The Anorexic Rabbit: Part 1
Frances Harcourt-Brown, BVSc MRCVS
Harrogate, N. Yorkshire, UK
Digestive physiology of rabbits
The rabbit's digestive system is adapted for a diet of fibrous food. All the teeth are open rooted and grow continuously. The chisel shaped incisors are used to slice through vegetation before it is ground between the cheek teeth and swallowed. In the hindgut undigested food, mainly fibre, is separated into two fractions. Large particles are sent distally through the colon to be excreted as hard faecal pellets. Small particles are sent proximally to the caecum where they are degraded by bacterial fermentation. The contents of the caecum form a fine paste containing microorganisms and the products of bacterial fermentation, such as amino acids, volatile fatty acids and vitamins. Periodically, soft pellets of this pasty material (caecotrophs) are excreted and re-ingested from the anus. Digestion of caecotrophs provides an additional source of nutrients for the rabbit.
The ascending colon of the rabbit is adapted for the separation of large and small particles. The proximal colon mixes and separates ingesta. Large particles accumulate in the lumen and are moved distally through the colon. Smaller particles accumulate at the circumferenceand are moved proximally to the caecum. The motility and function of the hindgut gut changes according to the type of faeces that are formed within it. The formation of hard faeces or 'hard faeces phase'coincides with feeding activity. The expulsion of caecotrophs is known as the 'soft faeces phase'. These phases of excretion follow a marked circadian rhythm. Caecotrophs are expelled at least four hours after feeding, during periods of rest.
The caecum contains large numbers of a variety of microorganisms. Volatile fatty acids are produced by the caecal microflora and their production is dependent on the composition of the caecal microflora and the type and availability of substrate. Like any continuous culture system, there are a number of homeostatic mechanisms in place. For example, bicarbonate secreted from the appendix acts as a buffer. Fluctuations in volatile fatty acid production and pH affect the population of caecal microorganisms. Alterations in the caecal microflora can result in the proliferation of pathogenic bacteria, notably Clostridium spp. and cause enterotoxaemia. Antibiotics, especially oral antibiotics can alter the balance of the microbial population and allow pathogenic species to proliferate.
Optimum gut motility is required to drive food through the digestive tract to supply the caecal microflora with nutrients and fluid. Gastrointestinal motility is under autonomic control. Dietary fibre has an effect on gut motility. Although it has no nutritive value, indigestible fibre is of prime importance because it stimulates gut motility. A high fibre diet also stimulates appetite and caecotrophy.
The effect of pain and stress on rabbits
Pain in any species is stressful and rabbits are particularly susceptible to the effects of stress. In rabbits, adrenaline can cause a marked and prolonged reduction in renal plasma flow and gastric ulcers are associated with stress. Stress can have a dramatic effect on digestive function. Stimulation of the sympathetic nervous system inhibits gut motility. Slow gastric emptying results in dehydration and impaction of the stomach contents. Hair, ingested during grooming, is incorporated in the impacted stomach contents that may be called 'trichobezoars' or 'hairballs'. Slow gut motility results in the accumulation of gas in the stomach and caecum and alters the secretion and absorption of water and electrolytes along the digestive tract. Reduced food intake reduces the supply of fluid and nutrients to the caecal microflora. A drop in blood glucose stimulates the mobilisation of free fatty acids and fatty infiltration of the liver. Oxidation of free fatty acids causes ketoacidosis. Rabbits are particularly susceptible to acidosis because some of the renal compensatory mechanisms and metabolic pathways that are present in other species are absent or restricted. Once ketoacidosis occurs, rabbits become increasingly depressed and anorexic. Fatal hepatic lipidosis develops rapidly and death typically occurs within a week of the onset of anorexia.
Causes of anorexia
Dental disease is the most common cause of anorexia in pet rabbits. Although dental problems in pet rabbits can be due to congenital malocclusion, most cases are part of a progressive syndrome caused by poor diet. Lack of calcium, vitamin D and dental wear have been implicated. The syndrome of acquired dental disease is characterised by an alteration in the shape, position and structure of the teeth and the development of malocclusion. Sharp spurs develop on the cheek teeth that penetrate surrounding soft tissues. Laceration and ulceration of the tongue is intensely painful and affected rabbits cannot eat or groom effectively. They are often salivating. Gut stasis is a common sequel. As dental disease progresses, distorted, weakened crowns break off leaving sore, infected gums that may be painful and prevent eating. Iatrogenic damage to the buccal mucosa or excessive shortening of crowns during dental procedures can lead to loss of appetite or eating difficulties following dental procedures.
Gastrointestinal hypomotility ('Gut stasis,' 'ileus,' 'trichobezoars' 'hairballs') is a secondary to pain or stress and it is important to identify and treat the underlying cause. Lack of dietary fibre is an important predisposing factor.
Examples of stressful situations include fright (e.g., predator attack, fireworks, trauma, transport), poor husbandry (e.g., rough handling, recent loss of a companion, a dominant companion, excess cold or heat), recent surgery (e.g., laparotomy, incisor removal). Examples of painful situations include areas of inflamed skin (e.g., urine scalding, penetrating foreign bodies), ulcerative pododermatitis, abdominal pain, oral pain or skeletal pain (e.g., arthritis, neoplasia, or osteomyelitis). The presenting signs of gastrointestinal hypomotility are a reduction or cessation of faecal production and anorexia. Small faecal pellets or mucus may be passed. In the early stages, the rabbit may seem bright and alert but, as the condition progresses, it becomes increasingly depressed and immobile. In the later stages, impacted stomach contents may be palpable.
Caecal impaction occurs in some cases of gastrointestinal hypomotility. Dehydration may play a part in the aetiopathogenesis of the condition. Caecal impaction can also be caused by ingestion of substances, such as clay cat litter, that are moved into the caecum but cannot be degraded by the microflora. Bulk laxatives that absorb water, such as methylcellulose or psyllium, can have a similar effect. An impacted caecum is also a feature of mucoid enteropathy. Caecal impaction is often mistaken for dental disease because the rabbit may pick at food, eat a little and then drop it uneaten. The onset is insidious. Affected rabbits adopt a hunched stance. The impacted caecum can be palpated in the ventral abdomen. It is visible on abdominal radiographs.
Gastric dilatation and intestinal obstruction occurs frequently in pet rabbits. Rabbits suffering from intestinal obstruction suddenly become totally anorexic, unresponsive, depressed and immobile. Gastric dilatation develops rapidly, especially if the obstruction is in the small intestine. The most common cause of obstruction is a pellet of felted hair. Longhaired breeds are most susceptible, especially during moulting when felts of hair accumulate and can be ingested during grooming. Dried pulses, foreign objects, tumours, tapeworm cysts, strangulations and intussusceptions are among other causes of intramural or extramural obstruction. If intestinal foreign bodies are removed promptly, there is a good chance of recovery. If the condition is not treated, the rabbit will die unless the foreign body is moving and happens to pass into the colon.
Mucoid enteropathy is characterised by the presence of copious mucus in the colon. Inflammatory changes are minimal. The caecum may be impacted. Gastric dilatation occurs in the terminal stages. In some cases there are respiratory signs. Mucoid enteropathy is usually encountered in juvenile rabbits kept in colonies although it is occasionally seen in adults. The aetiology remains unclear. Dysautonomia has been implicated in some outbreaks. Clinical signs include anorexia, abdominal distension, subnormal body temperatures, depression and a crouched stance. Hard faecal pellets are not produced. Diarrhoea can be present in the early stages. In the later stages, mucus, either on its own or mixed with faecal material is excreted or faecal production may cease completely. A feature of the mucoid enteropathy is tooth grinding, presumably due to abdominal pain. A solid impacted caecum is usually palpable. The disease is progressive and usually fatal.
Enteritis and enterotoxaemia are common causes of losses in colonies of commercial or laboratory rabbits but are rare in the pet rabbit. There are many infectious causes, such as coccidiosis, bacterial and viral enteric infections. In pet rabbits, the most usual causes are stress or inappropriate antibiotic therapy.
Chronic renal disease is a cause of anorexia and can be caused by Encephalitozoon cuniculi, neoplasia, interstitial nephritis or nephrotoxicity. Nephrolithiasis is common. Chronic renal failure in rabbits is manifested by anorexia, anaemia and weight loss. Polyuria and polydypsia may be present but often go unnoticed by the owner.
In rabbits, the kidney is an important part of calcium regulation. Calcium is readily absorbed from the gut and excreted by the kidney. Normal rabbit urine is turbid due to the presence of calcium carbonate deposits. Renal disease can impair the excretion of calcium leading to mineralisation of soft tissues including the kidneys and aorta. Overmineralisation of the skeleton may also be seen. These changes are also seen in cases of vitamin D toxicity.
Fly strike causes anorexia and is secondary to problems such as poor husbandry, dental disease, obesity or spinal problems.
Rhinitis not only affects the sense of smell but rabbits with blocked nostrils find it difficult to breathe and eat at the same time. Nasal foreign bodies can cause sudden anorexia, sneezing and discomfort
Neoplasia is most common in elderly rabbits and can cause anorexia. Lymphoma and uterine adenocarcinoma are the most usual neoplasms of rabbits.
Infectious or systemic diseases can cause anorexia. Examples includepasteurellosis, myxomatosis, viral haemorrhagic disease, acute encephalitozoonosis or hepatic coccidiosis.
Toxicity, especially lead poisoning, can cause anorexia. It is more commonly encountered in houserabbits
Management factors, e.g., change of diet, mouldy feed, and water deprivation can cause of loss of appetite. Loss of a bonded companion can cause anorexia.
Frances Harcourt-Brown, BVSc MRCVS