventually digested in the small intestine after cecotrophy, which occurs once or twice a day, usually at night. Over 100 additional strains of anaerobic bacteria have been isolated from the cecum mucosa of rabbits, and of those, very few have been identified.
Cecotroph Ingestion - aka "The Midnight Snack"
In healthy and physically able rabbits, the large, soft mucous covered cecotrophs are ingested directly from the rectum (“midnight snack”). Multiple factors, including stimulation of rectal mechanoreceptors, perception of the cecotroph odor, and the blood concentrations of various metabolites and hormones, all stimulate the rabbit to directly ingest the cecotrophs. Higher fiber diets increase cecophagy, whereas high-protein and low-fiber diets decrease cecophagy. Once eaten, the acidity of the stomach breaks down the protective mucous covering of the cecotroph. Continued digestion occurs in the small intestine where cecotrophs represent an important component of the diet providing protein, water and vitamins.