Urolithiasis in Dogs

Urolithiasis is a disease which results from formation of stones (calculi or uroliths) in the urinary tract. Crystals form in urine when the concentration of its constituents exceed a critical level of saturation. Dietary factors can profoundly influence urolith formation because dietary ingredients and feeding patterns influence the pH (acidity), volume and concentration of the urine.

More than 80% of uroliths in both dogs and cats are composed of magnesium ammonium phosphate (struvite) or calcium oxalate. However, a major difference between struvite urolithiasis in cats and dogs is that most struvite uroliths in cats are not associated with urinary tract infection (sterile struvite), while urinary tract infection is common in dogs with struvite urolithiasis. Other, less frequently seen mineral composites include ammonium urate, xanthine, cystine, calcium phosphate, silica, and dried, solidified blood (cats).²

Struvite Urolithiasis

In dogs, struvite urolithiasis is often associated with urinary tract infection. Bacteria create an increasingly alkaline environment and conditions which are ideal for the formation of struvite. Where infection is present, prolonged antibiotic treatment is necessary in addition to dietary and other measures. Bacteria commonly found to cause urinary tract infection are Escherichia coli, Staphylococcus species, Klebsiella pneumoniae, Pseudomonas aeruginosa and Proteus mirabilis.³

Treatment of infection-induced or sterile struvite urolithiasis involves either surgery to remove the uroliths or medical therapy. Medical therapy of infection-induced struvite typically involves the use of a calculolytic diet (such as Hill's Prescription Diet; Hill's Pet Nutrition, Topeka, Kansas, USA) and appropriate antimicrobial administration for 4 weeks after apparent urolith dissolution). A typical calculolytic diet acidifies the urine (increases struvite solubility), has decreased substrate for urease-producing bacteria (low protein), and decreased levels of magnesium and phosphorus.⁴

Calcium Oxalate Urolithiasis

It is not possible to dissolve calcium oxalate through diet and other means, and surgery is currently the only method of removing them in dogs. However, dietary changes can help to prevent recurrence of the condition. The goal is to reduce urinary saturation with calcium oxalate. These diets should promote increased urine volume, preferably through the addition of water to the food.

Cystine Urolithiasis

Cystine

Cystine occurs in dogs with an inherited defect in cystine (a component of many proteins) metabolism., resulting in cystinuria (presence of cystine in the urine). This leads to cystine urolith formation, since cystine is relatively insoluble, particularly in acidic urine. Dissolution and prevention of recurrence of cystine uroliths can be achieved through:

• Increased water intake to increase urine volume
• Reduction of dietary cystine to reduce cystine excretion
• Alkalinization of urine (with bicarbonate or citrate) to increase cystine solubility
• Administration of compounds such as D-penicillamine or 2-mercaptopropionylglycine (2-MPG), which convert cystine to a more soluble compound.

Ammonium Urate Urolithiasis

Ammonium urate uroliths occur mostly in Dalmatians and in dogs with portosystemic shunts when conversion of uric acid to allantoin in the liver is impaired. Increased urinary excretion of uric acid may predispose the dog to urolith formation.

Surgical relief of obstruction may be required in some cases. In others, dissolution and prevention of recurrence of uroliths can be achieved through:

• Restriction of dietary protein
• Supplementation with potassium citrate to promote neutral or slightly alkaline urine
• Increased water intake
• Administration of allopurinol which decreases uric acid production.

Identification of the mineral composition of uroliths is important because dietary treatment or management must be directed toward the specific type of urolith present.

References

1. D.R. Lane. Veterinary Nursing
2. Linda P. Case MS, Leighann Daristotle DVM, PhD, Michael G. Hayek PhD, Melody Foess Raasch DVM. Dietary Management of Urolithiasis in Cats and Dogs (Canine and Feline Nutrition Third Edition)
3. Gatoria IS, Saini NS, Rai TS, Dwivedi PN. Comparison of three techniques for the diagnosis of urinary tract infections in dogs with urolithiasis.
4. Nancy E. Rinkardt and Doreen M. Houston. Dissolution of infection-induced struvite bladder stones by using a noncalculolytic diet and antibiotic therapy