For a more general introduction to urolithiasis, see Urolithiasis in Small Animals.
Obstructive urolithiasis is most common in wethers, bucks, and rams, with commercial, exhibition, and pet animals at risk. There is a lesser incidence of disease in steers and bulls, with the animals most at risk being those in feedlots. Uroliths can also form in females, but because they do not have the narrow urethral tract found in males, obstruction and subsequent pathology is much less frequent. Blockage of the urethra is followed by urinary retention, which can lead to rupture of the urinary bladder or urethral perforation.
Uroliths are solid crystalline formations composed of organic matrix (sugars, protein, and cells) and inorganic crystalloids, including calcium, magnesium, and phosphate. These components are able to come out of solution and bond in situations of supersaturation.
There are four main stone types:
Matrix components may include uroepithelial cells from vitamin A deficiency, suture, tissue debris, blood clots, excess protein, or bacteria in the urine.
Urine supersaturation of mineral components occurs as a result of increased renal excretion, negative water balance, urine pH, and the presence or absence of crystallization inhibitors. The phosphatic and calcium carbonate stone types form in alkaline urine and in animals on high grain or legume diets. Urine pH likely has no impact on the formation of silicate or calcium oxalate uroliths; these form in animals grazing silicaceous pastures in the western USA and Canada or those grazing oxalate-containing plants, respectively.
Systemic illness and unpalatable water can result in reduced water intake, increasing urolith component concentration in urine. It is evident that diet and water intake are major contributors to the etiology of this disease, but other factors such as urethral diameter and individual metabolism also likely contribute.
The anatomy of male ruminants also contributes, not to stone formation, but to a predilection for obstruction. The urethra of male ruminants is long and sigmoid in shape, and small ruminants additionally have a 2–4 cm extension of the urethra beyond the glans penis called the vermiform appendage or urethral process. This is the most common site of obstruction in small ruminants, whereas the distal sigmoid flexure of the urethra is a common site in both cattle and small ruminants. Uncommonly, uroliths may form in the renal pelvis and lodge in the ureters or may form and obstruct females.
Obstructive urolithiasis should be considered as a differential diagnosis in all sick male ruminants, particularly sheep and goats. Historical findings are important in investigating possible cases of obstructive urolithiasis, and specific questions regarding diet, age at castration, progression of signs, any treatments given, and when last seen urinating normally provide valuable insight.
Presenting clinical signs include:
Many cases present on complaint of bloat or straining to pass feces.
A complete physical examination should be performed on presentation. Observation for grit on preputial hairs or pulsing of the urethra upon digital rectal examination are useful specific examination points for suspected urolithiasis cases. A large plaque of fluid (pitting edema) surrounding the prepuce is indicative of a ruptured urethra, whereas a pear-shaped abdomen with bilateral ventral distension suggests a ruptured urinary bladder, both of which are associated with a worse prognosis. A grave prognosis should be given for any male intended for breeding if there is a urethral rupture, because penile adhesion formation is likely and will interfere with breeding.
If the animal does not urinate during examination or when placed in a clean stall, this should be investigated by ultrasonography of the urinary bladder and/or exteriorization of the penis with examination of the vermiform appendage. Transabdominal ultrasonography with a 3.5MHz probe may reveal a distended urinary bladder (often >8–10 cm) or free abdominal fluid in cases of rupture. Abdominocentesis may be performed, and a sample creatinine level greater than double the serum creatinine identifies the fluid as urine.
Sedatives and local anesthesia can be useful to achieve exteriorization of the penis. Acepromazine (0.05–0.1 mg/kg, IV or IM), diazepam (0.1 mg/kg, slow IV) or midazolam (0.2 mg/kg, IV or IM) may serve to reduce anxiety and relax urethral tone. These all constitute extralabel drug use. For patients with cardiovascular compromise, acepromazine should be avoided. Alpha-2 agonists produce analgesia and sedation in ruminants but also cause increased urine volume from transient hyperglycemia. This sudden increase in urine volume may cause rupture in a previously intact urinary tract and is therefore contraindicated in cases of urinary obstruction. Lumbosacral epidural with 2% lidocaine (1 mL / 7 kg) can also facilitate exteriorization of the penis.
Survey radiography is particularly useful in cases of calcium carbonate urolithiasis because the uroliths are quite radiopaque, allowing for determination of the extent of the obstruction and potential for re-obstruction. In one study, calcium carbonate uroliths were visualized in 11/12 cases by survey radiography. Phosphatic stones, however, are less radiopaque and often missed on survey radiography due to the mass of the abdomen. The absence of uroliths on survey radiography does not rule out the presence of obstructive or nonobstructive uroliths.
Common findings on serum biochemical analysis include azotemia, elevated phosphorus, and elevated magnesium, which is associated with a poor prognosis. Ruminants are able to manage blood urea nitrogen (BUN) and potassium via the rumen and saliva, respectively, so these increases are not as large as those seen in obstruction of monogastric animals. Creatinine is therefore a more reliable indicator of impaired renal perfusion.
Urinalysis may be performed if a sample can be obtained. Protein and blood may be present, along with crystals of the obstructive stone type. It is common for crystals to be absent from the urine of obstructed animals. Hemoglobinuria, myoglobinuria, and hematuria are causes of red urine. These should be differentiated, by performing centrifugation of the urine, and differential diagnoses including copper toxicity, Brassica toxicity, cystitis, pyelonephritis, and leptospirosis should be considered.
Treatment goals for ruminants with urolithiasis are to establish urethral patency, provide analgesia, correct fluid and electrolyte imbalances, decrease urethral inflammation, and prevent infection.
Catheterization and retropulsion is a mainstay of treatment for obstructive urolithiasis in many species. However, the presence of the urethral diverticulum, a dorsal outpouching of the urethra at the level of the ischial arch, prevents simple retrograde catheterization in male ruminants. Various curved catheters have been used with some success in accomplishing retrograde passage into the urinary bladder in male small ruminants. Hydropulsion can be performed using a tomcat catheter after instillation of 0.5–1 mL 2% lidocaine into the urethra. Hydropulsion with small volumes of saline should be attempted very carefully so as to not force fluid past the stone, further distending and possibly rupturing the urinary bladder or causing urethral perforation. Surgery is often necessary to relieve these obstructions.
Fluid therapy should be instituted based on clinical findings and the results of serum biochemistry testing. After or during the relief of the obstruction, diuresis is important to replace hydration deficits, manage azotemia, and flush urolith components from the urinary tract. Normal (0.9%) saline makes an appropriate empiric first fluid for these cases. Additives should be used as indicated by serum biochemistry findings. Initial rates of fluid therapy should be slow until the obstruction is relieved and then increased to 80–100 mL/kg/day until the patient stabilizes.
Drug therapy may include the use of NSAIDs, opioids for pain management, and preoperative broad-spectrum antimicrobial therapy. Beta-lactam antimicrobial drugs make reasonable choices of these cases because of their spectrum of activity and their excretion via urine. These recommendations constitute extralabel drug usage, and drug labels should be followed for dosing and extended withdrawal times, even in pet animals.
Among the surgical techniques described for relief of urinary obstruction in ruminants are:
Vermiform appendage amputation can be performed in small ruminants, and is successful if it is the only site of obstruction. Following exteriorization of the penis, the vermiform appendage is sharply removed at the glans penis. There are almost always more uroliths, however, and these relieved animals are at high risk of re-obstruction.
Tube cystotomy is considered the gold standard treatment for cases of urethral obstruction, because it provides an alternate route for urine flow while allowing healing of the urethra. Briefly, a paramedian cystotomy is performed and a Foley catheter placed to divert urine and allow the urethra to rest for 3–5 days. Urination generally occurs in uncomplicated cases in 7–10 days. Tube cystotomy is associated with a higher cost than other surgeries for obstruction but has been reported to have 76%–90% short-term success at reestablishing urethral urine flow, with a 86% longterm success rate. The most common complication is the tube becoming displaced from the urinary bladder. Percutaneous and flank-approach placement of tubes have also been described.
Urinary bladder marsupialization creates a permanent or semi-permanent stoma from the urinary bladder to the skin, providing urine outflow that bypasses the urethra. Via a paramedian incision, the apex of the urinary bladder is brought to the body wall and a stoma created for urine diversion. This procedure is associated with complications of urine scald, mucosal prolapse, and ascending urinary tract infections. The reported success rates with urinary bladder marsupialization in two studies was 66% and 94%.
In a perineal urethrostomy, urination is established via a stoma created with the proximal urethra. The urethra is spatulated and sutured to the skin, creating the stoma. This procedure is associated with a high risk of stricture, which can be improved with excellent tissue handling and careful dissection to ensure that tension on the penis is relieved. The reported outcome is that 45%–78% will stricture within 8 months, making this generally an undesirable option for most pets, although some remain successful for years.
When stone material is obtained at or before surgery, urolith analysis is indicated to guide control and prevention measures. An initial presumption can be made based on appearance. Phosphatic stones occur in sand-like form or are easily crushable, while calcium carbonate uroliths are gold beads with a very stable structure.
A thorough review of the diet and management of herds or flocks with cases of urolithiasis should be performed. Animal purpose, water source, and stone type are all important considerations. Availability and type of hay, grain, pelleted feed, pasture, and mineral supplementation must also be determined. In general, prevention focuses on four main aspects of the development of obstructive urolithiasis:
The anatomy of the urinary tract in ruminants makes preventing and treating urolithiasis more challenging in these species. There are two approaches that can make the urinary tract less susceptible to obstruction: delaying castration to increase urethral diameter and prophylactically removing the vermiform appendage. Delaying castration has been shown to have a positive impact on urethral diameter in calves and lambs, providing a larger outlet for any uroliths that may form. Prophylactic removal of the vermiform appendage removes the most common site of obstruction in small ruminants. The purpose of the vermiform appendage is to spray semen during copulation. There have been no primary studies to evaluate whether removal negatively impacts fertility, but most experienced clinicians do not believe that this is a significant concern.
Increasing urine volume and dilution can be achieved by increasing water intake by providing clean, palatable, temperature-appropriate water and adding NaCl to the diet. Encouraging grazing and feeding a high-forage diet with limited grain and pelleted feeds also increases water intake. A high-roughage diet requires more water for mastication and digestion, therefore increasing urine output over that of concentrate diets. Further, meal feeding shunts water to the rumen at the time of consumption, releasing antidiuretic hormone and urine output.
Matrix components can be reduced by decreasing high-protein feeds and hays. Ensuring a solid trace mineral program with sufficient vitamin A reduces the likelihood of urinary tract metaplasia and cellular debris in the urinary bladder. Chloride salts, such as calcium chloride, sodium chloride, and ammonium chloride, can attach to the matrix binding sites and prevent formation of a nucleus.
Mineral component control is considered in light of the urolith type most likely to form in a particular species. Show animals and feeder animals are at most risk for phosphatic stones. Pets and other animals consuming alfalfa and other legumes are at highest risk for calcium carbonate stones. Both of these stone classes form in alkaline urine. Urinary acidification can be achieved with 200 mg/kg/day of ammonium chloride orally in the short term. Animals on ammonium chloride should have their urine pH evaluated periodically 5–7 hours after feeding to determine whether adequate acidification is occurring, with the dosage adjusted for the individual.
If clients insist on feeding grain or pelleted feed, the Ca:P ratio of the total ration should be held at 2–2.5:1 to limit phosphorus availability for phosphatic uroliths. An increase in this ratio predisposes to calcium carbonate uroliths, whereas a reduction predisposes to phosphatic uroliths. Loose trace minerals should be provided to all classes of animals.
Silicate and calcium oxalate urolith types are associated with specific plants on pasture. These should be identified and controlled or males should have limited access, with females used to graze high-risk pastures.
Obstructive urolithiasis is a significant cause of morbidity and mortality in male ruminants.
Treatment may require urethral catheterization, surgical correction, and supportive care.
Prevention may require increasing urine output, adjusting urine pH, and dietary changes to minimize urolith formation.