Rabbit calcium metabolism, "bladder sludge," and urolithiasis (Proceedings)

Calcium is the most abundant mineral in the mammalian body, and the major component of bones and teeth. In addition, calcium is involved in a variety of vital physiologic processes including blood coagulation, muscle contraction, membrane permeability, nerve conduction, enzyme activity, and hormone release. Calcium balance is achieved by the interaction of various control mechanisms which regulate its absorption and excretion. In most mammals, the interaction of these regulatory mechanisms maintains serum calcium levels within a narrow range. In contrast, rabbits have adapted a unique strategy in which most dietary calcium is absorbed in the intestine, and the excess is excreted in the urine. In this species, calcium levels may vary within a wide range, and in direct proportion to the dietary calcium intake. This unique calcium metabolism has implications for the health and husbandry of the rabbit.

Calcium metabolism in rabbits

Calcium metabolism in the rabbit differs in several respects from other mammals. In most species, calcium absorption is closely regulated to balance metabolic needs, and serum calcium concentrations are maintained within a narrow range, typically 1.25-1.6 mmol/L (5.0 – 6.4 mg/dL). Total serum calcium in the rabbit is 30-50% higher than in other mammals, and varies over a wide range from 3.25-3.75 mmol/L (13-15 mg/dL). The intestinal absorption of calcium in most mammals involves primarily vitamin D3-regulated active transport. In the rabbit, however, calcium is absorbed in direct proportion to the amount ingested in the diet rather than in accordance with metabolic need, and calcium absorption is relatively independent of vitamin D.

Most mammalian species grow only 1-2 sets of teeth in their lifetime. In contrast, a rabbits' teeth are constantly erupting at a rate of approximately 2 – 2.4 mm/week. The rabbit's increased life-long demand for calcium compared to other mammals is met by its efficient intestinal calcium absorption. In addition, during normal dental wear, calcium is released from the teeth, swallowed, and reabsorbed from the intestine.

In the rabbit, urinary calcium excretion increases in parallel to dietary intake. The fraction of calcium that can be filtered out of the blood is higher than in other mammals. The fractional excretion of calcium for most mammals is less than 2%; the range in rabbits is 45%-60%. When the reabsorptive capacity of the kidney is reached, calcium precipitates as calcium carbonate in the alkaline urine of the rabbit, causing cloudy or sludgy urine. When metabolic demand for calcium is increased by growth, pregnancy, lactation, or metabolic disorders, less calcium is excreted and the urine appears clear.

Hormonal regulation of calcium metabolism

The main hormonal regulators of calcium homeostasis are parathyroid hormone (PTH), calcitonin and active vitamin D3 (1,25 (OH)2 D3). Other hormones including estrogen, testosterone, prolactin, growth hormone, glucagon, and gastrin, as well as corticosteroids and other minerals also play a role in calcium regulation.

Parathyroid hormone

Parathyroid hormone is the principle hormone involved in minute-to-minute fine regulation of blood calcium in mammals. It is secreted by the chief cells of the parathyroid gland in response to decreases in serum ionized calcium or 1,25 (OH)2D3 concentrations. In the kidney, PTH acts on the renal tubules to increase calcium reabsorption and promote phosphorous excretion. By increasing the activity of alpha-hydroxylase in renal tubules, PTH is also a major stimulator of renal 1,25 (OH)2D3 synthesis, which increases duodenal calcium absorption. In bone, PTH stimulates osteoclastic bone resorption thus increasing the release of calcium and phosphate into the blood. PTH levels have been shown to rise post-prandially, stimulating the release of gastrin and digestive hormones which are postulated to affect serum calcium concentrations. The end result of these actions is a rise in serum calcium concentration, which inhibits further release of PTH by negative feedback. These actions also result in decreased urinary calcium excretion.

Rabbits exhibit a unique pattern of renal response to PTH. The rabbits' ionized calcium concentration is protected from hyper- and hypocalcemia by rapidly changing PTH and calcitonin secretion. Changes in PTH secretion are seen only at relatively high calcium concentrations, levels that are the physiologic norm for the rabbit. Despite having a high serum calcium concentration, rabbits have readily measurable levels of PTH which are dramatically reduced by infusion of calcium. This implies that the parathyroid gland and PTH actively contribute to calcium homeostasis in this species. In vitro perfusion studies of the rabbit nephron have demonstrated that PTH increases calcium transport in the cortical segment of the loop of Henle, but not in the medullary segment and causes an increase in calcium absorption in the connecting tubule. It is the distal nephron segments that play the key role in determining the calcium excretion in the final urine, and this content is regulated by the actions of PTH and calcitonin, and the presence of other minerals, particularly sodium.

Calcitonin

Calcitonin is produced by the C cells of the thyroid gland and released in response to increases in plasma calcium concentration. Release of calcitonin is also enhanced by estradiol, glucagons, and gastrointestinal hormones including gastrin, cholecystokinin and secretin. Calcitonin levels increase post-prandially, especially following a high-calcium meal. Neonates and pregnant and lactating animals also demonstrate increased calcitonin levels. Calcitonin decreases PTH-stimulated osteoclastic bone resorption and lowers the cytosolic calcium concentration in bone cells, thus decreasing calcium efflux from the pool of labile bone calcium. The hormone may aid in sequestering calcium and phosphate in bone in a form in which it could be available for fasting needs. Renal tubular reabsorption of calcium and phosphate are decreased by calcitonin. In the gastrointestinal tract, calcitonin causes decreased secretion of gastrin and gastric acid, and causes increased small bowel secretion of sodium, potassium, chloride and water. No major effect has been shown on regulation of intestinal calcium absorption. The end result of these actions is a lowering of serum calcium concentration.

There is marked species variability in the role of calcitonin in calcium homeostasis. In vitro studies with isolated rabbit nephrons have demonstrated specific calcitonin receptors in the kidney that are distinct from PTH receptors. The hormone increased net calcium flux in distal portions of the nephrons resulting in an overall calciuric effect in vitro. Still, the physiologic effects of calcitonin in the rabbit remain unclear. One group found that intramuscular injections of calcitonin decreased serum calcium concentration while another concluded that there was no change. No consistent effect has been demonstrated on urinary excretion of calcium in the rabbit.

Vitamin D

The precursors of active vitamin D3 come from the diet or are synthesized in the epidermis from 7-dehydrocholesterol. The conversion of 7-dehydrocholesterol is catalyzed by ultraviolet irradiation from sunlight. Dietary precursors are absorbed in the proximal small intestine and modified by hydroxylation in the liver and kidneys. In the kidney, activation of the enzyme 1-alpha-hydroxylase results in the final conversion of 25-hydroycholicalciferol to 1,25 (OH)2D3. Synthesis of active vitamin D3 is stimulated by PTH, and may also be increased by prolactin, estradiol and growth hormone. Increased serum calcium and vitamin D concentrations will decrease the rate of conversion to active vitamin D3. The main function of Vitamin D is to regulate the absorption of enough calcium and phosphorus to ensure adequate concentrations for bone mineralization. In most mammals, Vitamin D is the principal regulator of intestinal calcium and phosphorus absorption. The hormone is also necessary for osteoclastic bone resorption and mobilization of calcium from bone, and increases renal tubular reabsorption of calcium and phosphorus. Vitamin D also decreases the formation and secretion of PTH by the parathyroid gland.

When rabbits are fed a diet deficient in vitamin D, serum PTH levels rise, but there is no detectable change in serum calcium levels. Chronically vitamin D deficient rabbits showed no change in net intestinal absorption of calcium or phosphorus as compared to controls, while urinary excretion of both minerals was greatly decreased. These results emphasize the vitamin D-independent nature of intestinal calcium absorption in the rabbit, as well as the importance of renal conservation of calcium and phosphorus in vitamin D deficient rabbits. Vitamin D also plays a role in insulin secretion and glucose tolerance in the rabbit, and helps maintain the transplacental calcium gradient in pregnant does.

Other hormonal regulators of calcium homeostasis

In humans, both estrogen and androgens are known to regulate renal calcium transport. This regulatory effect may partly account for the negative calcium balance that results from deficiency of these hormones. Estrogens also increase intestinal absorption of calcium, elevate total serum calcium concentration, and may influence the conversion to active vitamin D3. Testosterone and progesterone both enhance reabsorption of calcium in the rabbit kidney. Testosterone has been shown to have a determinant effect in the formation of urinary stones in rats, while estrogen has a protective effect. The effects of gonadectomy on calcium homeostasis in the rabbit have yet to be determined. Sex steroids have been shown to play a role in decreasing the calcium content of atherosclerotic plaques in spayed rabbits fed a high-cholesterol diet.

Prostaglandins of the E series are potent activators of bone resorption. PGE1 causes increased synthesis of active vitamin D and subsequent hyperabsorption of calcium at the intestinal level. When administered intravenously to rabbits, PGE1 increased serum calcium concentration. PGE2 increases osteoclast numbers leading to increased bone resorption.

Exogenous steroid administration and endogenous glucocorticoids have been associated with decreased intestinal calcium absorption and increased renal calcium loss. Adrenal cortical steroids have actions that are antagonistic to Vitamin D and PTH, and can affect responses of plasma calcium, phosphorus, PTH and calcitonin. Administration of dexamethasone to rabbits resulted in calcium retention in the wall of the duodenum. Suckling rabbits treated with methylprednisolone exhibited decreased intestinal calcium. Another group found decreased serum 1,25(OH)2D3 concentration, decreased bone trabecular volume, and increased rate of bone turnover in methylprednisolone-treated rabbits.

Growth hormone (GH) is the principle hormone regulator of growth. It causes increased intestinal calcium absorption in humans, and has been shown to produce hypercalciuria by an unknown mechanism. GH may also produce hypercalcemia, possibly by stimulating 1, 25(OH)2D3 production.

Several digestive and pancreatic hormones also affect mineral homeostasis. Insulin deficiency results in decreased intestinal calcium absorption in association with low levels of vitamin D. Pharmacologic doses of glucagons produce transient hypocalcemia, but the physiologic role of this hormone is not clear. The hypocalcemic effect of glucagon may depend on the presence of the thyroid gland in rabbits. Gastrin, secretin, and vasoactive intestinal peptide have been shown to alter extracellular calcium concentrations in animals, but their physiologic role has not been determined. It has been proposed that the association of the digestive enzymes and calcitonin in the post-prandial period may affect the hypocalcemic effects of these hormones. This may also explain the hypocalcemia seen in some cases of pancreatitis.

Urolithiasis and "bladder sludge"

The urine of rabbits is normally cloudy in appearance and contains three main types of calcium-containing crystals: calcium carbonate monohydrate, anhydrous calcium carbonate, and ammonium magnesium phosphate. The color and turbidity of rabbit urine can vary from "grapefruit juice" to "coffee with cream". Calcium sediment can accumulate in the bladder like mud. It combines with mucus, protein and cellular debris, coming to rest in a dense layer along the ventral wall of the bladder. Over time "bladder sludge" can condense into a doughy consistency that it difficult to break up and resists going back into suspension. Bladder sludge may lead to cystitis, bladder distention, and other problems.

Etiology

Urolithiasis and "sludgy" urine are common in rabbits and may result from an exacerbation of normal physiologic calcium excretion. When rabbits are fed a high calcium diet, urinary calcium excretion increases, but urine volume remains constant, increasing the likelihood of crystal aggregation and stone formation. The alkaline pH of rabbit urine also increases the risk of forming insoluble calcium precipitates. Genetic predisposition, dehydration, metabolic disorders, bacterial or parasitic infections, and nutritional imbalances may also be predisposing factors. Temporary obstruction of the kidney in rabbits recovering from hydronephrosis resulted in crystal aggregation and stone formation within a few weeks. This suggests that any condition causing urinary stasis (arthritis, E. cuniculi infection, spinal trauma or deformity, bacterial infection, lack of exercise, obesity) may also predispose the rabbit to urolithiasis.

As with many exotic animal diseases, it appears that most instances of bladder sludge are the result of improper husbandry. Calcium intake may not be as important to the etiology as fluid intake and exercise. Rabbits that become obese on a calcium-rich, protein-rich dry diet tend to move around less and to hold onto more concentrated urine. Lack of access to fresh water, water that tastes bad, or water that is not changed daily leads to chronic mild dehydration and concentration of the urine. Rabbits without adequate space to move are prone to obesity and are less likely to eliminate urine sediment. Those that have concurrent metabolic bone disease, arthritis, back problems, lameness, bladder atony or perineal pain may also be prone to bladder sludge.

Symptoms

Affected rabbits may be asymptomatic. Urine can vary from normal, to cloudy, to thick and muddy; it may appear darker than usual. Hematuria is possible and may be intermittent. Affected rabbits may exhibit a change in litter habits: urination in an unusual area, at an unusual time, for longer than normal, or with increased frequency. Urinary incontinence and urine scald frequently occur. Rarely straining, tail-bobbing, posturing, or vocalization may be observed. Some owners mistake this for "constipation". Affected rabbits may exhibit anorexia, weight loss, decreased stool production, GI stasis, lethargy, depression, haunched posture, bruxism, or otherwise indicate that they are in pain.

Diagnosis

Affected rabbits frequently show no symptoms and may urinate normally. Mild cases of bladder sludge may be discovered as an incidental finding on x-rays. Abdominal radiography will reveal a diffuse to solid mineral opacity in the bladder, often completely defining its borders. Palpation of the bladder may reveal a mass with a doughy consistency. Abdominal ultrasound and endoscopy are occasionally used. It is also important to assess the kidneys, ureters, and urethra. Metabolic bone disease and dental problems are frequently found in rabbits with bladder sludge. Bladder sludge is occasionally associated with stones. Culture and sensitivity of the urine is often negative.

Treatment

Mild cases of bladder sludge respond to increased fluid intake, exercise, and dietary changes. Increasing fluid intake will cause more urine to be produced, dilute the solutes in the urine, and enable more sludge to pass. Subcutaneous fluids can be administered during the initial phase of treatment, 20-30 mL/kg q8-12 hr, and owners can be taught how to continue treatment at home. Increase the amount of water the rabbit will drink by flavoring the water with fruit juice. Find out which fruit juice the rabbit likes best by giving it directly, via syringe or in a bowl. Add its favorite juice to the drinking water. Change the water daily and monitor the water intake. Gradually reduce the proportion of juice in the water to the point that water consumption drops, and then return the quantity of juice to its previous level.

Get affected rabbits up and keep them moving. Take rabbits out for walks on a harness. Let them exercise outdoors. Make them use the stairs. Bladder sludge occurs more commonly in obese, sedentary rabbits. Exercise will help the rabbit lose weight, produce more urine and urinate more often. Agitating the bladder helps to stir up sediment, move it out, and reduces its tendency to settle. Urinating more frequently may improve muscle tone to the bladder, as well.

Reduce or eliminate pellets in the diet. Dry pelleted feed may contribute to chronic dehydration and the production of concentrated urine. Removing alfalfa-based hay and pellets will reduce the calcium load, protein and energy intake, and limit weight gain. Timothy hay based products are preferred. Add green moisture rich foods to the diet to force even more water consumption. The calcium contained in some fresh greens is bound by oxalate and is less bioavailable than previously believed. Dandelion greens, which are considered to be "high in calcium", actually have a diuretic effect. The benefit of increased moisture intake appears to outweigh the increase total calcium that greens add to the diet.

For mild cases of bladder sludge, and even in some cases where sediment appears dense, gentle manual expression of the bladder can be achieved without anesthesia. The author stands the patient on its hind limbs. The rabbit's chest is lifted and supported by the author's right hand, and its back is supported against the author's body. With the left hand the author gently agitates the bladder and expresses it. When expressing the bladder with the patient in this upright position, the heavy sediment in the bladder tends to flow out the urethra with the urine. Expressing the bladder may also help the bladder to return to its normal size and tone. This procedure can be repeated once or twice daily when the bladder is full. This "awake procedure" should not be utilized for rabbits that are in obvious pain.

For more severe cases, where urine sediment is extremely dense or the rabbit is in pain, the bladder should be catheterized and repeatedly flushed while the patient is anesthetized. The goal is to remove as much sediment as possible, to remove irritation and encourage the bladder to shrink to its normal size. The patient is premedicated with buprenorphine 0.02 mg/kg SQ for pain, and midazolam 0.25 mg/kg IM to improve relaxation. Fluids are administered SQ or via IV catheter. The patient is placed under general anesthesia with isoflurane. A urethral catheter is carefully placed. For bucks, a 3.5- or 5-fr catheter is recommended, and placing the catheter is routine. For does, a 5- or 8-fr catheter is advanced along the vaginal floor while simultaneously expressing the bladder. Once the catheter is placed, the bladder is repeated flushed with warm sterile saline and expressed until it is clean. Agitating the bladder will increase the output of sediment, however, the bladder should be handled gently as it is thin walled and prone to trauma. Cystotomy is an alternative to this method, however surgery can usually be avoided.

Urolithiasis may occur anywhere along the urinary tract. It does not indicate an emergency unless a stone has lodged in the urethra, impairing urination and distending the bladder. The bladder can be emptied by cystocentesis and, if possible, the stone should be flushed back into the bladder by urohydropropulsion. A cystotomy or urethrotomy is performed in routine fashion. If urethral blockage cannot be cleared, a urethrostomy or cystostomy can be established and maintained until another means of unblocking the urethra is found.

Post-operative pain meds are important for the bladder flushing procedure. Administer a second dose of buprenorphine 12 hours after the first, and give meloxicam 0.2 mg/kg PO q24 hr for 5-7 days. Hospitalize the patient for observation. Change the litter box frequently in order to monitor urination. If there is bladder atony, manually express the bladder several times daily. Antibiotics are generally not indicated for bladder sludge, but may be indicated after the bladder flushing procedure. The patient is discharged once he is able to urinate comfortably, and is eating and drinking normally.

Monitoring

The patient is rechecked and radiographed every 2-4 weeks until the problem has resolved. For mild cases improvement will be noted with fluids, diet change, and exercise alone. If conservative therapy fails, then offer manual expression and/or bladder flushing.

Dietary considerations

The unusual calcium metabolism of rabbits makes it essential to maintain a diet that is well-balanced with appropriate calcium concentration, calcium to phosphorus ratio, and vitamin D content. If too little calcium is consumed, secondary hyperparathyroidism and bone resorption may result, while if too much calcium is present, the risk of urolithiasis and bladder sludge is increased. A dietary calcium level of 0.22% supports normal growth, but 0.35-0.4% calcium is required for optimal bone calcification and growth rates in young rabbits. There should be less than 25 µg (1,000 IU) cholecalciferol/kg food to prevent soft tissue mineralization. The pet rabbit's diet should be chosen carefully to prevent selective feeding. If the rabbit rejects pellets and whole grain, calcium intake may be insufficient. Most commercial rabbit feeds contain calcium from alfalfa hay and calcium carbonate, which are very absorbable. While many vegetables are high in calcium, they often contain calcium oxalate, which is not metabolized by rabbits, and thus cannot contribute to urinary sludge or stones.

The ideal diet for pet rabbits should contain at least 15-16% fiber, 12-13 % protein, 1-3 % fat, 0.6-1.0 % calcium, calcium to phosphorus ratio of 1:1 – 2:1, and less than 1000 IU/kg vitamin D. A diet comprised of grass hay versus legume hay, a small portion of commercial pellets composed of timothy hay, fresh vegetables, and fresh drinking water has been recommended.

In the past, most of the emphasis concerning the prevention and treatment of bladder sludge and urolithiasis has been placed on the calcium content of the diet. Today, however, more emphasis is placed on water intake, exercise, and preventing obesity. Most instances of the disease can be addressed by improving these three factors.

Acknowledgement

The information on rabbit calcium metabolism comes from the excellent review in Veterinary Clinics of North America-Exotic Animal Practice 2008 11(1), by the author's good friend and coworker of many years, Christine Eckermann-Ross, DVM, CVA, CVH.

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