It is well known among vets that male castrated sheep and goats are prone to urolithiasis; however, it is perhaps less well known that it can occur in other species such as pigs, cattle and camelids. Urolithiasis is the blockage of the urethra with urinary calculi (stones), which usually occurs at either the sigmoid flexure or urethral process. Typically, treatment for urolithiasis is surgical intervention or euthanasia.
In its early stages (Figure 1), the disease can easily be missed by an inexperienced stockperson, resulting in owners often seeking veterinary advice too late in the disease process when the animal is showing more serious signs of urolithiasis (Table 1).
|Early signs||Late signs|
Stretching and arched back
Change in urination (presence of blood, decreased output, dribbling, prolonged urination)
Crystals on prepuce
Straining (can be mistaken for constipation)
Subcutaneous oedema around the prepuce due to urethral rupture
Dull/obtunded mentation and lethargy
In sheep, there are many different types of uroliths but struvite crystals are the most common and are associated with concentrate feeding that is high in phosphate and magnesium (Scott, 2015).
Other factors that predispose small ruminants to urolithiasis include castration at a young age, ringing after less than seven days, feeding of alfalfa (due to its high calcium content), clover pastures and high concentrate feeds of greater than or equal to 2.5 percent of the animal’s dry matter body weight for two months or longer (Harwood and Mueller, 2018).
Castration at a young age prevents the urethra from developing due to a lack of testosterone, so it consequently remains narrow. The common sites of blockage are at the sigmoid flexure or urethral process (Figure 2). There is a genetic predisposition in certain sheep breeds to urinary calculi based on the phosphorus excretion route, with either majority excretion through faeces or through urine. It is thought that there is likely to be a similar genetic predisposition in goats as well (Matthews, 2009).
Pigs often have urinary stones present in the bladder at post-mortem examination. These are insignificant in females, but in males, if they are sufficiently large enough to block the urethra, the pigs will present acutely with distended abdomens, weakness and death (Carr et al., 2018). Maes et al. (2004) describe a condition in neonatal piglets where urate crystals in the urinary tract cause clinical signs of splay leg, which progresses to paralysis and death.
Smears of this discharge will quickly enable the practitioner to determine between a diagnosis of crystals, infection (neutrophils) or tumours if extra cells are present on the slide
Sows may present with crystalluria, a white gritty discharge which contains calcium phosphate and struvite crystals. Smears of this discharge will quickly enable the practitioner to determine between a diagnosis of crystals, infection (neutrophils) or tumours if extra cells are present on the slide. For the latter, sending the smear away for tumour cell identification is helpful to assess the pig’s prognosis.
In cattle, urolithiasis is commonly seen in feedlot steers but can occur in any aged entire male animal. It is reported that cattle rupture their bladders more commonly than urinary leakage (seen with small ruminants) as a result of urinary calculi blockage at the distal sigmoid flexure (Gilbert et al., 2017).
In camelids, urolithiasis is more commonly seen in entire males and can occur in young camelids. Rectal palpation should reveal an enlarged painful bladder but caution should be taken when palpating as there have been reports of bladder ruptures in camelids and a common post-operative complication of urethral strictures (Fowler, 2010).
What diagnostic tools can be useful in identifying uroliths in farm animals?
Ultrasound can be a useful diagnostic tool for farm animals with suspected urolithiasis. The bladder in sheep typically sits within the pelvis, so the use of a standard linear probe such as one for cattle pregnancy diagnosis will be able to identify if the bladder is abnormally extending into the abdomen. Scott (2015) reports that for a 20 to 40kg growing lamb, a distended bladder is 6 to 8cm in diameter, whereas in a mature ram this would be 16 to 20cm in diameter.
The bladder in sheep typically sits within the pelvis, so the use of a standard linear probe such as one for cattle pregnancy diagnosis will be able to identify if the bladder is abnormally extending into the abdomen
Ultrasound can also show free fluid in the abdomen which occurs when the bladder becomes so distended that it leaks urine, as bladder rupture is rare in small ruminants. If the urethral blockage has been present for a few days, changes in the kidney such as renal pelvis dilation will be seen on ultrasound when examining the right kidney through a shaved right paralumbar fossa (Scott, 2017).
For new graduate vets who are often without an ultrasound machine, there are two equipment-free methods to aid diagnosis of urolithiasis: rectal palpation of the urethra and oral fluids. Rectal palpation, or digital palpation in small ruminants, enables the urethra to be palpated for pulsation or spasms. The urethra sits ventrally in the pelvis, and in blocked animals has obvious spasms. Giving oral fluids and asking the owner to monitor urine output on either shavings or a concrete yard are useful suggestions. However, action must be taken if the animal has no urine output within a few hours. This option is less preferable due to the obvious risk of worsening welfare if obstructed.
Biochemistry can be a helpful tool, with increased blood urea nitrogen (BUN) and creatinine levels indicating urolithiasis. Abdominocentesis can also be a useful tool to assess free fluid in the abdomen, and creatinine levels can be compared with serum creatinine levels to confirm uroabdomen. However, relying on external laboratories is not ideal due to the fast nature in which you need to act with these patients. It is worth considering referral to a farm animal hospital if in-house biochemistry is not available as referral institutes will be able to perform these diagnostic tests on site.
Relying on external laboratories is not ideal due to the fast nature in which you need to act with these patients. It is worth considering referral to a farm animal hospital if in-house biochemistry is not available
In small ruminants, urethral process amputation may alleviate some of the animal’s discomfort, enabling them to travel to a referral institute for surgery (Figures 3A and 3B). It is important not to sedate with alpha-2 agonists such as xylazine as they increase urine output. Referral institutes will be able to stabilise the patient appropriately and perform further diagnostics such as radiography to determine prognosis before commencing surgery.
Common surgical procedures that may be done include perineal urethrostomy, which is deemed a “salvage procedure” as the animal is prone to urine scald and urinary tract infections, or tube cystotomy (Figure 4) while the animal is under a general anaesthetic.
Management and prevention
Many lamb finisher pellets contain ammonium chloride mixed into the pellets to reduce the chances of urolithiasis in finishing commercial ram lambs. For the pet sheep, ammonium chloride is used post-operatively to acidify urine to dissolve struvite crystals. Matthews (2009) advises a target urine pH of around 5.5 to 6.0. One method of achieving this is to feed 10g ammonium chloride in 40ml water daily. It is important to note it is fairly unpalatable, and urine pH should be closely monitored to check urine is within the desired pH range.
It is well known that castrating at a young age increases the chances of urolithiasis, so if wethers are intended to be sold, it is important to educate owners about urolithiasis and delaying castration to allow some urethral development. Avoidance of concentrate feed is important to reduce the risk of urolithiasis as well as obesity and subsequent lameness issues seen in smallholder sheep and goats. However, it is worth bearing in mind the knock-on consequence of skin conditions in goats due to the lack of dietary zinc and subsequent oral supplementation that may be required. For all animals it is important to encourage water consumption to reduce chances of obstruction, and ensure water troughs do not freeze over in winter. For goats, water consumption can be encouraged by warming water, water fountains and floating balls.