Urinary Tract Stones

Lower urinary tract stones are common in cats. In addition to struvite stones, calcium oxalate, urate and cystine stones are all found in the bladder and the urethra. The urethra of the male cat tapers and is particularly narrow within the penis, so urethral obstruction due to stone formation is possible. The male dog has an os penis, a bone within the penis that has a groove along its length in which the urethra sits. This means that the urethra narrows abruptly where it enters the os penis and stones frequently become lodged at this site

Struvite stones can be medically dissoluted. Dissolution is aided by dietary therapy. Diets for the dissolution of struvite calculi are designed to result in urinary acidification, production of a large volume of dilute urine, and a low concentration of constituent crystalloids. This is typically achieved by feeding prescription diets that are specially formulated for this purpose; these contain calcium sulphate and dl-methionine to achieve a target pH of ≈6.0, low concentrations of magnesium and phosphate to reduce their concentration in the urine, and a low protein content which results in a reduction in the concentration of urea. The low urea level, in addition to reducing the substrate for the urease enzymes, results in the formation of more dilute urine.



The main symptom of urolithiasis is blood in urine and difficulty in passing urine, while the most frequently found symptoms are dullness, fever, or swollen abbdomen. The cat may have increased thirst, loss of appetite, vomiting occasionally. Other possible signs of urolithiasis may include acute pain, scratching, licking, difficulty in breathing, pale lips and gums, and stiff or unsteady gait.1

In addition to developing struvite stones, male cats are also vulnerable to the development of urethral plugs which cause acute urethral obstruction. Urethral plugs are predominantly composed of a proteinaceous matrix often with embedded struvite crystals. Plugs tend to form in male cats with signs of feline lower urinary tract disease of unknown cause (females do not become obstructed). Urethral plugs cause acute urethral obstruction with the life-threatening consequences. Death occurs rapidly if the obstruction is not relieved.

Kidney stones mainly affect old and male cats. Persians and Himalayans are predisposed to calcium oxalate stones. Many of the diets recommended for preventing calcium oxalate stones are relatively protein-restricted, but the benefit to this is uncertain. These diets have typically been supplemented with potassium citrate with the aim of producing a diet that results in urine being produced with a pH of 6.5–7.5. Treatment with hydrochlorothiazide has also been suggested as a means to reduce the risk of calcium oxalate recurrence in cats, although there are no studies evaluating the effectiveness in clinical patients.

Recent data indicate that a high-protein diet cannot be considered as beneficial for the prevention of calcium oxalate stones in cats, as previously assumed. Although a higher urinary volume was also associated with a higher protein intake, the increased urinary calcium concentrations, kidney calcium and oxalate excretion and urinary calcium oxalate values are potential risk factors for the formation of calcium oxalate stones.

References

  1. Stones in cats and dogs: What can be learnt from them? Harriet M. Syme
  2. Relevance of dietary protein concentration and quality as risk factors for the formation of calcium oxalate stones in cats. Nadine Paßlack et al.
  3. The Encyclopedia of the Cat, Angela Sayer





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