Uroliths (urinary stones) are common in cattle, sheep and goats. Urolithiasis is an important disease in feeder animals but may also be seen in breeders. It occurs in both sexes but partial or complete obstruction of the urinary tract tends to be more common in castrated males because of the relatively narrow urethra.
It is primarily a nutritional disease. Mineral salts from feed and water that are normally dissolved in urine, may form small stones (calculi) that lodge in and block the urethra, usually in the sigmoid flexure or the urethral process. The bladder distends and eventually the bladder or urethra ruptures. Death occurs after a few days from uraemia or septicaemia or both.
Risk factors that favour formation of uroliths include diets low in roughage or high in phosphorus, calcium, magnesium, oxalate or silicates; prepubertal castration; mineral-rich, inaccessible or unpalatable water; and conditions that favour dehydration and reduced urine output. Obstruction is usually at the sigmoid flexure of the penis. Complete obstruction will either be spontaneously relieved and the urolith(s) passed in urine, or the urethra perforates (urine escapes subcutaneously) or the bladder ruptures (releasing urine into the abdominal cavity). Bulls may incur rupture of the urethra when mounting other cattle.
Signs include reluctance to move, legs stretched out, straining to urinate, and little or no urine flow. If the bladder and urethra are still intact, the back will be arched and there may be bruxism (grinding of teeth), stretching and kicking at the abdomen. Affected animals may develop prolapse of the rectum from straining to urinate.
Rectal palpation in cattle will detect a hugely distended bladder. If the bladder has ruptured there will be urine accumulation in the abdominal cavity. This results in moderate abdominal distension and ballottement may induce a fluid wave at the opposite paralumbar fossa. Rectal palpation may detect a collapsed bladder.
If the urethra has perforated there is subcutaneous swelling along the prepuce and ventral abdomen (known as waterbelly).
The ammonia smell of the urine distinguishes it from other types of ascitic fluid. Calculi may be impacted in the urethral process at the tip of the penis in sheep and goats.
At necropsy the site of obstruction is identifiable by discrete calculi or granules compressed into a section of discoloured and necrotic urethral mucosa. If rupture of the urethra has occurred there will be urine in the tissues around the site of obstruction. If the bladder has ruptured there will be urine and possibly blood clots in the abdomen. Hydroureter and hydronephrosis may also be present.
Laboratory analysis of uroliths may assist determining aetiology.
Treatment is based on establishing a patent urethra to allow urination to occur and restoring fluid and electrolyte balance.
If the blockage is in the urethral process in sheep and goats, amputation of the blocked tip of the process with scissors may resolve the condition. Care should be taken to ensure there are not additional stones further back along the urethra or in the bladder.
If there is a complete obstruction and the bladder is still intact, perineal urethrostomy may be lifesaving. This can be performed under caudal epidural anaesthesia while the animal is standing if skills and resources are available.
Sometimes a ruptured urethra may resolve without intervention if a patent urethra is established. Incisions may be made into the subcutaneous tissues adjacent to the prepuce to facilitate drainage of urine. A 10 cm incision is made on each side of the prepuce, about 10 cm from and parallel to, the midline. It may also be necessary to perform a perineal urethrostomy to help drain the bladder and avoid further urine flow through the urethra while the rupture heals.
Animals with a perineal urethrostomy may then be transported for salvage slaughter.
Animals with a ruptured bladder and urine accumulating in the abdomen may be able to be salvaged with a perineal urethrostomy and drainage of urine from the abdomen using a trocar or teat cannula. This may allow the bladder to heal spontaneously. If facilities and expertise are limited or salvage slaughter is difficult then these animals should be euthanased.
Analysis of the uroliths is essential before adjustments to dietary mineral balance are made in order to ensure that appropriate changes are made.
Prevention of formation of urethral calculi is mainly dependent on providing a calcium to phosphorus ratio of 2:1 in the complete ration, and ensuring animals have ready access to good quality water supply. Additional measures may be useful in some cases such as supplemental sodium chloride (up to 4% of the total ration) to increase urine output, and urinary acidifiers that may prevent formation of some crystals.
Where a small number of cases have occurred it is important to implement dietary changes and check weater quality as soon as possible to try and prevent further cases from occurring.