Dietary management of urolithiasis (Proceedings)

Struvite and calcium oxalate are by far the most common types of uroliths in cats and dogs. Less commonly seen are urate and cysteine, although the prevalence within specific breeds may be high. The overreaching goals of nutritional management of urolithiasis, regardless of stone type, are to decrease urine specific gravity, decrease dietary stone precursors, produce appropriate urine pH and increase the concentration of stone inhibitors. These strategies decrease the risk of stone formation and may allow for dissolution, depending on stone type. When in doubt, urine dilution is likely the most important factor.

Struvite – Magnesium Ammonium Phosphate

Epidemiology/Risk Factors

Dogs

Struvite uroliths are the most common urolith in dogs less than one year of age. Females outnumber males. They are usually secondary to bacterial urinary tract infections (UTIs) caused by urease-producing bacteria (Staphlococcus ssp, Proteus ssp +/- Klebsiella, Pseudomonas). The urease splits urea to ammonia and carbon dioxide. The ammonia then combines with water to form ammonium ions. This process causes the urinary pH to increase and the solubility of struvite precursors to decrease. While struvite urolithiasis has been occasionally reported in sterile urine, bacteria can usually be cultured from various layers of the urolith or from the bladder mucosa even if the urine itself is sterile. Uroliths can form within 2-8 weeks after initiation of infection.

Cats

Unlike in dogs, the majority of struvite uroliths in cats are found in sterile urine. Breed predispositions have been suggested for Siamese, Persians and Himalayans

Treatment – Dietary Dissolution

Pure struvite stones can usually be dissolved using an acidifying diet (urine pH < 6.3) with reduced protein and phosphorus. Decreasing the urine specific gravity (<1.020 for dogs, 1.025 for cats) is critical to encourage dissolution and can be accomplished by increasing dietary sodium, severe protein restriction, or by increasing moisture (canned or pouched diets). High sodium diets should be avoided in animals with heart or renal disease.

All dogs and cats suspected of having infection-induced stones should be placed on appropriate antimicrobial therapy based on culture results and should remain on antimicrobials for 2-4 weeks after the stone is no longer visible on radiographs. A prescription diet designed for struvite dissolution should be used and the diet should be fed as 90-100% of daily calories during treatment to ensure efficacy. The risk of urethral obstruction increases as cystoliths become smaller and could be life-threatening. Some of the diets designed for dissolution (i.e. Hill's Prescription Diet s/d) are not appropriate for long term feeding (>3 months) and monitoring is necessary to minimize the risks of harm due to low protein intake and excessive acidification.

The urine should be cultured midway through the dissolution process and again 5-7 days after discontinuing the antimicrobial. Radiographs should be performed at least monthly.

Contraindications for Dietary Dissolution

Many animals have concurrent health problems that are contraindications for medical dissolution such as hyperlipidemia and/or a history of pancreatitis or hypertension and/or heart disease. All commercial dissolution diets are moderate to high in fat and high in sodium. The diets are also contraindicated in pregnant or growing animals and may not be ideal for clients known for poor compliance or with limited funds as several weeks to months of rechecks, radiographs, antibiotics, and a prescription diet may cost more than surgery in the long run, especially if the urolith ends up not dissolving completely because it is not 100% struvite.

Prevention Strategies for Struvite Urolithiasis (sterile or recurrent)

No specific dietary prevention is necessary in dogs with infection-induced struvite – measures to prevent/detect future UTIs in a timely manner should be instituted. For cats, suspected canine sterile stone formers or when infection cannot be completely eradicated, prevention strategies are similar to those of dissolution although it is critical to select a diet that is designed to be fed long-term. While dietary magnesium has been implicated in the etiology of struvite urolithiasis, more recent studies suggest that urine pH is much more critical than magnesium level (Buffington and Chew 1998). There are many prescription diets for both dogs and cats formulated to help prevent struvite urolithiasis; however, canned acidifying OTC diets are available for cats and may be helpful, especially if owner finances are tight.

Calcium Oxalate Urolithiasis

Calcium oxalate uroliths form when the urine is supersaturated with calcium and oxalate. Urinary calcium excretion is the product of intake, bone turnover, genetics and other factors and can be affected by drug therapy and various disease processes. Oxalate can be obtained from the diet or manufactured endogenously through the metabolism of ascorbic acid (vitamin C), glycine, glyoxylate and tryptophan. Calcium oxalate is the most common type of nephrolith in dogs and cats (and humans).

Epidemiology/Risk factors

Dogs

There are numerous risk factors that have been suggested for calcium oxalate urolithiasis including breed (Schnauzer, Yorkie, Lhasa, Shih Tzu, Bichon, Toy and Miniature Poodle), dietary oxalate concentration, obesity, hypercalcemia, hyperadrenocorticism, hyperoxaluria, steroid use, and diuretic use. Compared to dogs with struvite urolithiasis, dogs with calcium oxalate urolithiasis are older – 8-12 years - and are more likely to be male (Lekcharoensuk, Lulich et al. 2000).

Cats

Cats with calcium oxalate urolithiasis are usually middle-aged (mean 7.8 years, range 7-10 years) and males are affected more than females. Acidifying diets, high dietary oxalate concentration, hyperthyroidism, hyperparathyroidism and hypercalcemia can increase risk.

Treatment

Calcium oxalate uroliths will not dissolve, unlike struvite, so mechanical removal is necessary (urohydropulsion, cystotomy, lithotripsy). Recurrence is common (up to 50% within 3 years) and dietary therapy is generally one of the main methods of prevention if an underlying medical condition (i.e. hypercalcemia) cannot be identified and addressed.

Nutritional Prevention Strategies for Calcium Oxalate

     • Reduce urine specific gravity

     • Reduce dietary oxalate – Oxalate is commonly found in plant ingredients. Common ingredients high in oxalate include soy, spinach, beets, whole wheat, and potatoes (see www.ohf.org for a more complete list of oxalate containing foods).

     • Ensure adequate calcium intake –High dietary calcium levels can result in increased calcium excretion and could encourage stone formation. Low dietary calcium can actually increase oxalate absorption in the small intestine; calcium will bind oxalate in the intestine and produce an insoluble product that is then excreted in the feces.

     • Avoid acidifying diets – Oxalate stones can form at a wide range of pH and will not dissolve regardless of pH although crystals are most soluble in alkaline urine. Acidifying diets can increase the risk of calcium oxalate by inducing metabolic acidosis and subsequent mobilization of calcium and phosphorus from bone. Acidification also contributes to a decrease in citrate excretion, which inhibits crystallization of calcium oxalate in the urine by favoring the formation of calcium citrate. Overall, however, urine pH is much less important than it is for struvite.

     • Avoid vitamin C supplementation – Ascorbic acid is metabolized to oxalic acid before excretion.

     • Avoid high protein diets and rawhides – High protein diets (greater than 30% protein calories) may cause increased oxalate excretion due to the metabolism of excess glycine and tryptophan. High protein diets also tend to be acidifying. Rawhides and "animal part" treats contribute little nutritionally, but are a source of extra glycine that is then metabolized to oxalate.

Only one diet is appropriate for long-term feeding for calcium oxalate prevention in dogs – Royal Canin Veterinary Diet Urinary SO canine canned and dry. There are multiple veterinary therapeutic diet options for cats; most of these diets are formulated for prevention of both struvite and calcium oxalate.

Urate

The two main causes for urate urolithiasis are reduced activity of uricase, the enzyme responsible for converting uric acid to allatoin for excretion (i.e. Dalmations, Bulldogs, Jack Russell Terriers), and liver disease (e.g. portosystemic shunt) resulting in hyperammonemia and hyperuricemia and stone formation. In the case of the latter, managing the underlying disease process is the best way to prevent stone recurrence. Urate stones in cats are usually related to liver disease. For dogs with decreased uricase activity, diet is very important to prevent recurrence.

Uric acid and allatoin are the end products of the degradation of purines. Two common purines are adenosine and guanine, two nucleotides found in DNA. The cornerstone of management of urate stones are reducing dietary purines and decreasing urine specific gravity. Alkalinization of urine may also be beneficial.

Because nucleotides are part of DNA, the goal is to avoid metabolically active tissues that are likely to have more nuclei and thus more DNA. Dairy, egg and plant proteins tend to be lower in purines than meat proteins (especially organ meats) and are ideal sources of proteins for animals with a history of urate crystals or urolithiasis. Diets lower in protein tend to have less purines, but this is not a firm rule and protein source is likely more important than gross amount. Diets with higher amounts of plant ingredients have the added benefit of causing urine alkalinization.

Diets to consider for dogs include Royal Canin Veterinary Diet Urinary UC, Hill's Prescription Diet d/d egg and rice, Royal Canin Vegetarian canned and dry, and Purina Veterinary Diet Hypoallergenic HA. Hill's Prescription Diet u/d canned and dry cannot be recommended due to the very low protein and mineral concentrations. Low protein diets made for liver (Hill's Prescription Diet l/d canned and dry) or kidney disease (many diets) are generally used for urate stone prevention in cats. Pure urate stones can sometimes be dissolved utilizing a combination of diet and medication (i.e. allopurinol), but surgical removal is more common and effective.

Cystine urolithiasis

Cystine uroliths can occur in dog and occasionally cats that have a defect in the proximal renal tubules that prevent reabsorption of cystine and often ornithine, lysine and arginine as well. As cystine is the least soluble, it precipitates out forming uroliths. These tubular defects are usually inherited and are seen most commonly in English Bulldogs, Mastiffs, Dachshunds and Newfoundlands, but have been described in other breeds.

Prevention

Nutritional strategies for prevention of cystine uroliths include decreasing urine concentration, minimizing dietary methionine and cysteine, and alkalinizing urine. As cysteine and methionine are essential amino acids, care must be taken to make sure that deficiencies are not created. As plant-based proteins tend to be limiting in sulfur containing amino acids, vegetarian or mostly vegetarian diets may be good options. However, the quality of commercially available vegetarian diets (particularly OTC diets) is extremely variable and often questionable. Diets to consider for dogs include Royal Canin Veterinary Diets Vegetarian dry and canned, Royal Canin Veterinary Diets Urinary UC, and Purina Veterinary Diets Hypoallergenic HA as well as Hill's Prescription Diet l/d canned and dry. Many companies recommend the use of reduced protein renal diets for cystine urolithiasis in dogs; however, these diets have additional modifications (i.e. mineral alterations) in addition to much decreased protein, making them considerably less appealing as life-long diets. For cats, Royal Canin Veterinary Diets Hypoallergenic HP feline, a hydrolyzed soy diet, can be used but otherwise the options are limited to diets formulated for renal disease with all the caveats listed for dogs. As dogs utilize methionine and cysteine to make taurine, taurine supplementation is recommended for dogs with cystinuria. Cats are dependent on dietary taurine already, so this is less of a concern. Some dogs may also lose carnitine in their urine and carnitine supplementation can also be considered.

For dogs that cannot be managed on diet alone, drug therapy (2-MPG and occasionally D-penicillamine) is commonly used to dissolve or prevent further formation of cystine uroliths.

References

Lekcharoensuk, C., J. P. Lulich, et al. (2000). "Patient and environmental factors associated with calcium oxalate urolithiasis in dogs." J Am Vet Med Assoc 217(4): 515-519.

Stevenson, A., Rutgers, C. Nutritional management of canine urolithiasis. Encyclopedia of Canine Clinical Nutrition. Anisa SAS:EU, 2006:284-315.

Houston, DM, Elliott, DA. Nutritional management of feline lower urinary tract disorders. Encyclopedia of Feline Clinical Nutrition. Anisa SAS:EU, 2008:286-317