Urolithiasis in Small Ruminants
David E Anderson, D.V.M., MS, Diplomate ACVS
College of Veterinary Medicine
The Ohio State University, Columbus, Ohio
Urolithiasis is a common and frustrating problem in small ruminants for owners and veterinarians. Urethral obstruction most commonly is seen in pet goats and show lambs because of inappropriate nutrition. Although conventional management techniques often are satisfactory for treatment of urolithiasis in show animals because of their shortened life expectancy, owners of pet animals demand more sophisticated techniques that will allow for long-term survival of the patient.
FORMATION OF URINARY STONES
The composition of urinary stones varies with geographic location, but the most common uroliths are calcium apatite and phosphatic calculi (calcium hydrogen phosphate dihydrate, magnesium ammonium phosphate) . Silicate and calcium carbonate stones also are occasionally seen. Formation of phosphatic calculi is encouraged by high concentrate, low roughage, low Ca:P ratio, high magnesium diets and alkaline urine. Normally, phosphorus is recycled through saliva and excreted via feces in ruminants. High grain, low roughage diets decrease the formation of saliva and increase the amount of phosphorus excreted in the urine. High phosphorus diets overwhelm the salivary excretion mechanism and result in high urinary excretion of phosphorus. High calcium diets are effective at reducing the absorption of phosphorus from the GI tract. Lambs fed high phosphorus and high magnesium diets had a higher incidence of uroliths at necropsy (Table 1). Increased urine output and decreased urine pH may prevent the formation of uroliths by decreasing urine retention time and by dilution of solutes. Some breeds of sheep may be predisposed to urinary excretion of phosphorus (versus GI excretion; Texel, Scottish Blackface).
Early in the onset of urolithiasis in small ruminants, patients may demonstrate hematuria, dysuria, prolonged urination, urination by dribbling, flagging of the tail, and apparent abdominal pain (stretching out all four limbs, kicking at the abdomen, looking at the side, etc). Later in the course of the disease, patients demonstrate anorexia, apparent depression, recumbency, abdominal distention or preputial swelling. Terminally, patients may seizure, suffer cardiovascular collapse, or die suddenly. Physical examination reveals tachycardia, tachypnea, and elevated, normal, or decreased rectal temperature. Digital rectal palpation reveals pelvic urethral distention and excessive urethral pulsation. If preputial swelling is present, the skin may be hot early in the disease or cold to the touch as ischemic necrosis ensues. If abdominal distention is present, a fluid wave may be balloted. Transabdominal palpation may reveal an enlarged bladder. Serum biochemical analysis reveals elevation of BUN and creatinine, hyper- or normokalemia, hyponatremia, hypochloremia, elevated muscle enzymes (AST, CPK), and acidemia if the disease is long standing. The CBC will be normal early, but leukocytosis with a left shift will develop with time. Ultrasonography shows a distended bladder; occasionally the distended urethra can be traced distally until the urolith is encountered. Most uroliths in small ruminants lodge at the urethral process; the second most common site is at the distal sigmoid flexure.
Treatment of urolithiasis often is dictated by economic constraints and euthanasia may be elected by the owner. Perineal urethrostomy has been the mainstay of surgical treatment for urolithiasis for many years, but the poor long-term success of this procedure has lead to the development of alternatives. Options for medical management of urolithiasis are limited for clinically affected animals, but should be addressed for the group or herd. Urethral Process Amputation may provide a temporary correction of urethral obstruction, but recurrence of obstruction is likely. The sheep or goat is placed onto their tailhead and the penis is extended (a preputiotomy may be needed in immature animals to provide access to the penis). The urethral process is removed using a guillotine method (tongue depressor placed beneath the process and amputation done by rapid transection with a scalpel blade). Medical treatment should immediately be begun to try to prevent re-obstruction. Perineal Urethrostomy can be done with regional anesthesia induced by an epidural block, sedation with xylazine hydrochloride, and manual restraint. We often perform perineal urethrostomy with the patient under general anesthesia when we are treating pet animals or animals of high perceived economic value. A 6 cm incision is made on midline in the perineum approximately 4 to 6 cm ventral to the anus (sub-ischial urethrostomy). The penis is elevated to the surgical incision and stay sutures (No. 0 PDS, vicryl, or No. 1 chromic gut) are placed between the perineal fascia overlying the semimembranosus muscle and the tunica albuginea of the penis. A 2 cm incision is made on the caudal and ventral midline of the penis (overlying the palpable urethral groove). The exposed tunica albuginea is sutured to the subcutaneous tissue adjacent to the skin (No. 2-0 PDS or vicryl) and the urethral mucosae is sutured to the skin edges of the surgical wound (No. 3-0 monocryl or 4-0 PDS). Alternatively, the penis may be transected at the ventral aspect of the incision, the proximal stump is exited through the surgical wound, stay sutures are placed circumferentially around the penis from the tunica albuginea to the skin, and the urethral mucosa is sutured to the cut edge of the tunica albuginea. Perineal urethrostomy is indicated in all patients that have already ruptured the penile urethra. Tube Cystostomy is the treatment of choice for patients for which urethral process amputation is not useful and that have not ruptured the urethra. Although a tube cystostomy may be done with sedation (xylazine HCl, 0.03 mg/kg, IV) and local lidocaine anesthesia, I prefer to perform cystostomy with the patient anesthetized. This allows optimal aseptic technique and time to perform cystotomy, evacuation of the bladder, thorough lavage and cleaning of the bladder, and placement of the tube cystostomy. Although a closed, sterile collection system attached to the exposed end of the tube is optimal, these are difficult and more expensive to maintain. In my experience, these systems are not necessary. I leave the exposed end of the tube open with a "Heimlich" type valve attached to the end of the tube (wet Penrose drain). The tube cystostomy is maintained until urine is seen dripping from the prepuce for 48 hours. Then, the tube is clamped and urination is monitored. If the animal can urinate, is not painful, and is able to empty the bladder, the tube is removed. If the animal can only partially urinate and continues to retain urine, the tube is left unobstructed for 5 to 7 more days and the process is repeated. On average, I can remove a tube cystostomy 10 to 14 days after surgery. Prepubic Cystostomy is reserved for patients that have had multiple perineal urethrostomies with stricture formation and re-obstruction. This procedure is a "last resort" treatment for pet animals. After induction of general anesthesia, the patient is placed into dorsal recumbency, a 4 cm incision is made on ventral midline immediately cranial to the pubis, and the bladder is exited through this incision. The bladder wall is sutured to the linea alba, a 3 cm incision is made into the bladder, and the bladder mucosa is sutured to the skin. Chronic cystitis is the principle complication of this procedure. Urethral Translocation can be attempted to "by-pass" a ruptured urethra (or failed urethrostomy). This also is a last resort procedure for pet animals. The procedure is done with the animal in dorsal recumbency and under general anesthesia. A right paramedian approach is made to the abdomen and the pelvic urethra is dissected free and transected at the most caudal location that can be reached. The distal penis or prepuce is identified and freed of surrounding soft tissues (I prefer to use the prepuce if it is long enough). This segment is translocated into the abdomen through a stab incision in the linea alba and anastomosed to the pelvic urethra (No. 3-0 monocryl or 4-0 PDS). Neurogenic bladder atony, urine stasis, and chronic cystitis are the most serious complications of this procedure.
PREVENTION OF UROLITHIASIS
Male sheep or goats intended to be used as pets should not be castrated until they are 6 to 12 months old to allow for development of normal urethral diameter. All show animals that must be fed a relatively high grain diet should have salt 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 %). Horse feed should never be fed to small ruminants because the diet is not balanced for ruminants and is thus calculogenic.
Table 1. Dietary influences on formation of urolith(adapted from Poole DBR. Observations on the role of magnesium and phosphorus in the aetiology of urolithiasis in male sheep. Irish Vet J 1989:60-63).
Diet Magnesium % Phosphorus % Calcium % Other % Sheep with Urolith
1 0.12 0.36
2 0.27 0.36
3 0.35 0.3
4 0.35 0.5
5 0.42 0.36
6 0.50 0.3
7 0.5 0.5
8 0.57 0.36
9 0.63 0.11 0.48
10 0.63 0.63 0.48
11 0.63 0.63 1.08
12 0.63 0.11 1.08
13 0.63 0.37 0.78
14 0.65 0.5
67 - 100
15 0.25 0.29 3.4% SiO2 18
0.25 0.6 3.4% SiO2 30
0.25 0.6 3.4% SiO2,
1% NaHCO3 50
0.25 0.6 3.4% SiO2,
1% NH4Cl 23
1.. Poole DBR. Observations on the role of magnesium and phosphorus in the aetiology of urolithiasis in male sheep. Irish Veterinary Journal 1989:60-63.
2.. Cuddeford D. Role of magnesium in the aetiology of ovine urolithiasis in fattening store lambs and intensively fattened lambs. Veterinary Record 1987;121:194-197.
3.. Osborne CA, Sanna JJ, Unger LK, Clinton CW, Davenport MP. Analyzing the mineral composition of urolith from dogs, cats, horses, cattle, sheep, goats, and pigs. Veterinary Medicine 1989:750-764.
4.. Stewart SR, Emerick RJ, Pritchard RH. High dietary calcium to phosphorus ratio and alkali-forming potential as factors promoting silica urolithiasis in sheep. J Anim Sci 1990;68:498-503.
5.. Haven ML, Bowman KF, Engelbert TA, Blikslager AT. Surgical management of urolithiasis in small ruminants. Cornell Vet 1993;83:47-55.
6.. Van Weeren PR, Klein WR, Voorhout G. Urolithiasis in small ruminants. I. A retrospective evaluation of urethrostomy. Vet Quarterly 1987;9:76-83.
7.. Rakestraw PC, Fubini SL, Gilbert RO, Ward JO. Tube cystostomy for treatment of obstructive urolithiasis in small ruminants. Veterinary Surgery 1995;24:498-505.
8.. Stone WC, Bjorling DE, Trostle SS, Hanson PD, Markel MD. Prepubic urethrostomy for relief of urethral obstruction in a sheep and a goat. J Am Vet Med Assoc 1997;210:939-941.
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