Dental disease is considered as one of the, if not, the most common disorders seen in pet rabbits. This article provides a review of the scientific literature and an overview of the peculiarities of calcium homeostasis in the rabbit in an attempt to draw together current thinking on the cause of dental disease. A complete understanding of the aetiology and pathophysiology of rabbit dental disease is necessary for the veterinary practitioner to establish a proper therapeutic plan, prognosis and ultimately prevention of this common cause of morbidity and mortality in pet rabbits.
Oral cavity diseases are the most frequently reported disorders in small herbivorous mammals (Wiggs & Lobprise 1997a, Verstraete & Osofsky 2005). In rabbits, guinea pigs, chinchillas and degus, a wide range of local and systemic conditions which affect the mouth and oral cavity have been implicated including hereditary, infectious, metabolic, traumatic (foreign bodies included) and neoplastic disorders and additionally lack of chewing and poor dietary abrasive properties (Lindsey & Fox 1994, Harcourt-Brown 2002, Crossley 2003a).
Different forms of congenital, developmental or acquired dental disease are clinically manifested by hypersalivation, anorexia, chewing disturbances, change of feed component preferences and poor body condition. Dental disease can also be accompanied by the development of facial abscesses, moist dermatitis, epiphora, exophthalmia and damage of the temporomandibular joint. Factors that affect tooth positioning such as abnormalities of jaw width, length and height may also result in malocclusion, as may variations in tooth arrangement along the jaw, the degree of eruption, tooth rotation and tipping (Crossley 2003b).
Many clinical articles have been published regarding the diagnosis and clinical treatment of dental disease in rabbits. However, specific scientific information concerning the aetiology of dental disease remains scarce.
Physiology Of Calcium And Phosphorus Metabolism
Calcium and phosphorus absorption and excretion in rabbits
In most mammalian species, calcium is absorbed from the adult gastrointestinal tract by an active transport system in the brush border of enterocytes that involves calcium-dependent ATPase, regulated by the active form of vitamin D3. Calcium is moved from the intestinal lumen to the circulation by a paracellular process, i.e. down a chemical gradient through the enterocyte junctions in newborns and young animals (Bronner 1998, Tryfonidou et al. 2002, Bass & Chan 2006). In contrast, in both young and adult rabbits, almost all the available calcium is absorbed from the diet (independent of vitamin D3) by passive diffusion through the intestine wall (Buss & Bourdeau 1984, Kamphues et al. 1986, Barr et al. 1991). Only when dietary calcium is low, do rabbits use active calcium transport.
Factors which decrease intestinal absorption of calcium include pH of ingesta, presence of calcium ion binders (phytates, phosphate and oxalates), glucocorticoids and fat (Harcourt-Brown 2002, Marounek et al. 2009). Factors which facilitate calcium absorption include the presence of bile salts and lactose. In rabbits, increasing dietary phosphorus content (from 0·1 to 0·8%) at a constant calcium concentration (0·5%) results in a dramatic decrease in calcium excretion through the kidney and increased faecal calcium output because of the formation of CaPO4 complexes (Ritskes-Hoitinga et al. 2004). With excessively high-dietary calcium intake, intestinal absorption in rabbits is not reduced (as in other mammals) but calcium renal excretion is increased (Whiting & Quamme 1984).
Phosphorus is primarily absorbed in the small intestine via an active transport process, controlled by the active form of vitamin D3. A major factor influencing phosphorus availability from plant materials in non-ruminant animals is the presence of phytates and phytases (Mateos et al. 2010). Phytates are phosphorus-rich complexes that are not degraded by endogenous enzymes. In the rabbit, phytate phosphorus is well utilised because of phytase production by the microorganisms of the caecum (Marounek et al. 2009). Most of the phosphorus is recycled through soft faeces followed by coprophagy and, therefore, should result in an almost complete utilisation of phytate phosphorus (Mateos et al. 2010). In addition, natural phytases present in wheat and other ingredients, as well as in other exogenous sources, may facilitate phytate hydrolysis and absorption of phosphorus in the upper part of the digestive tract (Marounek et al. 2003).
In rabbits, increased dietary phosphorus intake causes increased urinary and faecal phosphorus excretion (Ritskes-Hoitinga et al. 2004).
The role of parathyroid hormone and vitamin D in rabbits
The metabolism of calcium and phosphate ions are closely related. The two most important hormones responsible for regulating the extracellular concentration of these ions are parathyroid hormone (PTH) and the active form of vitamin D3 (also known as calcitriol; 1,25-dihydroxyvitamin D3; [1,25-(OH)2D]; or 1,25-dihydroxycholecalciferol). The secretion of PTH is rigidly controlled by the extracellular concentration of calcium ions. PTH is responsible for the fine regulation of the serum calcium concentration on a minute-to-minute basis, by virtue of its effects on calcium mobilisation from bone and the rate of calcium (and phosphate) excretion into the urine.
Calcitriol is primarily responsible for regulating the quantity of calcium absorbed in the small intestine, and it also participates with PTH in the regulation of mineral mobilisation from bone. The effect of increased calcitriol requires hours rather than minutes, so that it may be more important in the long-term maintenance of calcium balance rather than the minute-to-minute regulation of the serum concentration of calcium ions.
The term vitamin D is used to refer to a group of closely related sterols. In the liver, cholecalciferol is converted to 25-hydroxycholecalciferol (calcidiol), which is further converted in the cells of proximal tubules of the kidney to the more active metabolite 1,25-dihydroxycholecalciferol (calcitriol). Calcitriol is also produced in placenta and in macrophages.
One of the primary roles of vitamin D is the regulation of calcium and phosphorus absorption and metabolism for bone health. This role is especially important during pregnancy and lactation because bones develop rapidly during this period. Vitamin D3 levels are very important for maintaining bone mineral density.
However, the best indicator of overall vitamin D status is the circulating concentration of 25-hydroxycholecalciferol, because it is the body's main storage form of vitamin D (Holick 2003).
In large herbivores (cattle, sheep, horses) and omnivores (rats, pigs, human), the biosynthesis of vitamin D3 from 7-dehydrocholesterol (ergosterol) occurs in the skin exposed to ultraviolet light (How et al. 1994, Holick 2003) and it is thought that this process also occurs in rabbits. A preliminary study of Fairham & Harcourt-Brown (1999) recommended exposing rabbits to natural daylight, however, there are no further scientific data to support this recommendation. However, clients should be encouraged to expose rabbits to natural conditions, as it supports normal behaviour and normal grazing.
The optimal dietary calcium content for growing and adult rabbits was suggested by Lowe (1998) to be 0·5 to 1%. This is in agreement with a later study using dexa scanning (for bone density analysis) that found 0·6 to 1% was required for optimal bone density (Norris et al. (2001). Further, the calcium to phosphorus dietary ratio should be at least 1-2:1 to achieve normal homeostatic balance (Harris 1994).
The minimum dietary phosphorus content has been quoted as 0·22% to 0·3-0·7% (Mathieu & Smith 1961, Mateos & de Blas 1998). However, a more recent experimental study by Ritskes-Hoitinga et al. (2004) showed that even a dietary phosphorus content as low as 0·2% with 0·5% calcium could cause some degree of kidney calcification. However, in clinical practice, it is hard to achieve such a low-phosphorus diet based on natural ingredients and optimal fibre content, and optimal dietary phosphorus recommendations should more practically be in the range of 0·3 to 0·4%.
The optimal dietary vitamin D content for rabbits was reported by Lowe (1998) as 800 to 1200 IU/kg. However, feeding rabbits even with 1000 IU/kg vitamin D can result in soft tissue calcification (Kamphues et al. 1986) and so extreme caution in using supplements for rabbits is required.
Physiological plasma/serum levels of calcium in rabbits
In contrast to other mammalian species, as a result of vitamin D-independent intestinal calcium absorption, the total circulating calcium concentrations fluctuate widely, i.e. if the dietary calcium intake is high, the total calcium concentration is also high (Brommage et al. 1988). Therefore, rabbits have relatively high concentrations of total calcium in plasma but the ionised fraction is more consistent and similar to other mammals (McLean & Hastins 1935, Norris et al. 2001). Circulating ionised calcium concentrations generally range between 1·67 and 1·76 mmol/L (Warren et al. 1989, Bas et al. 2004). It is therefore important to differentiate between total and ionised calcium when discussing calcium in rabbits.
Various authors describe different and wide physiological reference intervals for total circulating calcium or phosphorus concentrations (Table 1). When fed a high-calcium diet, adult rabbit serum calcium concentrations could increase rapidly above 4·5 mmol/L because of passive calcium intestinal absorption (Kamphues et al. 1986). However, higher calcium concentrations may also indicate undiagnosed renal failure or reduced glomerular filtration rates. Although the circulating calcium concentration in rabbits is higher than in most mammals, PTH values are comparable (Norris et al. 2001). However, unlike other mammalian species, the changes in PTH in rabbits occur at relatively high concentrations of calcium, suggesting that the parathyroid gland of the rabbit is set to respond to changes in ionised calcium within its own physiological range (Warren et al. 1989). Normal circulating values for calcium, phosphorus, PTH and vitamin D are given in a Table 1.
Table 1. The reference plasma/serum concentrations for selected plasma/serum parameters
Published wide reference intervals
Reference concentrations suggested by authors (VJ, SR)
Buss & Bourdeau 1984, Goad et al. 1989, Warren et al. 1989, Gillet 1994
Hewitt et al. 1989, Gillet 1994, Jenkins 2008‡Warren et al. 1989, Harcourt-Brown & Baker 2001, Bas et al. 2004, 2005
Dental disease is widely reported to be one of the most common diseases of pet rabbits (Harcourt-Brown 2007b, Jekl et al. 2008, Capello & Lennox 2011). Despite its prevalence in pet rabbits, dental disease is infrequently reported in laboratory rabbits. The frequent association of dental disease with an inappropriate diet (resulting in lack of wear) in pet rabbits appears to conflict with the lack of dental disease documented in laboratory rabbits fed only liquid food (Latour et al. 1998). However, any conclusions associated with the impact of liquid food on dentition may be misleading as specific examination of the oral cavity was not reported in the latter study. More recently, Okuda et al. (2007) reported a high prevalence of dental disease (100% of the 42 rabbits studied) in laboratory rabbits.
In rabbits suffering from dental disease, the pathophysiological relationship among orthodontic, periodontal and endodontic lesions is unclear (Verstraete & Osofsky 2005) and appears to be much more complex than in other mammals. Most likely, multiple factors are involved in the development of a syndrome of acquired dental disease such as traumatic injuries of the orofacial area, less abrasive diet, reduced chewing movements, altered chewing actions, changes in the structure and adaptability of the chewing muscles, temporomandibular joint disorders, inappropriate dietary phosphorus, inappropriate dietary calcium, malabsorption and systemic disease (Poikela et al. 2000, Harcourt-Brown 2002, Crossley & Aiken 2003, Fujisawa et al. 2003, Langenbach et al. 2003, Crossley 2005, Harcourt-Brown 2007a,2007b, Wolf & Kamphues 2009).
In pet rabbits, the most common aetiological theories for dental disease are genetic/inheritance factors, trauma, iatrogenic malocclusion, lack of abrasive food that leads to insufficient or abnormal dental wear (Crossley 1995) and underlying metabolic disease because of calcium and vitamin D deficiency (Harcourt-Brown 2007a).
Mandibular prognathism is inherited as an autosomal recessive (mp/mp) trait with incomplete penetrance (81%). Affected animals have reduced skull and maxillary diastema length, without significant deviation from the normal length of mandibles. Hereditary mandibular prognathism (brachygnatia superior, hypognatia) that leads to incisor malocclusion has been well described (Lindsey & Fox 1994). In affected rabbits, malocclusion of incisors usually first appears after the third week of life. Initially, there is an edge to edge bite with blunting of incisival cutting edges. Later, positioning of the lower incisor anterior to the uppers occurs (Fig 1). However, there have been no recent genetic studies in pet dwarf rabbit breeds to investigate the effect of this gene on dental disease development. There are also no differences between the prevalence of dental disease in dwarf and other rabbit breeds according to Harcourt-Brown (2006).
The most commonly seen hypodontia in rabbits appears to be caused by the absence of peg teeth (dentes incisivi minores), which could occur in all breeds. Sometimes, a reduced number of maxillary premolars are also seen. Neither of these congenital abnormalities is associated with dental disease development.
Other inherited diseases
Other diseases which affect the rabbit skull and dentition have been described; however, they are extremely rare, e.g. achondroplasia and osteopetrosis. Features of osteopetrosis include skeletal sclerosis together with hypocalcaemia, hypophosphataemia and osteoclasts of reduced numbers and abnormal cytology. The chief attributes of the osteopetrosis are incisor tooth abnormalities with an initial subnormal development and subsequent retarded and abnormal growth. The most frequently observed condition at birth is the absence of, or the rudimentary state of one or more of the teeth. The enamel of these abnormal teeth is often an opaque dirty grey colour or a dull yellowish white. The molar teeth are similarly affected. The commercial available mutants are used as an animal model for this disease (Pearce & Brown 1948).
Traumatic incisor, premolar or molar fracture may all occur as a consequence of falling from a height or gnawing/chewing on hard substances. Jaw or other skull bone fractures may also lead to incisor or premolar and molar malocclusion and subsequent elongation.
Iatrogenic tooth injuries are usually the result of improper clipping of teeth. Even though this technique should now be considered an improper veterinary procedure, it is still commonly performed in some veterinary practices. In referred cases, the author (VJ) sees that more than 10% of rabbits have had their incisors shortened with pliers, pincers, bone cutters, nail trimmers, cheek teeth molar cutters or scissors, which has resulted in more pronounced incisor malocclusion (unpublished data). Incisor shortening by this way results in complete incisor fracture, taking the tooth out of normal occlusion or, at best, causing partial enamel fractures at the place of clipping. In addition, enormous pathological forces on the tooth during tooth crown shortening could cause periodontal damage, alveolar bleeding, tooth torsion and clinical root fractures and pathologies (Jekl et al. 2008, Fig 3). Iatrogenic pulp exposure is also very common when adjusting the size of elongated incisor teeth, particularly when this is repeated; as the tooth shortening often permits the teeth to erupt faster with the result that the pulp cavities become longer (Crossley 2005).
Facial abscesses can also be the result of improper incisor shortening (Minarikova et al. 2012). Osteomyelitis associated with improper incisor correction occurs in 8% of rabbits with odontogenic abscesses in the author's (VJ) practice (unpublished data). As a consequence, shortening incisors with the instruments listed above should never be performed, (Crossley 2003a, Jekl et al. 2008, Capello & Lennox 2012).
Lack of tooth abrasion; the ‘lack of chewing’ theory
Diets provided to captive rabbits rarely match those eaten in the wild, as they are generally higher in energy and nutrient content, lower in fibre, possess less abrasive properties (Wolf & Kamphues 1996) and are often pre-processed into unnatural forms (Crossley 2005). The general result is a threefold reduction in duration of chewing (Wolf et al. 1997, & 1999, Crossley 2005). This leads to reduced tooth wear from reduced tooth on tooth contact during chewing, abnormal chewing patterns used when feeding on unnatural foods (decrease in lateral masticatory action) and albeit less importantly lack of naturally abrasive food content. Furthermore, when all factors are taken together there is often a 16-fold reduction in cheek tooth wear rate (Crossley 2005).
Crossley (1995) described that this lack of tooth wear resulted an continued eruption and abnormal incisor and cheek teeth coronal elongation. Increased curvature (bending of the axis) is based on this theory, caused by the increase in occlusal pressure and an increase in intrusive forces to the cheek teeth because of cheek teeth coronal elongation. Due to this pressure and abnormal forces mandibular cheek teeth bend in lingual and maxillary cheek teeth in buccal directions (Crossley 1995). This coronal elongation and tooth bending either lingually or bucally is caused, based on this theory, by insufficient lateral (side-to-side) jaw movements during mastication resulting from a lack of fibre. Moreover, physiologically, the enamel layer is thicker at the lingual aspect of mandibular and buccal aspect of maxillary cheek teeth, which, in association with improper jaw movements and lack of wear of those particular teeth exacerbates spike formation with further soft tissue damage and erosions/ulcerations.
Coronal elongation of the cheek teeth may also lead to functional prognathism of the mandible leading to incisor malocclusion in a rostro-caudal direction (Fig. 1). In some cases, cheek teeth malocclusion (or temporomandibular joint damage) results in lateral jaw dislocation with incisor malocclusion in a meso-distal (lateral) direction.
As the occlusal force increases on the elongating teeth the eruption rate decreases or coronal eruption is arrested completely. Within a two- to fourfold rate change there is some physiological capacity to cope and minimal change within the newly forming tooth; the changes being reduction in length of the pulp by dentin deposition and increased curvature of the tooth. Despite the change in eruption rate, tooth growth and deposition of tooth substance continue at the apex which leads to the compression of germinal tooth tissue which further promotes apical tooth growth (Crossley 2005). Apical tooth growth can be so severe that the tooth apices penetrate the mandibular or maxillary periosteum and compress the outer soft tissues (Figs 2 and 3). The rate of tooth growth, the intrusive force and the ability of apical bone to remodel have a major effect on what appears clinically with concurrent disease and dietary imbalances also influencing the outcome (Crossley 1995).
Even a minor degree of cheek tooth clinical crown elongation has serious detrimental effects on incisor occlusion and jaw function (markedly reduced chewing efficiency) both of which further impede tooth wear. Gingival and alveolar bone growth frequently accompanies elongation of the cheek teeth so there is little visible clinical crown elongation in many affected individuals and radiography is required to confirm the changes (Crossley 2005). However, this theory has not yet been scientifically proven in cheek teeth of rabbits or other small herbivorous mammals with elodont dentition.
Metabolic bone disease as a cause of dental disease in rabbits
The ‘selective feeding’ theory
Harcourt-Brown (2009) suggested that the diet of many pet rabbits is low in calcium and vitamin D because of the selective feeding behaviour of rabbits offered mixed rations. Selective feeding can develop because rabbits have a natural tendency to choose the higher energy food item with lesser fibre content and individual rabbits may develop preferences, picking out the favoured ingredients only and rejecting the remainder of the commercial mixed food, hay included (Carpenter & Kolmstetter 2000). Another reason suggested for selective feeding is the presence of medical problems, especially dental disease (Harcourt-Brown 2007b). In most clinical cases of dental disease, hay is the main food refused by rabbits. Dietary intake therefore becomes based on cereals (if traditional rabbit mixed pellet and cereal food is fed), especially with higher amounts of peas, maize, wheat bran (and similar ingredients), which have a reversed calcium to phosphorus ratio (1:3 and more) and/or is absolutely calcium deficient, and could therefore lead to the development of nutritional secondary hyperparathyroidism (NSHP). Diets based on cereals and commercial mixture are also non-abrasive resulting in decreased chewing movements and so may lead to tooth elongation. Not only do hay and grass provide appropriate fibre and dental wear for rabbits, they are also an appropriate source of balanced calcium and phosphorus. Clearly, the effects of selective feeding are reduced if the rabbit is offered complete commercial pellets, hay and grass rather than a mix of cereals and pellets or better still grass or hay only diets. This depends on full access to grass (virtually impossible for the pet hutch rabbit) or good quality hay. Larger breed rabbits, mutated in size away from the wild-type rabbit, may however, require a higher protein content; therefore some pellets should be added to their hay diet.
The ‘pet rabbits have vitamin D and calcium deficiency’ theory
The conclusion of Harcourt-Brown (2006) is that metabolic bone disease plays a major role in the development of dental disease in pet rabbits. These findings are supported by a study of morphological and radiographic features of prepared intact skulls of pet rabbits. As dental disease increased in severity, the signs of osteopenia in the skull were recorded more commonly leading to the conclusion that the loss of supporting alveolar bone rather than increased occlusal pressure allows apical elongation (Harcourt-Brown 2009). Indeed, root elongation was found in all rabbits with dental disease, even in those with normal reserve crowns, and was the first dental change noted to take place. The study also showed that progressive loss of alveolar bone is a feature of the progressive syndrome of dental disease. Similarly, in other species, alveolar bone is known to be particularly susceptible to metabolic bone disease and results in displacement of the teeth. In rabbits, the visual and radiographic changes that take place in the teeth and bones of the skull suggest that metabolic bone disease is the cause of root elongation. Forces generated during biting and chewing would cause root elongation because there is insufficient alveolar bone to support the apices of the teeth. Further, it can be argued that the curvature of the teeth takes place in response to forces generated during chewing and biting and as curved teeth do not meet opposing teeth correctly, this leads to malocclusion and spur formation (Harcourt-Brown 2009).
Harcourt-Brown (2006) also described that calcium and/or vitamin D deficiency is the most likely cause of the metabolic bone disease and bone loss, although other factors (e.g. chewing forces, genetic, hormonal influences) may also play a part. It is suggested that rabbits are susceptible to calcium deficiency because the continual eruption and rapid growth of all the teeth requires a high demand for calcium to mineralise new dental tissue. Indeed, the efficient absorption of calcium in the intestines of rabbits is a way of meeting that demand. Although vitamin D plays a minor role in intestinal absorption of calcium in rabbits, it is used to boost absorption of calcium if dietary levels are very low. In addition to its role in calcium absorption, vitamin D also has a direct effect on the mineralisation of bones and teeth.
The diet of pet rabbits can be severely calcium deficient because grains and legumes are often used as rabbit food. Some popularly fed fruit and vegetables (e.g. apples and carrots) also have low calcium content (Harcourt-Brown 1996). Undetectable (i.e. very low) vitamin D concentrations have been recorded in housed pet rabbits.
The primary risk factors for low bone mineral density, osteoporosis and osteopenia include vitamin D insufficiency, inadequate calcium intake, lack of exercise and other dietary factors. Another benefit of vitamin D is maintenance of optimal muscle strength. Vitamin D deficiency can cause osteomalacia, which is associated with muscle and bone pain. Vitamin D deficiency may play a role in calcium metabolism disorders of pet rabbits. Experimental chronic vitamin D deficiency in the adult rabbit results in intestinal phosphorus malabsorption with a resulting renal phosphorus conservation. Rabbits fed a diet with 1·0% calcium, 0·5% phosphorus and no vitamin D develop osteomalacia as a result of hypovitaminosis D (Brommage et al. 1988). Although some vitamin D-deficient rabbits are able to maintain homeostasis, in others the resulting hypophosphataemia leads to inadequate skeletal mineralisation and the classical signs of osteomalacia (Bourdeau et al. 1986, Brommage et al. 1988).
Experimentally, a low calcium diet induced NSHP in young rabbits resulted in hypocalcaemia, hypophosphataemia, elevated alkaline phosphatase activity and PTH and calcitriol concentrations (Mehrotra et al. 2006). However, no data regarding the level of dietary phosphorus or vitamin D were given. In a study by Wu et al. (1990), adult rabbits on a low calcium diet (0·1%) readily developed osteoporotic changes, however, they were also fed an inverted calcium:phosphorus in a ratio of 1:5.
Several other authors indicate that osteoporosis may result in decreased oral bone density and alveolar bone loss (Cai 1992, Bai et al. 2006). Ashcraft et al. (1992) described that osteoporosis affects the rate of tooth movement through the involvement of alveolar bone and consequent periodontal and dental problems influence the response even to the physiological orthodontic forces. Moreover, studies in rats showed that the age-dependent decrease in alveolar bone turnover activity, in response to mechanical forces, may also affect the amount of tooth movement (Sidiropoulou-Chatzigiannis et al. 2007).
Other aetiological factors for osteoporosis that have been observed under experimental conditions include limited movement (caged animals), ovariectomy, ageing and glucocorticoid administration (Strates et al. 1988, Rothfritz et al. 1992, Southard et al. 2000, Cao et al. 2004, Castañeda et al. 2008). Osteoporosis induced by ovariohysterectomy has particular relevance to the pet rabbit where this surgery is common and recommended to prevent uterine adenocarcinoma. It is also interesting to speculate whether stress, and hence endogenous glucocorticoids, could induce bone changes in pet rabbits.
Alternate view – pet rabbits have excess calcium and vitamin D
The results of Schroeder (2000), Kamphues (1991, 2001) and Kamphues & Wolf (2003) claim that commercial diets for rabbits actually contain enough or more calcium, phosphorus and/or vitamin D than is required by a rabbit (taking into account the peculiar calcium metabolism of the rabbit compared with other mammals). The severity of problems due to excess of vitamin D is more pronounced when calcium is also fed to excess.
Feeding rabbits with excess calcium and/or vitamin D or subcutaneous administration of vitamin D is not recommended because soft tissue metastatic calcification readily develops and could be the cause of acute death (Garibaldi & Goad 1988, Zimmerman et al. 1990, Eason 1993, Rajasree et al. 2002). Subcutaneous iatrogenic vitamin D administration of 52 mg/kg led within a few days to signs of cardiovascular insufficiency, manifested as ascites and lung oedema, anorexia, mucous diarrhoea, loss of weight and apathy (Peixoto et al. 2010). Arteries, heart, lungs and stomach were affected with ectopic mineralisation.
Confusingly, Aithal et al. (2008) showed that over-supplementation of growing rabbits with dietary calcium and vitamin D could lead to osteopenia, which was apparent in the long bone radiographs. Aithal et al. (2008) stated that this could be probably due to imbalances in the calcium-phosphorus ratio and vitamin D leading to reduced skeleton mineralisation.
Kamphues et al. (1986) suggest that in hypervitaminosis D, the osteoblasts and bone marrow undergo degeneration, leading to necrosis and calcification. After vitamin D withdrawal, osteoblasts reappear and become overactive, leading to over-ossification. Osteosclerotic lesions (dense epiphyses and metaphyses, thickened bony articular surfaces, dense and thickened cortical bone, etc.) were seen in rabbits with experimental hypervitaminosis D in another study (Jiang et al. 1990).
Kamphues et al. (1986) also describe that in rabbits fed high amounts of dietary calcium, high-calcium concentrations in aortic tissue and kidney were noted. Moreover, dietary vitamin D levels as high as 1000 IU/kg and more led, independently of the dietary calcium concentration, to enhanced calcium content in aortic tissue.
Calcium oversupply will result in large amounts of urinary sediment, which is reported to be one of the predisposing factors for urolith formation or sludgy urine and is indeed commonly noted in pet rabbits. However, in a recent experimental study (Clauss et al. 2011), rabbits fed with alfalfa hay only (dietary calcium 2·32%, dietary phosphorus 0·28%) without additional supplementation of vitamin D did not develop kidney calcification and had good health status. Higher dietary calcium content resulting in higher urinary mineral content might not be considered pathological as additional factors rather than just high-dietary calcium concentrations are required to trigger urolithiasis (Clauss et al. 2011). Nevertheless these findings indicate that individual pet rabbits may be on a calcium-rich diet.
Effects of dietary phosphorus on dental disease
It has been suggested that rabbits are tolerant of a high-phosphorus diet (Chapin & Smith 1967a,1967b) and are efficiently adapted to low calcium diets (Barr et al. 1991). Barr et al. (1991) concluded that there are appropriate intestinal and renal homeostatic adaptations to dietary calcium deprivation in the growing rabbit and that the renal mechanisms are more rapidly induced and ultimately more efficient than the intestinal pathway. As a defence mechanism in healthy animals, rabbits fed a low calcium diet develop elevated serum PTH and calcitriol concentrations and renal calcium excretion is decreased. In those rabbits fed a high-calcium diet serum calcitriol concentrations decreased and renal calcium excretion increased (Gilsanz et al. 1991).
When rabbits are fed a 0·6% calcium and 1·2% high-phosphorus diet they readily (within 3 weeks) develop NSHP (Bas et al. 2005). This is characterised by hyperphosphataemia, increased creatinine concentration, decreased calcitriol concentration and parathyroid gland hyperplasia (Bas et al. 2005). Associated hyperphosphataemia arises not only because of an inability of PTH to excrete the entire phosphate load but also because of decreased renal function (Bas et al. 2005). Continuous PTH secretion contributes to the development of osteoporosis. Confirmation of NSHP in rabbits is based on basal (intact) PTH measurement and on disease confirmation by various imaging methods and on bone histology. Ionised and total calcium concentrations will be low or normal. However, in advanced NSHP (dietary calcium to phosphorus ratio 1:7), Bai et al. (2006) described higher total calcium concentrations with high basal PTH concentrations reaching 201·3 pg/mL. PTH concentrations higher than 430 pg/mL could be explained by concurrent renal failure (Bas et al. 2005) even when fed a dietary calcium to phosphorus ratio of 1:2. Bai et al. (2012) fed rabbits with an inappropriate calcium to phosphorus ratio of 1:7 and all rabbits developed NSHP that was defined by hyperplasia and hypertrophy of all parathyroid glands with marked PTH elevation (175 pg/mL in contrast to 17·7 pg/mL in the control group). After 4 months of feeding such a diet, serum phosphorus concentrations decreased and calcium concentrations increased, which could be explained by the effect of the PTH on calcium reabsorption and phosphorus excretion in kidneys (Bai et al. 2012). Bai et al. (2012) also showed that kidney tubules were partially mineralised, however, there were no signs of kidney failure as described in degus (Gumpenberger et al. 2012).
In horses, loss of alveolar socket bone is an early change associated with NSHP and occurs before other skeletal changes (Krook & Lowe 1964). It seems, therefore, that it is the level of phosphorous, or the relative elevation of phosphorous to calcium that is more important in the development of calcium metabolic disorders in rabbits and small herbivorous rodents with elodont dentition rather than the low dietary calcium per se.
Dental disease in small herbivorous rodents with elodont dentition and the effect of high-phosphorus diet and improper calcium to phosphorus ratio
Dental disease is also present in other small herbivorous mammals kept as pet animals with a very high incidence in degus, guinea pigs and chinchillas (Jekl et al. 2008, 2011b).
The impact of pelleted diets with different mineral compositions on the incisor tooth structure, on the crown size of the mandibular cheek teeth, as well as the mandibular bone and cheek teeth density, in degus (Octodon degus) has been investigated experimentally (Jekl et al. 2011a,2011c, Gumpenberger et al. 2012). Degus in these studies were fed high-phosphorus diets with dietary calcium to phosphorus ratio 1:1 for 14 months. Dental disease in groups fed by this diet had obvious reserve crown elongation of all teeth and had significantly smaller relative cheek teeth and mandibular densities (osteoporosis) than animals fed normal diets. However, these osteoporotic changes were detectable by computed tomography. Moreover, disturbed mineral metabolism resulted in enamel depigmentation, enamel hypoplasia, enamel pitting and altered dentin morphology. High-phosphorus diets resulted also in parathyroid gland hyperplasia and kidney calcification as was also described in rabbits (Bai et al. 2012, Gumbenberger et al. 2012), which could further aggravate metabolic bone disease.
Results of these studies suggested that more attention should be focused on dietary phosphorus content when facing altered tooth structure in small mammals. Because of the same structural problems and coronal and apical teeth elongation, similar calcium metabolism (especially in degus – calcium excretion via urine, Hommel et al. 2011) it is suggested that the aetiology of such pathologies should be the same in small herbivorous rodents with elodont dentition as in rabbits.
Metabolic bone disease and other bone lesions
Jaws and teeth of elodont herbivorous mammals are specifically adapted to the prehension and processing of high-fibre diet. Because jaws are constantly remodelling because of various pressures acting during mastication and because of continuous teeth eruption, it seems, that these tissue are much susceptible to metabolic bone changes than other bones in the rabbit [similar to metabolic bone disease in reptiles and dogs (rubber jaw), where particular bones are more affected than others]. However, Harcourt-Brown (2006) described that rabbits affected with dental disease also developed decalcification of the vertebral skeleton. Southard et al. (2000) showed that in experimental osteoporotic female rabbits, mandibular bone mineral density decreases in relation to spinal density. On the basis of experimental studies in rodents (Jekl et al. 2011a) early and mild, and occasionally, even more severe changes may be detectable only by advanced imaging methods and/or histopathological bone examination rather than by radiography.
Developing a joint theory
It is suggested that dental problems in rabbits could be caused by a combination of one or other of metabolic bone disease and lack of wear. However, metabolic bone disease appears to play a more important role than was previously believed by many authors. The constantly growing rabbit tooth is continually under orthodontic forces – factors affecting tooth position, apart from dental tissue itself, includes the periodontal ligament, germinative tissue, bone and applied force (Wiggs & Lobprise 1997a). As trabecular bone (including that of the jaws) is in constant turnover the rabbit dental system is prone to impairment by any factor affecting bone and/or tooth.
Under physiological conditions, the attrition and eruption rate (2 to 4 mm per week for incisors and 3 to 4 mm per month for cheek teeth) are the same and dental tissues and alveolar and supporting bone are healthy. An intrusive force (pathological or physiological) which presses the tooth into the socket and causes activation of osteoclasts and subsequent bone resorption (apical tooth elongation) must therefore be present in either cases with reduced tooth wear or in cases with metabolic bone disease or their combinations.
The resulting enamel points (spikes) mechanically restrict the normal side-to-side jaw movements in a vicious self-perpetuating cycle, which affect the ability of the teeth to occlude and grind properly. Moreover, animals start to be more selective in feeding and refuse to eat hay and grass but prefer feeding on softer items which may have an improper calcium to phosphorus ratio or low calcium concentrations, leading to reduced and/or further improper wear and metabolic disturbances. Feed items are also chewed for less time resulting in improper wear (Reiter 2008). So, as the disease progresses, both metabolic bone disease and lack of wear play an important role in dental disease.
The relationship between systemic disease/systemic stress and oral bone loss/germinative tooth tissue pathology is a complex problem. Deterioration of tooth quality and abnormalities in tooth eruption (irregular hyperplasia of dentin arrangement, enamel loss, horizontal ridges on incisors, etc.) are present because of pathological changes of tooth germinative tissue or periodontal attachment (Fig 4), which could be caused by metabolic bone disease (Harcourt-Brown 2009, Jekl et al. 2011a, 2011c), trauma, inflammation, vascular damage or ageing changes (Wiggs & Lobprise 1997b). In degus, metabolic bone disease, apart from alveolar bone changes (osteoporosis), causes significant changes in dentin and enamel with subsequent dental disease development and/or general dental disease aggravation (Jekl et al. 2011c). Moreover, all systemic diseases, which influence calcium metabolism, should be included in the list of differential diagnoses (e.g. chronic kidney failure, hyperadrenocorticism, secondary hyperparathyroidism, hypervitaminosis D, hypovitaminosis D, acid-base disturbances).
The main aetiological factors for acquired dental disease may be grouped as:
Normal attrition plus metabolic bone disease and other systemic disease
Reduced attrition plus optimal bone health
Reduced attrition plus metabolic bone disease and other systemic disease
Abnormal eruption plus metabolic bone disease and other systemic disease
Tooth structure impairment plus any of the above
Rabbits have very efficient calcium absorption perhaps as an adaptation to evolving on a low calcium diet. They usually passively absorb all the dietary calcium available (independent of vitamin D) and the active vitamin D dependant calcium transport occurs only in rabbits fed a very low calcium diet. At relatively high-dietary calcium (for a rabbit) the intestine continues to absorb calcium and excretes the excess via the kidneys. The optimal dietary concentration appears to be 0·6 to 1% for calcium, 0·3 to 0·4 for phosphorus and 600 to 800 IU/kg for vitamin D. The calcium:phosphorus dietary ratio should be at least 1·5-2:1 for normal homeostasis. It is not recommended to feed rabbits additional calcium, phosphorus and/or vitamin D (calcitriol) because of the risk of soft tissue metastatic calcification, unless it is indicated in association with severe renal disease. It seems that the main aetiopathological factors for the development of subsequent nutritional metabolic disturbances are the reversed calcium to phosphorus ratio present in many commercial mix diets or consumed during selective feeding and high-dietary vitamin D, and not low dietary calcium per se.
It therefore also follows that in contrast to other mammals, pet rabbits with chronic renal failure (CRF) may develop hypercalcaemia with hypophosphataemia if maintained on a diet suitable for optimal calcium metabolism. Indeed, in clinical practice it appears that in some pet rabbits, hypercalcaemia, osteosclerosis and nephrocalcinosis are common findings in renal failure indicating that these rabbits are in fact receiving a normal/high amount of dietary calcium and/or phosphorous routinely in the pet environment. There is a need to establish an appropriate diet for rabbits with different syndromes of CRF, particularly a low-phosphorus diet to prevent further renal damage and metastatic calcification.
Metabolic bone disease together with the reduction in chewing movements/lack of wear that leads to insufficient or abnormal dental wear and tooth substance deterioration are responsible for the onset of dental problems in pet rabbits. Intrusive forces which press the tooth into the socket and cause activation of osteoclasts and subsequent bone resorption (apical tooth elongation) are present in cases with reduced tooth wear or in cases with metabolic bone disease or their combinations. Resulting enamel points (spikes), from whatever reason, mechanically restrict the normal side-to-side lateral jaw movements in a vicious self-perpetuating cycle. Apical pressure on intra-alveolar nerves and pressure on periosteum may lead to pain and disinhibition of grinding fibrous material. Moreover, animals start to be more selective in feeding and refuse to eat hay and grass resulting in reversed calcium to phosphorus ratio or low calcium diet. This leads to further improper tooth wear and metabolic disturbances. So, as the disease progress, metabolic bone disease, lack of wear and tooth substance deterioration play important roles in health impairment.
Deterioration of tooth quality and abnormalities in tooth eruption could be associated with a wide range of systemic disease and stress-related disorders. Enamel and dentin quality must be evaluated in all patients and it appears that pathologies of these tissues play an important role in pathogenesis of dental disease.
Selective feeding should be eliminated in all cases by offering hay, meadow grass and vegetables. Complete commercial pellets with proper calcium to phosphorus ratio could also be offered in a small amount. As a rough guide, ad libitum timothy based hay mixed with other types of hay, 1 cup of varied fresh vegetables and/or edible plants, and 25 g of a pelleted diet per kilogram of bodyweight per day is considered appropriate for a rabbit (Campbell-Ward 2012).
Care should be taken when assessing diet and assessing calcium and phosphorus content. Veterinary practitioners and clients should be aware, that some vegetables contain a proper calcium to phosphorus ratio in dry matter basis (e.g. kale – calcium 146 mg/100 g and phosphorus 89 mg/100 g; broccoli – calcium 105 mg/100 g and phosphorus 89 mg/100 g); however, they contain a very high water content (90%). It means that 100 g of fresh kale contain 10 g dry matter; 14·6 mg calcium and 8·9 mg phosphorus and that rabbits are mostly not capable of eating such large volumes to reach their physiological calcium demand (Harcourt-Brown 2013).
The authors would like to thank Mrs Frances Harcourt-Brown for critical reading of the manuscript and helpful suggestions.
Conflict of interest
None of the authors of this article has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.