During the past three decades, a tremendous amount of information has been generated regarding the etiology, detection, treatment, and prevention of canine urolithiasis. No longer is surgical removal the only option available when dogs develop urolithiasis, nor is surgical removal the "treatment" of choice" in all patients. Although we know a lot more information about urolithiasis in dogs than we did three decades ago, there is still a lot that we don't know and remains to be discovered. Nonetheless, our ability to medically manage this disease in dogs has dramatically improved since 1973, and new knowledge continues to be generated. The purpose of the lecture is to provide an overview and an update on therapeutic options available for the four most common mineral types of uroliths in dogs.
Distribution of mineral types of canine urolths:
In 2003, the distribution of canine uroliths (n = 28,629) submitted to the Minnesota Urolith Center (courtesy of Dr. Carl Osborne and The Minnesota Urolith Center) were as follows:
- purines (ammonium urate, sodium urate, uric acid xanthine) = 6%
Since 1981, the prevalence of calcium oxalate in dogs has continued to increase, and it is equal to that of struvite now. Successful long-term management of urolithiasis is dependent upon an understanding of each mineral type.
Struvite (magnesium ammonium phosphate) urolithiasis
- The majority of struvite uroliths in dogs are infection-induced.
- Urinary tract infections with urease-producing bacteria, such as staphlococci or proteus, result in urine becoming supersaturated with ammonium ions by the following reaction:
- When urine becomes supersaturated with ammonium ions (NH4+ ), these NH4+ can combine with magnesium and phosphate already present in urine, resulting in the formation of Magnesium Ammonium Phosphate uroliths.
- Therefore, successful management and dissolution of infection-induced struvite urolithiasis in dogs is dependent upon appropriate treatment of the UTI along with dietary intervention.
Medical dissolution protocol
o Successful dissolution of infection-induced struvite uroliths is dependent upon eradicating the UTI that caused it to occur in the first place.
- Appropriate antibiotic therapy, based on urine culture and sensitivity results, is a critical component of medical dissolution.
- A urine culture should be obtained pre-antibiotic treatment
- Once dogs are receiving the appropriate antibiotic, the urine should be re-cultured 5 to 7 days later to ensure that the urine is sterile.
- If the urine is not sterile by this time (5 to 7 days), you have a treatment failure (owner
- Noncompliance, inappropriate dose, dog spitting antibiotic out, etc) and you need to re-evaluate your therapy.
- Keep in mind that a urinalysis is not a sensitive way to evaluate whether the urine is
- Sterile or not; therefore, a urine culture is necessary for monitoring a UTI.
o If the urine is sterile after 5 to 7 days of antibiotic therapy, continue administering antibiotics during the entire dissolution period.
- Bacteria are often seeded throughout the entire stone, and therefore as you dissolve layers of the stone, you continually release bacteria into the urine.
o The main diet used for dissolution of struvite uroliths in dogs is Hill's Prescription Diet s/d, although Waltham S/O Lower Urinary Tract Support can also be use.
o Some very important points to keep in mind about Hill's Prescription Diet s/d are:
- s/d is very high in fat, and therefore it is contraindicated in any dogs with a history of pancreatitis or hyperlipidemia
- Although the development of pancreatitis in dogs consuming s/d is relatively uncommon, it can nonetheless occur.
- Dogs should be gradually transitioned from their current diet to s/d over the course of 7 to 10 days.
- If at any time the dog starts to vomit, discontinue s/d and monitor for pancreatitis
- s/d is very low in protein, and is not meant to be used long-term (>6 months)
- It is contraindicated to supplement dogs with methionine that are consuming s/d.
o The larger the stone, the longer it will take to dissolve
o Many dogs become asymptomatic long before all of their stones are dissolved.
o Recommend monthly rechecks and obtain at a minimum a lateral abdominal radiograph and a urine culture
o Once stones no longer visible on radiographs, continue dissolution protocol (antibiotics and s/d) for an additional "insurance" month to ensure microscopic crystalline material not visible on radiographs dissolve.
Prevention of recurrence
- The single most important thing to do to prevent recurrence of infection-induced struvite urolithiasis is to prevent UTI and retreat promptly if it recurs.
- If a dog had infection-induced struvite urolithiasis, once stones are successfully dissolved, I usually do not recommend feeding an acidifying therapeutic diet, but rather suggest they feed whatever diet they were feeding prior to developing stones or some other maintenance diet.
o Many of the breeds that we see with increased risk for developing struvite uroliths are also breeds with increased risk for developing calcium oxalate uroliths.
- If a dog had rare sterile struvite stones, then dietary management may be necessary to prevent recurrence.
- The following mineral types are classified as purine uroliths: ammonium urate, sodium urate, calcium urate, uric acid, and xanthine.
- By far the most common mineral type is ammonium urate.
- Normally endogenous and dietary purines are metabolized to allantoin in the liver, which gets excreted in urine.
o This pathway is impaired in dogs with liver disease, especially portosystemic shunts. [the remainder of this section will focus on dogs without portosystemic shunts].
o A few breeds of dogs (ie Dalmatians and English Bulldogs) can have impaired conversion of uric acid to allantoin without any obvious liver dysfunction
- It is not the result of a deficiency of the enzyme uricase, as once thought
- One hypothesis is that the hepatic membranes in these dogs are partially impermeable to uric acid, thereby impairing its conversion to allantoin
- The proximal renal tubules in Dalmatians and English Bulldogs may reabsorb less uric acid than those from other breeds not predisposed to purine stones
Medical dissolution protocol
o Not only is the amount of protein in the diet important, but also the type of protein
o Protein sources should be low in purines
o Diet should be alkalinizing
- Hill's Prescription Diet u/d meets these criteria
- Keep in mind it is contraindicated to give this diet to dogs with pancreatitis or hyperlipidemia
o Allopurinol is a xanthine oxidase inhibitor, and it increases the amount of xanthine in urine and decreases the amount of uric acid present in urine.
- It should only be administered in non-azotemic dogs with normal liver function.
o The following tests should be evaluated every month during the dissolution protocol
- double contrast cystogram (possibly could use U/S)
- urinalysis and urine culture
o When stones no longer visible on radiographs (or U/S), continue u/d diet and allopurinol for an additional "insurance" month
Prevention of recurrence
- Recurrence rates for urate urolithiasis are as follows (Bartges J. VCNA Small Anim Pract 29:161-192,1999)
o 1 year recurrence rate is 33% to 50%.
o Greatest risk of recurrence is for dogs 1 to 6 years of age.
o Risk of recurrence (and urine uric acid excretion) decreases in dogs > 6 years of age.
- Long-term management strategies for dogs with urate urolithiasis consists of:
o Diet (protein-restricted, low in purines, alkalinizing, canned diet)
o Control UTI if present
o Frequent monitoring (q 1-2 months) during "active" phase to detect stones when they are small enough to remove through a urinary catheter or via voiding urohydropropulsion
o Use of low dose allopurinol is controversial
- Even at low doses, dogs can develop xanthine stones, and xanthine stones are a more aggressive stone type than are urate.
- Lulich (oral presentation at UC Davis Uroll Colloquim, May 2000) found the following:
- Using just canned Hill's Prescription Diet u/d resulted in recurrence rates of 40-50%
- Adding water to canned Hill's Prescription Diet u/d (1-3 cups/day) resulted in no recurrence of uroliths
- Long-term use of water instead of allopurinol is safer, cheaper and more physiologic