Rabbits are herbivores and their digestive process depends on hind gut fermentation whereby their large well developed caecum uses beneficial microbes to break down digestible fibre into usable nutrients. Lack of fibre in the intestines, as occurs with malnutrition, anorexic rabbits, or those that are unable to eat for a medical reason, can compromise the functioning of the intestines and potentially affect the animal's ability to heal and recover from illness (Rees Davies, 2006; Meredith, 2011).
In small herbivores such as rabbits, the gut is designed for processing of fibrous low-nutrient vegetation (Meredith, 2011). Good functioning of the gastrointestinal (GI) tract requires that it is never empty so when the animal is anorexic or unable to eat, life threatening consequences can occur (Harcourt-Brown, 2002a). Not only can its rate of healing be compromised, a lack of appropriate nutrition can lead to devastating effects from gut stasis that can be a risk to the rabbit's life (Rees Davies, 2006).
Assisted feeding should be considered in rabbits that are unable or unwilling to eat normally, this can help ensure that intestinal motility is maintained throughout the recovery process. In a clinical environment, the unique digestive processes of small herbivores must be considered when developing a nutritional plan and assisted feeding may be necessary to ensure healthy intestinal functioning. The composition of foods used in assisted feeding must meet the rabbit's unique nutritional needs and ensure that adequate fibre is provided.
Rabbit digestion
Rabbits are a popular pet and like most other herbivores, they need a continual supply of fibrous vegetation to maintain a healthy digestive tract (Leng, 2008). Like horses, the digestive process of rabbits involves hind gut fermentation where the majority of nutrient breakdown occurs by microflora within the large intestine.
As with many other herbivores, rabbits must ingest a significant volume of vegetation each day to meet their nutritional needs and to accomplish this, they have a very well developed GI tract (Stevens and Hume, 1995; de Blas and Wiseman, 2010). The volume of digesta within a healthy rabbit's GI tract makes up nearly a fifth of its body weight and the caecum itself holds just under half of the total GI volume (Portsmouth, 1977). Rabbits have the largest caecum of any mammal relative to its size and this enables it to extract a very high percentage of nutrients compared with other monogastric herbivores (Stevens and Hume, 1995).
The success of the rabbit's digestive process is largely due to caecal fermentation (de Blas and Wiseman, 2010). This process begins with particle separation within the fusus coli, a muscular area dividing the proximal and distal colon (Figure 1). The fusus coli mixes and separates digesta, collecting insoluble fibre within the lumen and allowing small soluble fibre particles to settle around the perimeter (de Blas and Wiseman, 2010). The perimeter of the proximal colon then undergoes a process of reverse peristalsis sending the soluble fibre back up the proximal colon and into the caecum. At certain times of the day, particularly when the animal is feeding, the fusus coli stimulates the expulsion of insoluble fibres from the lumen of the proximal colon into the distal colon where it is dehydrated and formed into dry faecal pellets (de Blas and Wiseman, 2010). During this time, soluble fibres collect within the caecum and a large variety of caecal microflora, including species such as Bacteroides, Bifidobacterium, Clostridium, Strepto coccus and Enterobacterferment the fibre, producing by-products such as volatile fatty acids (Padilha et al, 1999; Leng, 2008; de Blas and Wiseman, 2010).
During daylight hours, food intake diminishes and intestinal motility slows down. The fusus coli stimulates the colon to change to a soft faecal phase and fermented digesta is expelled from the caecum into the proximal colon where it is dehydrated and formed into a soft faecal pellet coated in thick protective mucous (de Blas and Wiseman, 2010). This faecal pellet is called a caecotroph and it is rich in microbial proteins and fatty acids. The caecotroph passes easily through the distal colon and is excreted. The efficient processing of nutrients within the caecum would be wasted since the colon cannot easily absorb these nutrients; however, to maximize nutrient absorption, the rabbit ingests the caecotroph directly from the anus, swallowing it whole. The caecotroph is then held in the near pH neutral fundus of the stomach for a number of hours (Figure 2).
In contrast to the daylight process of caecotrophy, the nighttime hours are dedicated to ingestion of high amounts of fibrous vegetation. The ingested caecotroph that has been sitting in the stomach now dissolves and mixes with newly ingested vegetation. The nutrients held within the caecotroph are then exposed to stomach acids and digestive enzymes as they pass through the body of the stomach and small intestine, where they are finally absorbed (Marounek et al. 1995; Stevens and Hume, 1995). The insoluble fibres move quickly through the GI tract and are excreted as hard faeces (de Blas and Wiseman, 2010).
Basic nutritional requirements of rabbits
The rabbit's practice of caecotrophy enables extraction of nutrients at a much greater efficiency than other monogastric herbivores and means that rabbits can thrive on relatively low-nutrient vegetation (Cheeke, 1987). It also is a system that depends on optimal GI motility and this is facilitated by the ingestion of long indigestible fibre combined with vegetation that has a high moisture content, or sufficient water ingestion.
Rabbits have a high surface to body weight ratio and as a result, they have a relatively high daily energy requirement. To meet their energy needs, they need to eat about 8% of their body weight per day in high-fibre vegetation (dry weight) or about 5% of their body weight in concentrates (dry weight) assuming that the feedstuff has an energy density of between 2100 kcals/kg and 2500 kcals/kg for maintenance to growth, respectively (Jenkins, 1999).
Protein requirements for rabbits relate strongly to the nitrogen content and overall digestibility of the diet as well as the efficiency of microbial amino acid synthesis and relative ingestion of caecotrophs by the rabbit. In optimal conditions, the crude protein requirement is generally between 12 and 17% of the total dietary energy, from maintenance through to growth, respectively (de Blas and Wiseman, 2010). Caecotrophs are a significant source of dietary protein, and in the healthy rabbit that is regularly ingesting caecotrophs they can help to overcome poor quality protein diets; the diet itself does not necessarily have to have a crude protein content as high as the required ranges (de Blas and Wiseman, 2010). Rabbits also have an ability to retain nitrogen within the caecum for conversion to amino acids, but this may not be a sufficient protein source for growth or reproduction (Cheeke, 1987; Feugier et al, 2006). Very high protein foods are not ideal in rabbits, especially in growth (Gutierrez et al, 2003). Studies suggest that there is an adverse effect to excessively high protein levels during growth and development as well as an increased potential for proliferation of pathogenic bacteria within the colon in the presence of high protein levels (Padilha et al, 1999; Gidenne and Fortun-Lamothe, 2002; Leng, 2008). This further highlights the rabbit's dependence on high fibre, low nutrient vegetation as a staple of the diet (McNitt et al, 1996).
The importance of fibre cannot be underestimated for maintaining intestinal functioning in the rabbit. As a herbivore, rabbits ingest a number of different types of plants to meet their nutritional needs. Plants contain varying amounts of carbohydrates and fibre. Soluble fibre is composed of digestible carbohydrates and fine fibrous particles that are easily fermented in the intestine. Insoluble fibre is composed of indigestible plant cellulose but it serves an important role in providing bulk and transporting moisture within the intestine. Both soluble and insoluble fibres are important in rabbit digestion for different reasons and each can affect GI functioning in different ways.
The long insoluble fibres in hay move through the rabbit digestive tract much faster than finely ground or soluble fibres (de Blas and Wiseman, 2010). This process of separating the soluble and insoluble fibres is one of the main ways that GI motility is maintained and thus these fibres are an essential component of the rabbit diet (de Blas, 1992). Rabbits are only able to digest around 14% of the insoluble fibre.
Insoluble fibre is essential for maintaining GI function in the rabbit. Fibre absorbs and transports water and functions to maintain intestinal motility and balance pH levels throughout the GI system. The coarse indigestible fibres are passed through the digestive tract undigested, forming hard faecal pellets that are not ingested by the rabbit.
It is important that diets contain high levels of fibre. Evidence suggests that low fibre/high carbohydrate diets can be detrimental to the rabbit's health as these can change the pH of the GI tract, affect the passage of digesta within the GI tract and can potentially create an enviroment where proliferation of pathogenic micro-organisms can take place (Sakaguchi et al, 1992; de Blas and Wiseman, 2010). Recommended minimum fibre levels for rabbits vary depending on the source. Some studies suggest a minimum fibre content between 14–16% but other studies show that diets that have less than 20–25% fibre can result in decreased gut motility leading to a higher incidence of enteritis, diarhhoea, prolonged retention of digesta within the caecum and decreased formation of caecotrophs (Cheeke, 1994; Kelly and Willis, 1996; Jenkins, 1999). This evidence suggests that high fibre diets are desirable particularly those that include a high proportion of insoluble fibre and a relatively low carbohydrate content (Brooks, 1997; de Blas and Wiseman, 2010).
Microbial breakdown of soluble fibre within the well functioning caecum produces fatty acids and, for the most part, these fatty acids help to supplement the rabbit's total dietary fat. Rabbits have been found to preferentially select a diet that is around 10% fat when given the choice of very low or very high fat diets (Cheeke, 1987). This range is easily supplied by feeding a balanced diet that includes a variety of vegetation.
Feeding of pet rabbits
For the normal healthy adult rabbit, fresh timothy, lucerne, bermuda or similar grass hay should be provided ad lib. In growth or reproduction, alfalfa, clover or a similar leguminous hay can be substituted instead to provide a higher protein and calcium level. For all rabbits, a variety of fresh green vegetation such as dark green leafy vegetables, dandelion leaves, herbs and carrot tops should also be offered at a rate of approximately one to two handfuls per day depending on the size of the rabbit. Higher carbohydrate foods like peas, beans, grains, sweet carrots, pumpkin and corn as well as fruits should only be offered as an occasional treat, if at all, and only in small amounts so as not to compromise GI health (de Blas and Wiseman, 2010).
Commercial pellets are concentrated diets composed of chopped vegetation and other ingredients designed to increase palatability or alter the concentration of key nutrients. Pellets are relatively clean and convenient and as such, many rabbit owners find them to be convenient alternatives to hay-based diets, but they should be used with care. Pellets were originally developed for commercial production of rabbits where their concentrated, easy to digest energy promoted fast growth (McNitt et al, 1996). Modern pellets come in a range of types and these can vary in content and nutritional profile so it is important that pet owners are advised carefully about their rabbit's nutritional needs so that a suitable diet can be chosen. Choosing a suitable pellet can be a challenge in some areas of the world where speciality brands are limited. It is important for pet owners to be aware of quality pellets in their area and in the absence of quality products, it may be useful to promote a more varied diet including a hay-based diet supplemented with a small amount of fresh vegetables and some pellets to ensure a balanced diet with plenty of fibre.
Rabbits are selective feeders and will choose to eat easily digested feeds so for normal healthy rabbits feeding of concentrates should be restricted so that the rabbit is encouraged to eat hay.
Natural cycles should be maintained if at all possible and in healthy rabbits a natural circadian rhythm can be encouraged by offering the bulk of the daily requirement once per day in the late afternoon hours to encourage eating throughout the evening and very early morning hours, after which caecotrophy can take place in the mid morning to early afternoon hours as would normally occur in wild rabbits (Ruckebush and Hornicke, 1977).
Nutrition and intestinal health
There are a number of intestinal disorders attributed to incorrect diet. Excessively high carbohydrate diets can cause proliferation of Clostridium spp, changes in GI pH and decreased GI motility (Cheeke, 1994). Insufficient fibre or anorexia can adversely affect the viability of caecal microflora which can lead to decreased intestinal motility, malabsorption, diminished immune response, intestinal pH changes, gas build up or the proliferation of pathogenic bacteria (Harcourt-Brown, 2002b; Bauer et al, 2006; Rees Davies, 2006).
GI hypomotility, is known as gut stasis, and can result in painful gas build up, constipation, GI impaction by hair balls, diminished microbial action, delayed absorption of nutrients, and can lead to hepatic lipidosis, devastating illness and even death (Cheeke, 1994; Harcourt-Brown, 2007). Gut stasis can be caused by a number of factors and is largely preventable (Box 1).
Box 1.Some causes of gut stasisStressPainDental problemsDehydrationInability to eatInsufficient dietary fibreIntestinal blockage
Anorexia is a serious problem seen in rabbits within a clinical environment and there are a number of significant consequences that can occur as a result (Harcourt-Brown, 2002a; Rees Davies, 2006). It is important, therefore, to ensure that adequate nutrition is available to combat these problems and prevent them from occurring.
Retrobulbar abscess case reportSignalmentPickle is a 7-year-old neutered male rabbit.Previous historyPickle has a long history of dental problems that required repeated dental treatments to trim his teeth.Presenting conditionRetrobulbar abscess (abscess behind the eyeball) requiring enucleation. Pickle had not been eating for 24 hours before being admitted to the hospital for treatment.Nursing goals
- To maintain a good level of analgesia through repeated nursing assessments and observations
- To maintain nutritional status through an assisted feeding regimen until voluntary feeding is resumed.
Pre-surgeryBefore surgery could proceed, it was important to ensure that Pickle's gastrointestinal mobility was normal. The plan was to begin assisted feeding but before feeding could take place, he had to be well hydrated, free of pain and able to eat and digest properly. To achieve this Pickle was given intravenous fluids (100 ml/kg over 24 hours), pain relief (meloxicam at 0.5 mg/kg daily by subcutaneous injection) and prokinetic drugs (metoclopramide by subcutaneous injection). Once he was stabilized, he was fed 20 ml/kg of Supreme RecoveryPlus orally through a feeding syringe four times daily. As soon as he started to defecate normally, surgery was planned.Post surgeryOnce surgery was complete and Pickle was fully awake, the assisted feeding regimen was continued to avoid post-operative gut stasis. Assisted feeding of Supreme RecoveryPlus was continued at 20 ml/kg four times daily. Fresh food and a bowl of Supreme RecoveryPlus was placed in his enclosure so that he could eat on his own if he felt well enough to do so. During the routine observations to assess pain relief Pickle was seen lapping the Supreme Recovery and also using grinding movements to chew the fibrous particles it contains. Assisted feeding was required for the first day, and then Pickle was observed eating fresh leaves and pellets on his own. As soon as he appeared to be eating independently, the assisted feeding was stopped. Pickle was discharged 2 days post operatively.DiscussionRabbits often do not eat if they are in pain so providing pain relief was important to encourage Pickle to start eating on his own. For the time that Pickle was not interested in eating, it was important to make certain he was given enough nutrients and fibre to ensure good gastrointestinal motility. Supreme RecoveryPlus worked very well in this case as it is high in fibre and palatable so Pickle was able to maintain good digestive function and quickly recover from the stress of the illness.
Assisted feeding
Like other animals, rabbits may need nutritional support when in hospital, and it is important to avoid periods of anorexia as this can result in devastating health consequences for the animal (Harcourt-Brown, 2002a). Rabbits in hospital should be encouraged to eat whenever possible (Harcourt-Brown, 2002b). Tips for getting them to eat include relieving pain, reducing stress and providing various palatable feeds such as fresh hay, vegetables, grass, herbs or pellets. Water should be provided in a wide bowl rather than a bottle to encourage drinking and adequate hydration (Tschudin et al, 2011). It may be necessary to provide fluid therapy to rehydrate the rabbit both to stimulate the appetite by making the rabbit feel better, but also to hydrate the digesta within the GI tract so that impaction is less likely to occur from eating dry foods (Brown and Goodman, 2006; Harcourt-Brown, 2011).
When the rabbit is not able or willing to feed on its own for more than 12 hours, assisted feeding may be required. Assisted feeding techniques vary depending on how long the feeds are likely to take place. In a rabbit that is otherwise healthy, syringe feeding for a day or two can be all that is required to get the rabbit eating again (Figure 3). This method is an easy and non-invasive method of feeding and can provide immediate support that the client can easily carry on at home, if required.
For feedings that are expected to take place over several days, a nasogastric tube can be placed. This tube is inserted into the oesophagus by way of the nasal passage and the end is secured in place on the external nose using tape, glue, suture or a combination of all three (Brown, 2010). It is generally easy to place but complications can occur including misplacement and flooding into the trachea during feeding. It is fairly well tolerated by the rabbit but the diameter of the tube can prove challenging to provide the required daily nutrients so the largest size tube possible will help to ensure that the diet does not have to be overly diluted with water in order to pass easily through the tube.
For longer-term feeding, oesophagostomy tubes are a good option. They are easy to place with sedation and have few serious complications. The benefit of these types of tubes is that they are large enough to pass thicker foods which have the benefit of being more concentrated/less diluted compared with that required to fit through a thinner tube. For rabbits, more fibrous, bulky foods can be fed this way (Paul-Murphy, 2007).
In very long-term cases, gastrostomy tubes can be placed although the technique does require special equipment, the rabbit must be anaesthetized and there are considerations related to anatomical variations in the rabbit compared with placement in dogs and cats (Smith et al, 1997).
The type of foods fed to rabbits undergoing assisted feeding is of utmost importance. Without an ability to regulate their own fibre intake, they are reliant on a balanced diet that has a suitable protein, carbohydrate and fibre level. If they are unable to ingest caecotrophs, the overall digestibility of proteins may not be as much as it could be so a slightly higher protein level may be advantageous in assisted feeding formulae (Paul-Murphy, 2007).
There are a handful of products used for assisted feeding in small herbivores. Some of the more common brands include Supreme Petfoods Recovery range (Supreme Petfoods, Suffolk, UK) (Box 2) and Oxbow Animal Heath Critical Care range (Oxbow® Animal Health, Murdock, NE, USA) These types of formulae are generally more advantageous than hydrating ordinary pelleted concentrates both in terms of ease of use and in terms of the nutritional profile.
Box 2.The Supreme Recovery range of critical care dietsSupreme Petfoods Recovery
- High fibre — 19%
- Added fructo-oligo-saccharides (soluble fibre)
Supreme Petfoods RecoveryPlus
- Based on timothy hay with a high concentration of insoluble fibre
- High fibre — 25%
- Includes added vitamin C — 1000 mg/kg
- Plus added probiotics
The Supreme foods
The Supreme RecoveryPlus foods are complete diets for rabbits, guinea pigs, chinchillas, degus and other small herbivores (Figure 4). They are rehydratable so are easily mixed when needed. The paste can be fed in a dish, via a syringe or thinned to pass through the thinnest of tubes.
RecoveryPlus in particular contains added vitamin C, which is necessary to support guinea pigs and degus. It also provides an excellent concentrate of insoluble fibre and probiotics which are essential for maintaining intestinal motility. The Supreme Recovery range includes fructo-oligo-saccharides which are an excellent prebiotic soluble fibre promoting a good environment for GI microflora to develop, which can help establish healthy digestion.
Mandibular abscess case reportSignalmentSpeedy is a 5-year-old dwarf lop rabbitPresenting conditionSpeedy was referred to the rabbit clinic with a large mandibular abscess. History of poor husbandry, inadequate diet creating pockets of infection in the gum. The owner had also noted anorexia and lethargy. Speedy was not handled very often and the owner did not realize how bad the swelling under her chin had become.Nursing goals
- To maintain a good level of analgesia through repeated nursing assessments and observations
- To maintain nutritional status through an assisted feeding regimen keeping in mind that the rabbit does not like having its mouth touched.
Pre-surgical treatmentEmergency treatment was applied in the form of intravenous fluids and analgesia. Speedy was anaesthetized and radiographs and computed tomography images of the head were taken. These showed marked mandibular osteomyelitis associated with dental disease of the premolar teeth on the lower left mandible. Surgery to debride and remove the infected tissue and affected teeth was required. After Speedy was stabilized overnight with further analgesia, fluid and nutritional support.Providing nutritional support was particularly challenging due to Speedy's abhorrence to having her head touched. However this was an important part of her care so day and night nursing staff gently persevered and managed to administer the required amount of Supreme RecoveryPlus at the rate of 20 g/kg of body weight per day, diluted with warmed water. After the first feeding session Speedy was reported to appear hungry and protested less at having the syringe placed in the mouth.Post surgical recoveryThe following day surgery was performed and supportive care continued. Despite gut stimulants and analgesia there were only a small amount of faeces being passed postoperatively until the third day, when a few more were noted. Assisted feeding during the recovery period was important to help Speedy to regain her appetite. Supreme RecoveryPlus was fed orally by syringe. Nursing staff helped reduce Speedy's stress by administering a bed bath in the form of damp cotton cloth being stroked over the fur to simulate grooming, as rabbits meticulously maintain their fur which is something Speedy was unable to achieve. Speedy seemed to respond to the nursing care and companionship. Nose scratches and crushing green leaves helped to stimulateolfaction-appetite receptors and within 5 minutes she was eating dandelion leaves, spring greens and a little piece of broccoli. The frequency of Supreme RecoveryPlus feeds and fluid therapy were reduced once eating and urination increased in frequency. Speedy remained in the hospital for a total of 8 days until she was maintaining her own weight from voluntarily eating and the wound healing process was satisfactory.DiscussionMaintaining gut motility before and after surgery is important to prevent gut stasis and hypomotility. Rabbits often do not eat if they are stressed so reducing Speedy's stress with grooming and companionship helped here to encourage normal appetite. Good nutrition was maintained by hand feeding Supreme RecoveryPlus and this also provided the needed hydration and fibre to maintain normal gut motility.
Conclusion
Rabbits have a unique digestive process that depends on a herbivorous diet high in fibre and low in carbohydrates. Lack of fibre resulting from an inadequate diet can compromise the functioning of the intestines and potentially lead to devastating consequences to the rabbit's health and well being. Proper gut functioning requires a constant supply of high fibre feed so when the animal is anorexic or unable to eat, assisted feeding should be considered. Developing a nutritional and assisted feeding plan for rabbits must take into account their unique nutritional needs and ensure that adequate fibre is provided. Using a quality formulated critical care diet, such as Supreme's RecoveryPlus, for small-herbivores is ideal for a number of cases and can help prevent GI motility disorders.
Key Points
- The rabbit requires a continual supply of fibrous vegetation to maintain good gastrointestinal health.
- During periods of fasting, the rabbit's gastrointestinal health can be severely compromised.
- Gut stasis can be a life-threatening condition.
- It is vitally important to maintain good intestinal functioning with a continual supply of high fibre foods.
- Assisted feeding can provide a healthy supply of nutrients to the rabbit that is unable or unwilling to eat.
- The success of assisted feeding plans are highly dependent on the feeding route, frequency of feedings, and the quality of the diet fed.
- Supreme Recovery is a quality diet that can be fed to rabbits that are unwilling or unable to eat properly.