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The composition of uroliths (also called “urinary stones” or “calculi”) varies with the part of the country that the animal lives in. The most common uroliths are calcium apatite and phosphatic-based calculi (for example, calcium hydrogen phosphate dihydrate, and magnesium ammonium phosphate or struvite). Silicate and calcium carbonate uroliths also are occasionally seen.
Urethral obstruction caused by uroliths (Figure 1) is most commonly seen in show or pet goats and lambs that have a high grain, low roughage diet. Diets that are high in grain, phosphorus, and magnesium and low in roughage (hay or fresh grass) and calcium will increase the risk of phosphate urolith formation. Normally, a ruminant will remove phosphorus from their body by excreting it into saliva and then out through the feces (manure). High grain, low roughage diets decrease the formation of saliva, so extra phosphorus must be removed from the blood by the kidneys and then excreted in the urine. When diets are too high in phosphorus, the urine phosphorus levels become excessively high, and the phosphorus settles and consolidates into stone-like pellets that can be too large to pass. These uroliths increase the risk of urinary tract infection and may result in life-threatening blockage of the urethra. Some breeds of sheep (for example, Texel and Scottish Blackface) may be predisposed to stone formation because they tend to excrete phosphorus through the urinary tract rather than the saliva and feces.
Early clinical signs include:Blood in the urineStraining to urinateDecreased urine productionPainful urinationProlonged urinationDribbling urineTail flaggingAbdominal pain (stretching out all four limbs, kicking at the abdomen, looking at the side)
Late clinical signs include:Loss of appetiteLethargy (apparent depression)Abdominal swelling (from a ruptured bladder)Swelling around the prepuce (the skin covering the penis)
Once the animal is severely ill, they may lay on their side and not get up, and may eventually seizure or die suddenly.
Most uroliths in small ruminants lodge at the “urethral process” or “vermiform appendage” — a small tube-like extension of skin and urethra at the tip of the penis (Figure 2); the second most common site is at the “distal sigmoid flexure” — an S-shaped curve in the lower half of the penis. Uroliths trapped in the urethral process can often be felt during physical examination.
As toxins build up, blood tests may show high concentrations of blood urea nitrogen (BUN), creatinine, and potassium, which are normally excreted in the urine. Other blood work changes may include high concentrations of muscle enzymes and low concentrations of sodium and chloride. Acid may also build up in the blood stream.
Uroliths are often diagnosed with ultrasound of the belly. The bladder and possibly the urethra can be distended (overfilled with fluid) on ultrasound, and sometimes the stones can be seen. Some uroliths are visible on radiographs (x-rays), which can be used to determine the number and location of stones. Special contrast studies can be performed with radiographs to determine if the bladder has ruptured (Figure 3).
The urethral process is a very short, narrow tube-like structure on the tip of the penis, and because of its location and size is the most common site for uroliths to obstruct. “Amputation” or removal of this process with a scalpel blade may allow uroliths within this process to pass, but recurrence of obstruction is likely, particularly if uroliths are also present higher up in the urethra. If this is the case, additional surgical procedures are needed.Perineal Urethrostomy:
The urethra and its surrounding tissues can be easily felt in the back end (“perineum”) of sheep and goats 2-3 inches below the anus and just behind the rear legs. When uroliths block the penis or when the urethra has ruptured downstream from this area, a new opening can be made in this location to allow the animal to urinate like a female. This procedure is sometimes performed under heavy sedation with regional anesthesia (nerve blocks), but in valuable animals or pets it is often performed under general anesthesia. An incision is made through the skin and into the urethra, and the new urethral opening is sewn directly to the skin. The animal then urinates down and backwards instead of forwards.Tube Cystostomy:
In animals with urethral obstruction or rupture, urine must be diverted away from the urethra to allow swelling to subside and tissues to heal. Tube cystostomy involves surgically placing a rubber tube in the bladder and exiting the tube through the belly wall adjacent to the prepuce (Figure 4). This procedure is performed in conjunction with urethral process amputation and surgical removal of uroliths from the bladder. Once the swelling in the urethra has resolved, any uroliths that were not removed at surgery often pass in the urine either through the urethra or the tube. Once the animal is healed, the tube can be removed.
Although a tube cystostomy may be performed with sedation and local nerve blocks, general anesthesia is often used. This permits optimal sterile technique and time to remove uroliths from the bladder or repair it if it has ruptured, and allows the surgeon to thoroughly flush the abdomen before placing the tube. Once the tube is in place, it can be attached to sterile collection system or covered with a one-way valve to allow urine excretion.Prepubic Cystostomy:
For animals that have strictured (narrowed) perineal urethrostomy sites and subsequent re-obstruction, a permanent opening can be made between the bladder and the belly wall near the prepuce. This procedure is also called bladder “marsupialization”.Urethral Translocation:
This complicated procedure involves attaching the urethra in the belly to the lower half of the penis or prepuce.It can be attempted to “by-pass” a ruptured urethra (or failed urethrostomy).
Because of potential complications, advanced urethral surgeries may require specialized training, particularly when the patient is a valuable breeding animal or beloved pet. Your veterinarian will recommend/refer you to the appropriate facility with an jiyuushikan.org board-certified veterinary surgeon experienced in these techniques.
Breeding males should be rested sexually for 1-2 weeks after urethral process amputations and for 2 months after other urethral surgeries to reduce the chance of surgical complications.
Prevention of urolithiasis involves never feeding horse grain to small ruminants because the diet is not balanced appropriately for them and therefore increases the risk uroliths formation. All show animals that must be fed a relatively high grain diet should have salt (sodium chloride) added into the ration at a rate of 2 to 5 % (alternatively, ammonium chloride can be used at a rate of 0.5 to 1 %) to increase the amount of urine formation or acidify urine. Diets high in calcium may reduce the amount of phosphorus absorbed; owners should always speak to their veterinarians before adding any supplements to the feed.
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Photos provided courtesy of Susan L. Fubini, DVM, Diplomate jiyuushikan.org and Peter C. Rakestraw, VMD, MA, Diplomate jiyuushikan.org.