Urinary calculi or stones may form in any part of the equine urinary tract, but the most common site is the bladder (cystic). Calculi are formed by material dissolved in the urine (solutes) being precipitated upon a collection of bladder or other cells, such as red or white blood cells. The factors favoring this precipitation are not well understood but include urine pH; alkalinity increases the formation of carbonate calculi; and the concentration of urine solutes. This can be affected by diet, water intake and loss. The concentration increases when the horse is deprived of water or loses water excessively, as in sweating or diarrhea. If the diet or water fed to a horse has a high mineral content, this also increases solute concentration, while a high-concentrate, low-roughage ratio may allow the deposited solute to cement together more easily. Usually only one calculus occurs at a time, often composed of calcium carbonate.
All breeds and both sexes are equally likely to develop calculi, athough in mares they become very large before symptoms appear. These are similar to those seen in cases of cystitis, which often is present at the same time. Affected individuals urinate more frequently, with straining and dribbling of urine. Less commonly there may be mild recurrent colic, loss of condition and stilted gait. Occasionally a calculus passes into the male urethra, causing acute obstruction of urine flow.
Veterinary help should be always sought in cases in which there is obvious difficulty in passing of urine.
Diagnosis of cystic calculi involves urine analysis (the changes are similar to those of cystitis), passage of urinary catheter, and occasionally in the mare, passage of an endoscope into the bladder. Surgical removal of the calculus is the only effective method of treatment. The approach and type of surgery is determined by the size of the stone and the sex of the patient. Some cases may also require treatment for concurrent cystitis.
Prognosis for cases treated successfully by surgery is guarded because the affected horse may remain predisposed to chronic cystitis and calculus formation. Preventive measure are limited to correct dietary management, particularly in regard to moneral and concentrate proportions, adequate sources of drinking water and prompt veterinary attention to any case of suspected cystitis.