Page 749 - Small Animal Internal Medicine, 6th Edition

CHAPTER 43 Canine and Feline Urolithiasis 721

xanthine stone formation (particularly if the animal is not is often recommended, such as those diets marketed for
being fed a purine-restricted diet), gastrointestinal upset, kidney disease (e.g., Hill’s k/d; Royal Canin Renal LP; Purina
VetBooks.ir and hepatotoxicity. NF Kidney Function, Purina, Vevey, Switzerland). Anecdot-
ally, a commercially available hydrolyzed soy protein diet has
If urate-containing calculi are found in non–
hyperuricosuric breeds, a search for an underlying portovas-
protein intake with low purine content (Royal Canin Feline
cular anomaly, such as a portosystemic shunt (PSS), should been fed to these cats in hopes of providing an adequate
be pursued. Urate uroliths have also rarely been reported Hypoallergenic Hydrolyzed Adult HP). No evidence of the
from dogs with portal venous hypoplasia (PVH; also referred effectiveness of any of these approaches has been evaluated
to as microvascular dysplasia). Dogs with underlying liver in controlled trials in cats with urate urolithiasis. As with all
disorders are likely predisposed to urate urolith formation cases of urolithiasis, feeding a high-moisture diet is the
because of hyperammonuria and hyperuricosuria, which cornerstone of urolith prevention management. Periodic
result from the reduced ability to convert ammonia to urea imaging using ultrasonography—because urate can be dif-
and uric acid to allantoin. Correction of the vascular dis- ficult to see on plain radiography—is also important to
order should be addressed, if possible, to help prevent urate monitor for recurrence.
recurrence. In patients with inoperable PSS or PVH, diets
marketed for liver disease may be used to help decrease CALCIUM PHOSPHATE CALCULI IN CATS
the urinary ammonium urate and help control any signs of AND DOGS
hepatic encephalopathy. If a portovascular anomaly cannot Apatite (calcium hydroxyl phosphate)-containing calculi in
be found, DNA testing for the hyperuricosuric genetic dogs is usually found as a so-called “tag-along mineral”
abnormality should be considered. because it usually is associated with struvite uroliths, either
mixed throughout or as single layers. Apatite-containing
URATE UROLITHIASIS IN CATS uroliths are not very common in cats but can be seen in
association with struvite, CaOx, or by themselves. Unlike
Etiology brushite, apatite is generally less soluble in alkaline urine.
After CaOx- and struvite-containing uroliths, urate uroliths Brushite (calcium hydrogen phosphate dihydrate) is another
are the third most common mineral type submitted to most form of calcium phosphate that is much less common in
stone laboratories from cats. There is no gender predilection dogs and very rare in cats. Brushite tends to occur as mul-
for cats with urate urolithiasis; recurrence rates are variable. tiple small uroliths. Prevention measures for these uroliths
Specific breeds such as the Egyptian Mau and Siamese have in cats and dogs are unknown. Similar to CaOx manage-
been reported to be at higher risk. The pathophysiology of ment, a search for predisposing disorders that could result
feline urate urolithiasis is largely unknown. Unlike dogs, in hypercalcemia is recommended. Often it is recommended
genetic studies have not been published, and it is thought that the dog or cat be evaluated and managed similarly to
that most cats do not have underlying hepatic dysfunction those patients with CaOx uroliths. A high moisture diet that
such as a PSS. However, in one study, most cats seen by produces a neutral urinary pH is advised.
primary care veterinarians with urate urolithiasis did not
have advanced diagnostic studies to investigate the presence CYSTINE AND SILICA UROLITHIASIS IN
of a PSS fully, but a normal physical examination, clinico- CATS AND DOGS
pathologic findings, and no clinical signs (e.g., ptyalism,
neurologic signs, lethargy) suggestive for a hepatopathy were Etiology and Management
present. Cystine- and silica-containing uroliths are uncommon in
dogs and very rare in cats. They represented 1.3% and 6.6%,
Urate Urolithiasis Management respectively, of all the canine uroliths we analyzed. Cystine
In any cat that has been diagnosed with urate urolithiasis, a urolithiasis is much more common in dogs compared with
full complete blood count (CBC) and serum biochemical cats. Cystinuria in dogs results from a mutation in 1 of 2
profile are warranted. If clinical signs and/or clinicopatho- genes, SLC3A1 (type I-A; autosomal recessive inheritance,
logic abnormalities (e.g., microcytosis, high liver enzyme and II-A; autosomal dominant inheritance) and SLC7A9
activities, abnormal liver function parameters) are suggestive (type II-B; autosomal dominant inheritance), which encode
for a hepatopathy or vascular anomaly, further diagnostic subunits required for the dibasic amino acid transporter
studies such as abdominal ultrasonography and measuring system that enables cystine reabsorption form the glomeru-
serum bile acid concentrations are recommended. If urate lar filtrate. Furthermore, an androgen dependent type III
uroliths are diagnosed in a cat with no history, clinical signs, cystinuria has been reported in several breeds of intact male
and clinicopathologic abnormalities suggestive for a hepa- dogs. Cystine urolithiasis has also been reported more com-
topathy, the costs of further tests can be discussed with the monly in intact male dogs. Recently, a mutation in the
owner, because not all cats will require further evaluation. In SLC3A1 gene has also been reported in the cat.
older cats, further testing may not be necessary. Dietary management includes feeding a diet high in
To prevent recurrence of urate uroliths in otherwise moisture and low in protein (e.g., Hill’s Prescription Diet u/
®
®
healthy cats, a high moisture diet that is restricted in protein d , Royal Canin Veterinary Diet Urinary U/C™); or feeding