SMALL RUMINANT UROLITHIASIS

Photos courtesy of Drs. Susan L. Fubini and Peter Rakestraw

The urinary tract includes the kidneys, ureters (small tubes that drain urine from the kidneys to the bladder), the bladder, and the urethra (the tube that carries urine from the bladder and out of the animal).  Urolithiasis — the formation of “stones” or concretions of mucus, protein, and minerals in the urinary tract — is a common problem in male small ruminants and a frustrating one for owners and veterinarians.

Formation of Uroliths
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 pet goats and show lambs that have inappropriate nutrition. For example, 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 its 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.

Figure 1.  These uroliths — also called bladder stones or calculi — were surgically removed from a sheep that had been fed an inappropriate diet. 

Diagnosis
Early clinical signs include:

• Blood in the urine (hematuria)
• Straining to urinate
• Decreased urine production
• Painful urination (dysuria)
• Prolonged urination
• Dribbling urine
• Tail flagging
• Abdominal pain ( stretching out all four limbs, kicking at the abdomen, looking at the side)

Late clinical signs include:

• Loss of appetite
• Lethargy (apparent depression)
• Abdominal swelling (from a ruptured bladder)
• Swelling around the prepuce (the skin covering the penis)

Once the animal is severely ill, it may lay on its side and not get up, and eventually it may seizure or die suddenly.

Physical examination findings may include increased heart rate, rapid breathing, and possibly a large bladder.  If the bladder has ruptured into the belly, a fluid wave can sometimes be felt (“balloted”) when pressure is put on one side of the abdomen. 

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. 

Figure 2:  The urethral process, or vermiform appendage, is a tube like extension (arrows) off the tip of the penis.  Uroliths can become trapped within this tube, blocking urination. 

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).

Figure 3.  Contrast material (red arrows) is leaking from the bladder (green arrows) in this animal.

Treatment Options
Surgery is usually required to remove uroliths or relieve the blockage.  Farm management practices should also be reviewed to determine whether diet or other factors could increase the risk of urolithiasis within the herd.

Urethral Process Amputation — 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 to allow the animal to urinate. This surgical procedure is called a perineal urethrostomy. 
Perineal urethrostomy 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.
Complications of perineal urethrostomy include stricture formation (closure of the hole) in some animals. Also, the animal can no longer be used for breeding since semen does not  pass through the penis.

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.

Figure 4.  An incision has been made into the belly (green arrows) to open the bladder and remove the uroliths.  Then the cystostomy tube (black arrows) is placed into the bladder and exiting out the belly wall before the abdomen is closed.