Communicating With Clients About Nutritional Management of Canine Uroliths

The presence of a stone (urolith) anywhere in the urinary tract is known as urolithiasis. The terms “stone” and “urolith” are used interchangeably in this article. Urolithiasis is the development and resulting effects of mineral deposits precipitating in the urinary tract. The type of stone can be narrowed down according to its location in the urinary tract (see GLOSSARY).¹ Rather than being a single disease with 1 cause, urolithiasis stems from various underlying conditions. In this sense, urolithiasis is considered a syndrome caused by different pathophysiologic factors—familial, congenital, or acquired—that make mineral precipitation in the urinary tract more likely, leading to stone formation.

Of the various causes of lower urinary tract disease, urolithiasis accounts for 15% to 45% of cases.² Recent studies demonstrate that urolithiasis affects approximately 33% of canine patients.^3-5 The rate of urolithiasis recurrence is high.^3,6

This article describes urolithiasis in general, the unique characteristics of calcium oxalate and struvite stones, and a hypothetical dialog with a client whose dog had calcium oxalate stones to help the client understand the condition and the value of modifying the dog’s diet to help prevent future stones.

Urolithiasis Overview

Pathophysiology

Various minerals may precipitate within the urinary tract, resulting in stone formation. However, most (approximately 70%) uroliths in dogs are composed of either struvite (magnesium ammonium phosphate hexahydrate) or calcium oxalate (monohydrate or dihydrate).^7,8

Uroliths form through a complex interaction of stone-forming substances, urinary pH, urine concentration, high concentration of crystalloids in urine, and reduced water consumption, as well as decreased urinary frequency associated with supersaturation of urine with salts and combined with a high dietary supply of minerals and proteins.

Stone formation begins with the precipitation of microcrystals in urine, which contain a variety of salts, minerals, and other solutes. The microcrystals can join and grow larger, eventually forming macrocrystals. Under typical physiologic conditions, inhibitory mechanisms prevent crystal formation (e.g., solute undersaturation; presence of urinary inhibitors, such as molecules that reduce calcium oxalate and calcium phosphate supersaturation; absence of a solid matrix on which macro- and microcrystals can form). Absent these factors, the process continues until crystals develop into a urolith.⁹

Clinical Signs

Clinical signs of crystals and uroliths can include hematuria, pollakiuria, stranguria, and dysuria; however, these signs are also seen in other lower urinary tract diseases (e.g., urinary tract infections), making it difficult to pinpoint the exact cause based on clinical signs alone.

Small uroliths may partially or completely obstruct the urethra, which may lead to obstructed urination resulting in bladder distention; abdominal discomfort; paradoxical urinary incontinence; stranguria; clinical manifestations of postrenal azotemia; and, in severe cases that are not treated swiftly, bladder rupture leading to uroabdomen.¹

Diagnosis

Diagnostic evaluation of dogs with recurrent or persistent lower urinary tract signs should include urinalysis, urine culture (if inflammatory cells are evident on microscopy or if struvite urolithiasis is suspected), diagnostic imaging, blood panels, and serum biochemistry to screen for underlying causes.

Treatment

Urolithiasis treatment can involve surgical and/or medical management, which can include dietary modifications aimed at dissolving struvite uroliths or preventing both struvite and calcium oxalate urolith formation.¹⁰ To ensure effective management, lower mineral concentration of urine with lithogenic crystalloids must be promoted by¹⁰:

• Increasing the urine solubility of crystalloids
• Increasing the volume of urine where crystalloids are dissolved or suspended
• Reducing the quantities of lithogenic crystalloids in urine

General Nutritional Considerations

To make crystalloids more soluble in urine frequently involves adjusting the diet to alter urine pH levels, which helps create a urinary tract environment that is less likely to promote mineral crystallization. Some nutritional considerations vary according to the type of stone being treated. More specific recommendations for each type of stone are described later.

Water Intake

Adjustments to fluid consumption can influence the prevention and or management of urinary stone formation. Keeping urine dilute helps prevent urinary struvite and calcium oxalate bladder stones (BOX 1). Urine specific gravity in dogs with uroliths, especially calcium oxalate stones, should remain dilute at 1.025 or less.¹² One way to increase the volume of urine in which crystalloids are dissolved or suspended is to increase dietary moisture, which helps reduce the amount of lithogenic crystalloids in urine. Large urine volumes resulting in more frequent urination can often remove stone-forming substances and harmful bacteria from the bladder.^1,2,12 Dilute urine also makes it harder for solutes to reach the concentration needed to form stones.

Protein Intake

For some dogs with struvite stones, low-protein diets are recommended to minimize formation of precursors. Evidence indicates that moderate- to high-protein diets may also be effective if appropriate antimicrobial therapy is administered. For dogs with calcium oxalate stones, foods with elevated protein levels may contribute to calcium oxalate uroliths due to increased calcium excretion and decreased urine citrate excretion. Excessive protein in the food should be avoided for dogs with a history of calcium oxalate uroliths.¹⁵

Urine pH

The ideal diet depends on the type of stone being treated and, thus, the ideal urine pH.

Treats

In addition to a diet formulated to satisfy the nutritional requirements, treats should be limited to less than 10% of calorie intake. Careful consideration of treat ingredients is also necessary to avoid conflict with the overall dietary strategy.

Calcium Oxalate Stones

The formation of calcium oxalate stones involves genetic, dietary, and environmental factors, although the precise mechanism remains unclear.1 Systemic hypercalcemia increases the risk for calcium oxalate stone formation, which often results in hypercalciuria.¹ A proposed risk factor is oxalate metabolism; hyperoxaluria can be secondary to increased dietary oxalate intake or decreased oxalate-degrading bacteria (Oxalobacter formigenes) in the colon.¹

Because magnesium, phosphate, and citrate help prevent calcium oxalate stones from forming, when their concentrations drop in urine, calcium oxalate stones are more likely to develop. However, findings that calcium and oxalate levels are similar between healthy dogs and most dogs that get calcium oxalate stones indicate that there are other causes of stone formation.^1,16

A total of 47% of bladder stones in dogs are calcium oxalate (FIGURE 1).⁸ Calcium oxalate urolith risk is higher in middle-aged, neutered male dogs.⁸ Breeds that are predisposed to developing calcium oxalate stones include miniature Schnauzers, bichon frises, cairn terriers, Chihuahuas, standard poodles, Pomeranians, Japanese chins, Brussels griffons, Jack Russell terriers, miniature pinschers, Lhasa apsos, Yorkshire terriers, and Maltese.^8,17 Veterinary team members can educate clients who own dogs of these breeds about the potential for calcium oxalate stone occurrence and recurrence, despite the time between stone formation varying significantly according to the individual patient, diet provided, and breed of dog.

Figure 1. Microscopic appearance of calcium oxalate uroliths in urine sample. Sittichai Kammayeengoen/shutterstock.

Treatment and Prevention

Calcium oxalate stones cannot be dissolved and so must be surgically removed before management can focus on preventing recurrence.^1,8 In many dogs, calcium oxalate stones recur within 3 years.^1,8 Because calcium oxalate stones cannot be dissolved, the focus of dietary management is preventing stone formation.

Water

Increased water intake is useful for helping prevent stone recurrence. Feeding a canned formulation or adding water to dry foods helps increase dogs’ water intake and aids in diluting urine. Dilute urine reduces the concentration of stone-forming substances, such as calcium, oxalate, and uric acid, which makes stone formation much less likely^1,15; achieving supersaturation of solutes is harder when urine is dilute. Furthermore, producing a larger urine volume leads to more frequent urination, which helps expel microcrystals and minerals from the lower urinary tract before they can grow into larger, problematic stones.^1,2,12

Urine pH

Urine pH does not significantly affect the solubility of calcium oxalate. However, because acidosis can lead to increased calcium concentrations within the urine, the recommended target urine pH is from 6.5 to 7.^1,18 Another preventive option is incorporation of potassium citrate into suitable diets to increase urine pH, which is believed to increase urine citrate excretion.1 However, urine pH greater than 7.5 may promote formation of calcium phosphate uroliths and should be avoided.

Metabolic acidosis leads to increased calcium in urine because bone releases calcium during buffering (alkalinizing). Highly acidic urine (pH < 6) has been shown to promote calcium oxalate crystallization and adhesion in canine kidney cells and improve the likelihood of calcium oxalate stone formation in the bladder.¹² Thus, foods that promote formation of acidic urine (unbalanced diets and diets high in animal proteins) should be avoided.¹⁹

Protein

Foods with elevated protein levels may contribute to calcium oxalate uroliths due to increased calcium excretion and decreased urine citrate excretion. Hypercalciuria can occur in healthy dogs fed high-protein foods. Excessive protein should be avoided in food for dogs with a history of calcium oxalate uroliths. In addition, any protein fed should be highly digestible.¹⁵

Calcium and Oxalic Acid

Reduction of calcium and oxalate in foods and treats being fed is necessary for the management of calcium oxalate crystals.

Sodium

Sodium chloride can increase thirst and urine volume. However, excess sodium also increases urine calcium excretion and therefore should be considered a risk factor for calcium oxalate urolithiasis. High sodium levels in food may also contribute to hypertension, especially in dogs that have a sensitivity to salt.

Vitamin C

Acidifying urine is not recommended for dogs with calcium oxalate uroliths; thus, providing supplemental ascorbic acid is not recommended, especially given that ascorbic acid is also a precursor of oxalates. To avoid increasing the amount of oxalate and calcium in the urine, supplementation of vitamins C and D is not advised.^1,2,12 Avoiding vitamin C supplementation is recommended because vitamin C is converted to oxalate. Avoiding vitamin D supplementation is recommended because vitamin D may increase gastrointestinal absorption of calcium, thereby increasing urine concentrations of oxalate and calcium.^1,18

In vitro, Lactobacillus species have been shown to degrade oxalate; however, whether these supplements help prevent calcium oxalate uroliths remains unclear as their effectiveness against calcium oxalate stones has not been tested in vivo.²⁰

Therapeutic Diet Options

Veterinary teams should recommend a high-moisture diet with moderate levels of calcium and low-oxalate ingredients (TABLE 1); appropriate treats may include zucchini, broccoli, and cucumbers.^1,2,12 If a high-moisture diet cannot be fed, the veterinary team can consider a dry diet with increased sodium to help with urine dilution. Choosing diets labeled with low relative supersaturation for calcium oxalate is recommended.¹²

Struvite Stones

Struvite stones are composed of magnesium, ammonium, and phosphate minerals. Among dogs, struvite stones are commonly cystoliths; 44% of stones analyzed are struvite (FIGURE 2).^8,12 Struvite stones are more common in young female dogs, consistently associated with urinary tract infections caused by urease-producing organisms (e.g., Staphylococcus, Pseudomonas, Klebsiella).^1,21

Figure 2. Microscopic appearance of struvite uroliths in urine sample. Pee Paew/shutterstock.

Treatment and Prevention

Unlike calcium oxalate stones, struvite stones can be dissolved. The 2 objectives of medical management of struvite urolithiasis are to 1) stop further growth of the macrocrystals that lead to uroliths and/or to prevent crystal reformation after dissolution or surgical intervention and 2) promote urolith dissolution and reformation by correcting or controlling underlying abnormalities. Maximum struvite solubility is achieved when urinary pH is maintained below 6.⁴

Antimicrobial Therapy

Medical management for dogs with struvite stones includes antibiotic administration and provision of a calculolytic diet. Veterinary teams should perform abdominal radiography and urinalyses routinely during dissolution to ensure that the stone(s) is/are dissolving. Antibiotics should be chosen according to urine culture and susceptibility results and are typically prescribed for 2 to 4 weeks beyond radiographic/ultrasonographic resolution of the stone.^1,2

Water

It is prudent to increase water when attempting to dissolve and/or manage struvite uroliths. Choosing a high-moisture diet such as canned or stew formulations is recommended. Dry foods can be fed with water added to increase moisture content. Increased water intake is aimed at achieving a specific gravity of 1.020 or less. In addition, increased water intake leads to increased urination, thus reducing crystal retention time and minimizing crystal growth. Adequate water intake may be the most important factor for preventing urolith recurrence.²²

Urine pH

Struvite stone formation occurs in urine with an alkaline pH. The alkaline urine allows phosphate ions to exist in a trivalent state, in which they more readily form crystals.^12.23 Maintaining urine pH below 6.4 is thought to facilitate dissolution of infection-associated struvite stones.¹² However, further research should evaluate the clinical significance of urinary acidification in canine patients.

Protein

The influence of dietary protein on dissolving struvite stones in dogs with infection-induced urolithiasis remains unclear. Some dissolution diets are low in protein to minimize precursors such as urea, ammonium, and phosphorus in addition to promoting low urine specific gravity via medullary washout^12,24; however, low-protein diets are not recommended for dogs with certain conditions (e.g., cachexia, sarcopenia, poor muscle condition score). Although low-protein diets may be effective, evidence indicates that struvite stones may also be dissolved by feeding moderate- to high-protein diets, provided that appropriate antimicrobial therapy is administered, which may play a more critical role than protein restriction.

Magnesium

Struvite crystals are likely to form when there is excess magnesium in the urine. Avoiding excess dietary magnesium can decrease the concentration of magnesium in the urine.

Therapeutic Diet Options

Along with antimicrobial therapy, dietary strategies should always be used (TABLE 1).²⁵ Because it is common for dogs to have urinary tract infections that facilitate formation of struvite stones, nutritional management is aimed at dissolving the crystals. Choosing a high-moisture diet with controlled levels of magnesium, phosphorus, and moderate amounts of protein, along with appropriate antimicrobial therapy, can aid dissolution.^1,12

Studies demonstrate that a stone composed of only struvite has a mean dissolution time of 31 to 35 days when treated with diet and antimicrobials.²¹ Stones that do not dissolve within 8 weeks despite appropriate medical therapy are more likely to be of mixed composition, although some struvite stones may take up to 4 months to fully dissolve.^12,24,25 After dissolution of an infection-induced struvite stone, prevention should focus on early detection and management of future urinary tract infections. Diets formulated to manage struvite urolithiasis will not prevent recurrence in dogs with a urease-producing urinary tract infection.^11,12 However, those diets may delay or decrease urolith formation in the presence of an undiagnosed urinary tract infection.

Client Communication

Nutritional management education is the responsibility of the veterinary team. Discussing nutritional management with clients ensures their understanding of how uroliths form and the role nutrition plays in preventing their development or recurrence. Veterinary team members should take a medical history from the client to determine what clinical signs the dog is exhibiting. Clinical signs, along with urinalysis and identification of crystals, help formulate a diagnosis.

The following scenario is a hypothetical conversation between a veterinary nurse and a client to help the client understand their pet’s condition, surgical treatment, and follow-up expectations and care to prevent recurrence.

Communication Scenario

Mr. Jones presented Hank, a 5-year-old neutered miniature pincher, to the clinic after noting that Hank was asking to go out to urinate much more often but was not producing much urine during those trips.

Diagnostics pointed to calcium oxalate uroliths, and after surgical removal of the uroliths, the veterinary nurse discussed nutritional management with Mr. Jones to help prevent stone recurrence in his dog.

Veterinary nurse: Hi, Mr. Jones. Hank is doing well after his surgery to remove the calcium oxalate stones. I would like to discuss the value of modifying Hank’s diet after his diagnosis and surgery to prevent future stones. Would that be all right with you?

Mr. Jones: Yes, what modifications do you recommend?

Veterinary nurse: We recommend transitioning Hank’s diet to a therapeutic diet formulated to help prevent recurrence of calcium oxalate crystals because calcium oxalate stones may recur within 3 years in about 50% of dogs. We want to begin with a canned formula because increased water intake helps prevent recurrence. We also want the target pH of Hank’s urine to be between 6.5 and 7, which we will check at follow-up visits. How does this sound so far?

Mr. Jones: Well, Hank has always eaten a dry food, so I am not sure he will eat canned food.

Veterinary nurse: Thank you for reminding me that Hank has always eaten dry food. I understand your hesitation to begin feeding a new type of food. Hank has had surgery, and we want to be sure he eats well! How would you feel about adding water to the dry therapeutic food we are recommending? That would help increase his water intake and aid in diluting his urine, which would decrease the amount of substances in his urine that can lead to stone formation and increase how often he urinates, thus helping remove crystals from the bladder. Would you be open to adding water to his dry food?

Mr. Jones: Yes, that sounds better for Hank. Can I still give him carrots and green beans? He loves those as treats.

Veterinary nurse: Great. Thank you. Regarding treats, I understand how important it is for us to give treats to our pets. Unfortunately, carrots and green beans are not recommended because they contain oxalate, which is one of the building blocks for Hank’s type of stone. What would you think of offering either his kibble as a treat or low-oxalate veggies such as zucchini or even a small amount of applesauce? We cannot give more than 10% of Hank’s daily energy requirement as treats, but I can calculate how much to feed each day and ensure that part of the calculation includes low-oxalate treats. What are your thoughts on this plan?

Mr. Jones: That sounds like a great idea. I love giving Hank treats. Thank you!

Veterinary nurse: Wonderful! I will write down the recommended food and the amount to feed Hank, along with a measuring cup so we are aligned on how much Hank should receive. I will also write down the amount of zucchini or applesauce that you can give. Remember, we cannot stray from the type or the amount of treats because we are working to prevent the crystals from recurring, and we also do not want Hank to gain any weight.

I will give you my direct number here at the hospital should you have any questions, and I will put our plan in Hank’s medical record in case another team member talks with you.

Do you have any questions for me?

Mr. Jones: No, that sounds like a good plan to me.

Veterinary nurse: Great! I will follow up tomorrow to see how Hank is doing and if you have any questions about his recovery and nutrition plan.

Summary

Increasing water intake and adapting diet play a valuable role in the management of struvite and calcium oxalate stones in dogs. Discussing with clients the type of stone affecting their dog and how diet helps manage and/or prevent future stones increases their understanding and compliance.