Urolithiasis in dogs

Urolithiasis consists of the presence of aggregates of crystals resulting from the precipitation of solutes present in the urine and accounts for 1.5-3% of disorders seen in clinical practice.Uroliths (urinary tract calculi/stones) can be distinguished based mainly on their site (kidney, ureter, bladder or urethra), on their composition and on the pathogenic mechanism that causes them. Affected dogs may be asymptomatic (with stones being an incidental finding) or show various signs depending on the location and size of the stone: dysuria, macrohaematuria, including oliguria and anuria when the lower urinary tract is involved, or hyperthermia and colic when the upper urinary tract is involved. The diagnosis is usually confirmed by radiographic and/or ultrasound examinations (Table 1).

FORMATION AND GROWTH OF URINARY TRACT STONES

In order for urinary calculi to form, the urine has to be supersaturated with a given element.The formation of a urinary tract stone begins with the microscopic precipitation of a small nucleus.The process by which a crystal forms from a pure solution is called homogeneous nucleation, whereas the process by which a nucleus is formed starting from pre-existing surfaces (cell debris, casts, other crystals) is called heterogeneous nucleation; this latter form of nucleation requires a lesser degree of supersaturation._¹

The relative supersaturation index (RSS) is currently the best accredited method for evaluating the risk of stone formation.The growth, formation and dissolution of urinary crystals depends on the concentration of the minerals making up the crystal (e.g. calcium and oxalate), which are free to react with each other._² These free fractions can be calculated and their product is called the activity product.

The method envisages the measurement ofpH, calcium, magnesium, sodium, potassium, ammonium, phosphate, oxalate, citrate, sulphate, uric acid and chloride in a 24-hour collection of urine.The data obtained are analysed using specific software that calculates the ion activity product and divides it by the thermodynamic solubility product (K_sp), thereby obtaining the RSS.The thermodynamic solubility product is the maximum amount of a specific salt that can be dissolved in a solvent (water) at a constant pH and temperature (e.g.pH 6.0, 37°C).

• An RSS < 1means that the urine is undersaturated, crystals do not form and existing stones can be dissolved.
• An RSS > 1 means that the urine is supersaturated and crystals can form but not dissolve.A distinction can be made between:
  • metastable supersaturation in which heterogeneous nucleation is possible, as well as the growth and aggregation of crystals (although the activity of inhibitors tends to prevent or slow down crystallisation)
  • unstable or labile supersaturationcharacterized by higher concentrations of minerals in the urine.In this case, nucleation will occur and the activity of the inhibitors will be inadequate.

The boundary between the metastable and the labile zone is given by the formation product (K_f); beyond this point a substance can no longer be kept in solution.Studies have shown that the RSS for the formation product is 2.5 for struvite and 12 for calcium oxalate.

EPIDEMIOLOGY

Recent epidemiological data (Table 1) confirm that from the 1980s to date there has been a constant increase in calcium oxalate stones._³ There are undoubtedly various reasons for this, some of which may be the formulation of diets, the greater spread of predisposed breeds, better breeding conditions and genetic studies that might have led to a reduction in the prevalence of other forms of “metabolic” urolithiasis.

Table 1.  Prevalence and characteristics (radio-opacity, contour and shape) of the various types of urinary tract stones.

STRUVITE

Struvite stones are composed mainly of magnesium ammonium phosphate hexahydrate; in dogs they are often mixed and may contain smaller amounts of calcium apatite and carbonate apatite.Struvite urolithiasis is often caused by the presence of urinary tract infections due to urease-producing bacteria.

Infection-induced struvite stones

The bacterial species most frequently involved in dogs are Staphylococcus spp., while Proteus sppare most frequently involved in humans. Ureaplasma urealyticum has been isolated from struvitestones inboth humans and dogs_⁴ Although micro-organisms, such as Klebsiella and Pseudomonas, are capable of producing variable amounts of urease, they are not often associated with struvite urolithiasis; the same can be said of Escherichia coli which infrequently produces urease.

In the presence of urease-producing bacteria, the urea contained in urine is split into ammonium and bicarbonate ions. The ammonium ion forms, with magnesium, magnesium ammonium phosphate, while the bicarbonate increases the urine pH, rendering the struvite less soluble.In addition, the NH₄⁺ ions damage the glycosaminoglycan (GAG) layer covering the urothelium, aiding adhesion of both bacteria and crystals to the mucosa (Fig. 1), as well as the growth and retention of the urinary calculi themselves.The inflammatory response and the increased concentration of proteinaceous products will contribute to the formation of the matrix.

Struvite urolithiasis occurs mainly in female dogs (76.6%) since females are more prone to urinary tract infections. Shih Tzus, Bichon Frises, Poodles, Miniature Schnauzers and Pekingese are among the breeds most frequently reported to be affected_^.4

Cases of struvite urolithiasis have been found in dogs with alkaline urine in the absence of bacteriuria or detectable amounts of urease.In these cases, it is believed that dietary or metabolic factors may be involved. Alkalinising diets rich in magnesium and phosphorus can cause sterile struvite urolithiasis in predisposed subjects. Pathological conditions, such as primary hyperparathyroidism and distal renal tubular acidosis, may cause alkaline urine and, in the latter case, hypercalcinuria and reduced citrate excretion.

Treatment
The objectives of medical treatment are to stop further growth of the struvite stones and to promote their dissolution, controlling, correcting and/or eliminating the predisposing causes or determinants.The stones can be dissolved through combined therapy: increasing the volume of the urine, decreasing the urinary concentration of the precursors (urea, phosphorus and magnesium) and treatment of the infection.

Diet – Some diets formulated to dissolve struvitestones contain low amounts of phosphorus, magnesium and proteins.These diets have been supplemented with sodium chloride to stimulate thirst and induce polyuria. Furthermore, the low protein content causes a decrease in urea synthesis, with a resulting reduction in the concentration of urea in the renal medulla and an increase in diuresis.

The blood-chemistry findings will bedecreased urea, magnesium, phosphorus and serum albumin values, as well as a slight increase in alkaline phosphatase activity._^4,5 These diets are designed for dissolution therapy (lasting a few weeks or months) rather than for long-term prophylaxis.They are contraindicated in patients with cardiovascular and renal diseases or hypertension, in growing puppies, in pregnant or nursing bitches and in geriatric patients.As they are relatively rich in fats, it is advisable to monitor animals at risk of pancreatitis closely, as well as those with alterations in lipid metabolism or subjects prone to primary hyperlipidaemia (Miniature Schnauzers).

Antibiotic treatment – The role of urinary tract infections in the formation of struvite stones in dogs underlines the need for a treatment aimed at the elimination or control of urease-producing bacteria.It is recommended that therapeutic doses of antibiotics, selected based on the results of antibiograms, are administered.Antibiotics whose mean urinary concentration (MUC) is at least four times higher than the minimum inhibitory concentration (MIC) are to be preferred and bactericidal antibiotics are to be preferred over bacteriostatic antibiotics.It is important to consider that diuresis reduces the urinary concentration of drugs.

Antibiotic administration should not be suspended as long as there are still stones in the urinary tract.Indeed, it has been shown that pathogenic bacteria, trapped inside the urolith, are protected against the action of antibiotics and can remain viable for long periods of time, before being released during a stone dissolution protocol._¹ For this reason antibiotic therapy and the administration of special diets must last for at least 1 month after radiographic verification that the stones have been dissolved.In this way the likelihood of recurrences due to the permanence of small stones (diameter < 3 mm), not detectable by a radiographic examination, is reduced.It is advisable to monitor the patient on a monthly basis during the dissolution protocol.On average it takes 3-6 months to dissolve stones.At the end of the protocol, cultures should be performed at intervals to verify that the UTI has been eradicated.

Urease inhibitors – These are agents that, by reducing the enzymatic hydrolysis of urea, decrease the concentration of NH_4⁺ and the alkalinity of urine.Acetohydroxamic acid is both a competitive and a non-competitive inhibitor of bacterial urease.Treatment with acetohydroxamic acid, at doses of 12.5 mg/kg every 12 hours, inhibits the growth of struvite stones and promotes their dissolution. Higher dosages, although potentially capable of dissolving the stones, can causereversible haemolytic anaemia, blood dyscrasias and alterations in bilirubin metabolism.Teratogenic effects have also been shown.Since acetohydroxamic acid is primarily excreted in the urine, it should not be administered to patients with kidney disease or patients with hyperuraemia.Generally, an effective dissolution protocol is started based on the results of an antibiogram, and urease inhibitors are not necessary.In any case, AHA can be useful in patients with UTI caused by antibiotic-resistant urease-producing bacteria associated with persistent struvite stones.

Urinary acidifiers – Urinary acidification in the absence of dietary changes is not effective in dissolving infection-induced struvitestones. Because of the amount of urease produced by pathogenic bacteria, it may not be possible to acidify the urine by administering urinary acidifiers at doses that do not cause systemic acidosis.Urinary acidifiers should not be administered together with acidifying diets or in the presence of metabolic acidosis. Methionine and ammonium chloride are among the active substances which are most widely used._⁶

Prevention
Eradication or control of urinary tract infections remains the most important factor in the prevention of recurrences of infection-induced struvite urolithiasis.If a urinary tract infection persists or is recurrent, the recommended treatment is prophylactic doses of antibiotics that are excreted in high concentration in the urine (e.g. amoxicillin, enrofloxacin, trimethoprim-sulfa), which must be chosen based on sensitivity tests.

Given the efficacy of diet in inducing the dissolution of struvite stones, dietary changes could be relevant even for preventing recurrences.The role of diet in the prevention of struvite stones has, however, not yet been fully defined. Although many bacteria cannot live in urine with a pH below 5.5, it is quite difficult to acidify urine to values lower than this.In clinically healthy dogs, diets for the dissolution or prevention of struvite stones lead to the production of urine with pH values from 5.8 to 6.4. This pH range is not capable of preventing new bacterial infections.In addition, the consumption of diets that promote diuresis could predispose dogs to the development of urinary tract infections, since many bacteria cannot survive in highly concentrated urine. Furthermore, the prolonged administration of these diets could lead to the formation of calcium oxalate stones in predisposed breeds.

Sterile struvite stones

Treatment
The medical management of sterile struvite urolithiasis is based on the use of diets that dissolve the calculi and urinary acidifiers.Antibiotics and urease inhibitors unless there are secondary urinary tract infections.Sterile struvite stones usually dissolve in less time than required for the dissolution of infection-induced struvite stones (2-4 weeks).Protein restriction is not essential for the dissolution of sterile struvite stones in dogs, while urine acidification to pH values of around 6 has proven effective in inducing the dissolution of these uroliths.

Prevention
Sterile struvite stones have a greater tendency to recur compared to infection-induced struvite stones in which the urinary tract infection was eradicated or controlled.Diet plays an important role in affected dogs.The prophylactic value of the concomitant restriction of phosphorus and magnesium in the diet has not been determined.The administration of urinary acidifiers should be taken into consideration when the urine pH remains alkaline in spite of the dietary therapy.

CALCIUM OXALATES

Calcium oxalate can occur in two forms: calcium oxalate monohydrate (whewellite) and calcium oxalate dehydrate (weddellite) (Fig. 2). The monohydrate salt is the more frequent form in dogs.Certain breeds are at risk:Bichon Frise, Miniature Schnauzer, Shih Tzu, Pekingese, Lhasa Apso, Yorkshire Terrier, Maltese and Pomeranian._³ Calcium oxalate stones are found more frequently in males (69%) than in females (31%).They are concomitant with infection in 50% of females and in 30% of males. It has been estimated that around 50% of affected subjects have a recurrence within 3 years. In order for the stones to form, the urine must be supersaturated with the precursors of the uroliths.

Some predisposing factors have been identified:

Hypercalcinuria
The concomitant absence or presence of hypercalcaemia and the mechanism through which the excess urinary calcium occurs must be determined.

Normocalcaemic hypercalcinuria may be secondary to an excessive intestinal absorption of calcium (absorptive hypercalcinuria) or to reduced renal tubular absorption (hypercalcinuriafrom renal loss).

• Absorptive hypercalcinuria is caused by increased intestinal absorption of calcium.In this case, it is accompanied by an increase of calcium filtered calcium through the glomeruli and by a reduction in the renal reabsorption of calcium due, in part, to the suppression of parathormone once the calcium levels have reached the upper limits of the normal range.The hypercalcinuria is accompanied by an increase in absorption of oxalic acid which, in its turn, will cause hyperoxaluria.In around half of affected humans, there is an increase in the concentration of vitamin D3.The same phenomenon has been observed in a group of Miniature Schnauzers.
• Hypercalcinuria from renal loss is less frequent than the form described above.It is characterized by renal loss with a reduction in serum calcium ions and increased concentrations of parathormone and calcitriol:the intestinal calcium absorption increases, bringing the calcium levels within the normal range, contributing to hypercalcinuria.Unlike in patients with hypercalcinuria from excessive absorption, the loss of calcium through the urine does not decrease with fasting.

Hypercalcaemic hypercalcinuria is caused by increased glomerularfiltration of mobilised calcium, which overcomes the normal tubular reabsorption mechanisms.It occurs in diseases that cause hypercalcaemia:neoplasms, primary hyperparathyroidism, hyperthyroidism, sarcoidosis and granulomatous diseases._⁷

Hyperoxaluria
Oxalic acid in the urine results from the hepatic metabolism of ascorbic acid and of some amino acids (glycine and serine) derived from both endogenous and dietary sources (tryptophan and hydroxyproline).In humans, mice and swine, part of the oxalate introduced with the diet is degraded in the intestinal tract by the normal bacterial flora (Oxalobacter formigenes). Some patients with calcium oxalate nephrolithiasis have no or reduced amounts of Oxalobacter formigenes in the intestines; this situation could be caused by prolonged antibiotic therapy._⁸ The oxalate is filtered freely through the glomeruli and undergoes bidirectional transport in the renal tubules.The increase in urinary oxalate levels promotes the formation of calcium oxalate stones.Patients with increased intestinal absorption of calcium or who consume low calcium diets are prone to increased intestinal absorption of oxalate, hyperoxaluria and consequent formation of calcium oxalate stones.

Hypocitraturia  
Hypocitraturia is present in around 15-63% of people with calcium oxalate urolithiasis. Urinary citrate inhibits the formation of these calculi since, bound to calcium ions, it constitutes a relatively soluble salt (calcium citrate), reducing the amount of calcium available to bind with the oxalate.Hypocitraturia has been observed in dogs with calcium oxalate stones.In humans, citrate is filtered by the kidney and around 75% is reabsorbed in the renal tubules.The amount of citrate reabsorbed is heavily conditioned by acid-base homeostasis.Metabolic acidosis reduces the urinary excretion of citrate, increasing its tubular reabsorption, while metabolic alkalosis increases its excretion. In dogs, urinary citrate excretion is very limited and less than 1% of the filtered citrate is excreted._¹

Magnesium
Magnesium binds with oxalate, reducing the urinary saturation of calcium oxalate; in contrast, when the levels of magnesium in the urine are low, there is a higher risk of urolithiasis.

Nephrocalcin 
Nephrocalcin is a glycoprotein synthesised by the cells of the proximal tubule and of the ascending limb of the loop of Henle.Nephrocalcin can be absorbed by calcium oxalate crystals, forming a macromolecular layer that inhibits the growth of the crystal. Nephrocalcin is a powerful inhibitor of the aggregation of calcium oxalate monohydrate crystals._⁹ The nephrocalcin identified in the urine of subjects with calcium oxalate urolithiasis is structurally different from that found in the urine of healthy patients.Indeed, it lacks sufficient amounts of carboxyglutamic acid residues and is 10 times less effective in preventing crystal growth._⁹

Tamm-Horsfall protein
Tamm-Horsfall protein is a mucoprotein synthesised by cells of the ascending limb of the loop of Henle and the distal tubule; it binds to the surface of the crystals and is a stabilising agent. In humans, Tamm-Horsfall mucoprotein tends to polymerise at acid pH, thus reducing its capacity to interact with the crystals.

Pyrophosphate
Pyrophosphate is an inhibitor of calcium oxalate in the urine.The amount of pyrophosphate in the urine is related to the dietary intake of phosphorus.Although the role of pyrophosphate in canine urine is unknown, it has been shown that diets low in phosphorus increase the risk of calcium oxalate urolithiasis.Phosphate ions bind to calcium forming complexes that cannot be absorbed; a lower intake of phosphorus in the diet can, therefore, help to increase the amount of calcium absorbed.In addition, diets low in phosphorus can stimulate the production of calcitriol, which promotes the intestinal absorption of calcium and phosphorus, while diets high in phosphorus appear to cause an increase in pyrophosphates in the urine.

Treatment
Calcium oxalate stones cannot be dissolved:patients with such stones will have to undergo surgery. Medical treatment should be aimed at preventing relapses.

Diet – considering that the solubility of calcium oxalate practically does not change as the pH changes, dietary measures are aimed at the correction/elimination of the predisposing or determining causes (see above).The following are, therefore, indicated:

Avoid excessive restriction of dietary calcium – for the same amount of oxalate introduced with the food, subjects fed with low calcium diets have higher urinary oxalate levels.In fact, calcium forms complexes with phosphate, citrate, sulphate and oxalate anions, preventing their absorption.In the case of restriction of calcium intake in the diet, the formation of these complexes is reduced, allowing greater absorption of oxalate and an increase of its concentration in the urine.

Reduce foods containing oxalic acid – contained in some types of fruit and vegetable.

Avoid a reduction in phosphorus – this causes a greater synthesis of calcitrioland consequently greater absorption of calcium and phosphorus in the bowel. Furthermore, the phosphorus introduced with food forms some insoluble complexes with calcium in the intestines, limiting its absorption.

Avoid a reduction in magnesium– in vitro studies have shown that the addition of magnesium reduces the supersaturation of calcium oxalate because of its bondwithoxalic acid.There are studies indicating that dogs fed with diets rich in magnesium are two or three times less likely to develop calcium oxalate stones.These data must be considered with caution since the oral administration of magnesium oxide has been associated with an increase in the urinary excretion of calcium.

Sodium– the sodium content in diets formulated to prevent calcium oxalate urolithiasis has been a very controversial and much discussed topic.The dietary intake of sodium is correlated with increased tubular excretion of calcium ions with consequent hypercalcinuria.Dietary sodium restriction was initially recommended based on this observation._¹⁰ More recently, it was verified that sodium supplementation actually increased the daily urinary excretion of calcium and sodium (mg/kg/24 hours) but decreased both the urinary concentration of calcium (mg/dl) and the RSS due to the induced polydipsia and polyuria.Thus, the risk of precipitation was affected more by the urinary concentration of calcium than by the daily amount excreted.

Avoid a reduction in vitamin B6 – since this aids the transamination of glycoxalate to glycine.

Avoid vitamin C supplementation – since this is converted into oxalate and an excess of vitamin C could cause hyperoxaluria.

Avoid vitamin D supplementation – since it could cause greater absorption of calcium in the intestine.

Potassium citrate - The administration of potassium citrate is controversial.A potassium citrate supplement was recommended at oral doses from 50 to 75 mg/kg b.i.d. if the urine pH was acidic or if the calcium oxalate crystals persisted in spite of dietary therapy.Potassium citrate supplementation did not significantly increase the urinary excretion of citrate in healthy dogs; this supplementation caused urinary excretion of citrate and lowered the RSS levels of calcium oxalate in the urine in only three Miniature Schnauzers.In another study, orally administered potassium citrate did not increase the urinary concentration of citrate but caused a dose-dependent increase in the pH of the urine._¹¹

In any case, the manipulation of urine pH and alkalinisation (pH>7) could lead to the precipitation of calcium phosphate.

Thiazide diuretics – Thiazide diuretics reduce the urinary excretion of calcium; they promote natriuresis by inhibiting the sodium-chloride co-transport on the luminal surface of distal tubules. By inhibiting sodium transport, thiazides decrease the intracellular concentration of sodium.This promotes calcium-sodium exchange in the basolateral membrane of the tubule, leading to reabsorption of the calcium from the tubular fluid.These diuretics can help to reduce recurrences of calcium oxalate urolithiasis.Since hydrochlorothiazide is a more powerful natriuretic than chlorothiazide, it is plausible that it lowers the urinary calcium to a greater extent.

The administration of hydrochlorothiazide (2 mg/kg b.i.d. per os) should be considered in dogs with persistent or recurrent crystalluria in spite of dietary therapy._¹² Thiazide diuretics should only be administered in cases of normocalcaemic hypercalcinuria.They cause subclinical volume depletion.This effect can be prevented by increasing the sodium content in the diet and the water intake; thus the benefits of hydrochlorothiazide must, therefore, be evaluated in association with high sodium diets. The possible side effects – dehydration, hypokalaemia and hypercalcaemia – compel caution when prescribing this diuretic.

PURINES

Purines are the metabolic products of purine nucleotides:(i) oxypurines (hypoxanthines, xanthines, uric acid, allantoin), (ii) amino purines (adenine, guanine) and (iii) methyl purines (caffeine, theophylline, theobromine).In the majority of dogs, allantoin is the end product most present in the urine and it is also the most soluble purine derivative.

URATES

Ammonium acid urate (ammonium urate and ammonium biurate) is the monobasic ammonium salt of uric acid and it is constituent of the purine stone seen most often in dogs.

The risk factors for urate stone formation in dogs are an increase in the urinary concentration of uric acid, an increase in the renal excretion of ammonium ions or their production by urease-producing bacteria, an acid urine (due to lesser solubility of uric acid), the presence of promoters or the absence of inhibitors of precipitation/aggregation (e.g.Tamm-Horsfall protein).

The consumption of protein-rich diets by healthy dogs has been associated with high urinary excretion of uric acid and increased urinary saturation of uric acid, sodium urate and ammonium urate.Furthermore, since the acidity of the urine is a risk factor for urate stone formation, the administration of foods that acidify the urine (diets containing large amounts of protein or  acidifiers) may constitute an additional risk factor._^13,14

Dalmatians – The capacity of Dalmatians to convert uric acid into allantoinis much lower than that of other breeds or crossbreeds.Indeed, the conversion of uric acid into allantoin in Dalmatians is so inadequate that the urinary excretion of uric acid in these dogs is 10-60 times higher than that found in other breeds.Dogs other than Dalmatians have a serum uric acid concentration below 0.5 mg/dl and they eliminate around 10-60 mg of uric acid per day in their urine.Dalmatians have a serum uric acid concentration from two to four times greater than other dogs and they eliminate around 400-600 mg of uric acid per day.In spite of this, the concentration of hepatic uricase in Dalmatians is equivalent to that in the other breeds:this strengthens the hypothesis of the existence of a defect in the transmembrane transport system of uric acid in the liver.This alteration and the lesser reabsorption in the proximal renal tubules cause hyperuricosuria and hyperuricaemia with consequent crystalluria and possible formation of stones.This condition has an autosomal recessive type of transmission (CFA03).

Although all Dalmatians show hyperuricosuria, around 97% of subjects with urate stones are male.All observations indicate that this predisposition is due exclusively to anatomical causes (width/length of the urethra and the obstructive potential of the os penis)._¹⁵

The relatively low incidence of cases of urolithiasis indicates the presence of additional predisposing factors, for example a decrease in the concentration of inhibitors of crystallisation: one study showed reduced urinary excretion of Tamm-Horsfall protein in stone-forming dogs compared to in those that did not have stones._¹⁶

Other breeds – Although urate urolithiasis is commonly found in Dalmatians, in approximately 30-60% of the cases, this type of stone is found in other breeds without liver damage:the English Bulldog, Miniature Schnauzer, Shih Tzu and Yorkshire Terrier.Urate stones are found more often in males than in females in these breeds too.The causes of stone formation have not been determined.

Liver dysfunction– Impaired liver function, porto-systemic shunts or other vascular abnormalities predispose to urate urolithiasis._¹⁷ The failure of conversion in the liver of uric acid into allantoin and of ammonium into urea (hyperuricemia and hyperammonaemia) will lead to greater urinary excretion of uric acid and ammonium (hyperuricosuria and hyperammoniuria) with resulting predisposition to urolithiasis.

Treatment
Urate stones can be dissolved with the use of medical therapy and some expedients aimed at reducing the urinary concentration of the precursors and increasing their solubility.On average, it takes 3-4 months of a combined protocol for urate stones to dissolve.

Diet – Diets with a low purine or precursor content are based on a considerable restriction in protein intake; in particular, they should not contain fish and glandular organs, which are especially rich in purines.The most advantageous protein sources include eggs and dairy products.Protein restriction aids the formation of alkaline urine containing low concentrations of ammonia and ammonium ions, reducing the risk of formation of ammonium urate.In addition, the decrease in levels of urea alters the tonicity of the renal medulla, decreasing the capacity to concentrate urine.

The optimal urine pH for the greatest solubility of urates is 7.0-7.5. This goal can be obtained by administering specific diets and with the supplementation of alkalinising agents such assodium bicarbonate (25-50 mg/kg b.i.d.) and potassium citrate (50-150 mg/kg b.i.d.).

Allopurinol – The dissolution protocols involve the administration of xanthine oxidase inhibitors such as allopurinol.Allopurinol is a synthetic isomer of hypoxanthine and inhibits the action of xanthine oxidase, decreasing the production of uric acid.In dogs, its half-life is 2.5-3 hours and its bioavailability is not altered by the administration of food.The recommended dosage is 15 mg/kg b.i.d.Since allopurinol is excreted through the kidneys, the dosage should be reduced in the case of renal impairment.Xanthinuria is the most frequent side effect.The formation of xanthine crystals occurs above all in dogs treated with allopurinol which are not prescribed a low protein diet.

Prevention
The prevention protocol envisages a specific diet and the administration of urine alkalinisers in order to reduce the concentration of uric acid and ammonia in the urine. Prophylactic administration of allopurinol is not, however, recommended, given the risk of xanthinuria (Fig. 3).

XANTHINE

Xanthinuria is a rare metabolic disorder in which there is excessive excretion of xanthine in the urine.It can result from a deficiency of the enzymes that catalyse the last stages of the purine degradation pathway (hereditary form) or, more commonly, it ensues from the administration of allopurinol (iatrogenic form).

Xanthine is an intermediate product of purine metabolism; it results from the oxidation of hypoxanthine and is then transformed into uric acid, which is eliminated through the urine.Both reactions are catalysed by the enzyme xanthine dehydrogenase, found in the liver, intestine and in human breast milk.The deficiency/inhibition of xanthine dehydrogenase causes excessive urinary excretion of hypoxanthine and xanthine.The very poor solubility of the latter in the urine, at any pH, can lead to the formation of stones.

In veterinary medicine, very few cases have been studied.The hereditary form is suspected in animals which do not have a history of allopurinol administration.It has been reported in the Cavalier King Charles Spaniel, in the Wirehaired Dachshund and in two cats.

Treatment
The treatment in the course of xanthinuria involves surgical removal of the stones in the case of obstruction and correction of the fluid, electrolyte and acid-base imbalances that may be associated.Prevention protocols do not give satisfactory results.At present there is no specific therapy and since xanthine’s solubility is virtually unaffected by the pH of the urine (for pH values in the physiological range), alkalinisation, which is effective in the case of hyperuricosuria, in this case fails.As a matter of fact, the solubility of xanthine (around 0.5 mmol/L at pH 5.0) does not increase significantly following alkalinisation (0.9 mmol/L at pH 7.0), constituting a limitation in treatment.The only management really indicated is, therefore, restriction of dietary purine intake and, above all, a high and constant water intake._¹⁸

CYSTINE

Cystine is an amino acid filtered by the glomerulus and 99-100% reabsorbed from the proximal tubule through an active transport mechanism.Cystinuria is a hereditary disease characterized by defective reabsorption of cystine and other dibasic amino acids in the renal tubules.

Cystinuria can occur without the loss of other amino acids or it may be associated with the loss of: lysine, ornithine and arginine. The degree of cystinuria varies among individuals and may decrease with age.In dogs, cystine calculi form more often in male subjects (98%), developing in both young subjects and in adults 4-5 years old._¹⁹Cystinuria is rare in females and is not associated with urolithiasis.Cystine stones have been reported in various breeds:Newfoundland, English Bulldog, French Bulldog, Bassett Hound, Dachshund, Staffordshire Bull Terrier and Chihuahua.The transmission inthe Newfoundland dog is autosomal recessive: the nonsense mutation is a cytosine to thymidine transition at nucleotide 111 of exon 2 of the SLC3A gene._²⁰ This mutation has not been detected in other breeds, confirming that the pathogenesis of cystinuria in dogs is heterogeneous.Agenetic test is available for Newfoundland dogs and Labrador Retrievers.Cystine’s low solubility at normal urine pH values (5.5-7.0) leads to its precipitation in the form of crystals and stones.

Treatment
Cystine stones can be dissolved through medical therapy which involves some strategies useful for reducing the urinary concentration of the aminoacid and increasing its solubility.

Diet – Low protein diets and the administration of urine alkalinisersare recommended.This can lead to a 20-25% decrease in cystine excretion.The degree of protein restriction is still controversial, given the risk of dilated cardiomyopathy associated with carnitine deficiency and reported in dogs with cystinuria fed with low protein diets._²¹ Cystine’s solubility increases when the pH of the urine reaches values between 7.0 and 7.5; this degree of alkalinisation can be obtained through a suitable commercial diet and/or supplementation with potassium citrate or sodium bicarbonate. Dissolution of cystine stones has been reported even without the aid of the diets described above. As in every case of urolithiasis, an increase in urine volume is desirable.

Tiopronin - Drugs containing sulphydryl groups that interact with cystine, converting it into a more soluble compoundthrough a disulphide exchange reaction, are prescribed for cystinuria; the drug takes the place of one of the cysteine residues of the cystinemolecule.The compounds indicated for use are D-penicillamine and tiopronin (2-MPG). Since D-penicillamine is associated with a high incidence of side effects, its use is limited.Tiopronin is used more and is currently recommended at a dosage of 20 mg/kg b.i.d.This treatment has led to dissolution of cystine stones in around 60% of the cases in a period varying from 1 to 3 months._²² Although well tolerated, several side effects have been reported for tiopronin too, including increased aggressiveness, myopathy, proteinuria, anaemia, thrombocytopenia, increased concentrations of liver enzymes and cutaneous disorders.²²

Prevention
Cystine stones recur frequently in a period varying from 1 to 36 months.The administration of a diet characterized by modest protein restriction and the administration of 2-MPG at a dose of 15 mg/kg b.i.d.have proven to be effective methods of prevention.Since the tendency to develop cystinuria decreases with age, the prevention protocol may be stopped in older dogs, especially if they have not had episodes of urolithiasis for years.

CALCIUM PHOSPHATE

Calcium phosphate stones are usually composed of hydroxyapatite and brushite; more rarely they are formed of whitlockite and octacalcium phosphate.

Pure calcium phosphate stones are not very common in dogs (0.3-0.%), while they often make up mixed stones with struvite and calcium oxalate (38%).

The breeds at highest risk are:Bichon Frise, Miniature Schnauzer, Shih Tzu, Pekingese and Lhasa Apso._³

All factors that reduce calcium phosphate solubility predispose to the formation of stones.Calcium phosphate solubility in the urine depends on the following variables:

• urinary concentration of hydrogen ions:the pH of the urine affects the solubility of some forms of calcium phosphate.Alkalinisation (>7.5) of the urine considerably decreases the solubility of hydroxyapatite, while brushite is less soluble in acid urine;
• urinary concentration of calcium ions:hypercalcinuria reduces the solubility of calcium phosphate and can cause an increase in the RSS;
• urinary concentration of inorganic phosphorus;
• urinary concentration of inhibitors of calcium crystallisation:nephrocalcin, pyrophosphates, magnesium, citrate.These inhibitors provide 30-40% of the capacity to inhibit calcium phosphate crystallisation in normal human urine.In particular, pyrophosphates raise the upper limit of saturation at which the spontaneous precipitation of calcium phosphate occurs and delay the growth of hydroxyapatite crystals, as they are adsorbed onto the surface of the crystals and block the active growth sites;
• urinary concentration of crystallisation enhancers.

These factors are identifiable in several disorders or diseases in which hypercalcaemia, hypercalcinuria and hyperphosphaturia occur, such as primary hyperparathyroidism, hypervitaminosis D, neoplasms, Cushing’s syndrome, distal renal tubular acidosis and normocalcaemic hypercalcinuria.

Treatment
The patient must be thoroughly evaluated to detect possible causes of hypercalcaemia; when the aetiopathogensis is determined, the treatment consists of resolving the underlying causes. In the other cases, medical treatment can be started.Since calcium phosphate stones are often small and numerous, the risk of urethral obstruction may increase.Consequently, removal of the stones should be considered.

Diet – No optimal diet exists for the prevention of calcium phosphatestones.Water intake should be increased and it is generally recommended that the diet used for the prevention of recurrent calcium oxalate stones is followed, with special caution to avoid excessive alkalinisation of the urine, which could lead to decreased phosphate solubility and further precipitation of crystals.

OTHER STONES

Other less common forms of urolithiasis have been identified.

SILICA – this mineral is present in very small amounts in animal proteins.The food source is usually vegetal: rice or soy hulls, which may be added to food to increase the fibre content, or wheat gluten, which is used as a cheap source of protein.The ingestion of soil can contribute to the formation of these rare stones, for which there is no dissolution protocol.

DRUGS – fluoroquinolones, tetracyclines, sulphonamides, primidone and allopurinol._²³

MELAMINE – this is an organic base that cannot be metabolised by mammals and has no nutritional value in dogs and cats.Given its nitrogen content, it is used in some countries as a fertiliser.In industry, it is used in the production of plastic or materials used for kitchen utensils.Its use as a food additive is illegal in Europe and in the USA.In China it is used as an additive in animal foods.Its combination with cyanuric acid has caused episodes of lethal nephrotoxicity in pets and urolithiasis in dogs.The crystals are rounded, yellowish-brown with radial streaks and give rise to yellowish-grey stones._²⁴

References

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