The urinary tract of the dog is complex and the potential for problems is large with varied clinical symptoms including dysuria, stranguria, haematuria, pollakiuria and increased time to urinate (Shaer, 2010). Symptoms can be caused by many issues, including congenital defects, neoplasia, trauma, infection and/or uroliths.
As with cats, the most common urinary tract condition in dogs is cystitis. Figures from Lulich et al. (2000) attributed 40 percent of urinary tract conditions to cystitis, 24 percent to incontinence and 18 percent to “other causes”. Uroliths were attributed to 18 percent of cases. A 2017 analysis of over 75,000 uroliths submitted to the Canadian Urolith Centre over a 16-year period found that calcium oxalate and struvite were the two most common stones seen, account-ing for 81 percent of all stones analysed (see Figure 1).
The same study also noted that medium and large breeds were most at risk of struvite stones, which are most often the result of a urinary tract infection, and small breeds were more at risk of developing calcium oxalate stones.
According to Moore (2007), although the physical appearance of many uroliths can give a good indication of the composition of the stone, different stone types do form in a wide variety of shapes, sizes and colours. Additionally, each calculus may have up to four different layers; from inside to out, these are: the nidus, the stone, the shell and surface crystals. Each layer could be composed of different mineral types and therefore, the external appearance of a calculus rarely gives an indication of the full composition of the entire urolith, the very centre of which (the nidus) is the key to accurate diagnosis and treatment.
The nidus could be quite different from the bulk of the stone. For this reason, when sending a stone for analysis, it is recommended to use a laboratory which will use a combination of structural and morphological tests to determine the exact composition of the urolith (Basiri et al., 2012).
Using X-ray to determine the physical appearance of uroliths is also not 100 percent reliable as it can’t always detect them and if it does, the shape might be misleading. It’s possible to find multiple crystal types in one bladder so identifying the composition of one specific type of crystal is an unreliable indicator of the composition of a urolith, since crystals present could be completely different from the composition of a stone taken from the same bladder, no crystals may be seen, or multiple crystal types might be found in the same urine sample.
Urinalysis
It’s preferred to use a first-morning sample for analysis because it is most concentrated. For a detailed quantitative analysis, a 24-hour sample should be taken. If it’s not possible to carry out urinalysis within 20 to 30 minutes of the sample being taken, the urine can be refrigerated and stored for up to six hours before the validity of results is compromised (Padilla et al., 1981; Hesse and Neiger, 2004). The refrigerated urine should be brought to room temperature and thoroughly mixed before analysis. If crystals are observed in stored samples, this should be validated by re-evaluation of a fresh urine sample (Albasan et al., 2003).
The optimum sampling method is cystocentesis, but if examining a free catch sample brought in by the owner, it is important to establish when the sample was taken. Low quantities of crystals are normally present in urine; the risk of crystals forming spontaneously increases with storage time and changes (including cellular disintegration, bacterial growth and pH changes) are more pronounced the longer the storage time and the greater the temperature fluctuations (Archer, 2005). It is important to note that post-prandial urine is usually alkaline: the “alkaline tide” occurs while acid is being secreted into the gastric juice. Stress also alters urine pH; respiratory alkalosis due to hyperventilation can raise urine pH (Chew, 2004).
Relative supersaturation
Relative supersaturation (RSS; Figure 2) is a method pioneered by Royal Canin to measure the risk of struvite or oxalate crystal formation within a bladder, in animals fed any given diet. Each crystal has a specific saturation level below which the environment within the bladder makes it unlikely that crystals will form. RSS takes into account 12 parameters including the pH of the urine, the mineral content (specifically measuring five positive and five negative ions) and the volume of urine produced by feeding the specific diet.
When feeding a dog (or cat) that has crystals in the bladder, the RSS level needs to be undersaturated to dissolve struvite crystals and prevent them reforming, and to prevent calcium oxalate reforming crystals once they have been removed. In healthy animals, or once the bladder is completely free of crystals, a diet with metastable-supersaturation levels can be fed.
Feeding a diet which causes oversaturation (also known as labile supersaturation) will result in spontaneous crystallisation if the animal is predisposed. Houston et al. (2017) found various predisposing factors, such as breed, gender, neutered status and potentially weight, although body condition scores were not available for most of the 75,000+ submissions.
Struvite
Struvite calculi, composed of magnesium ammonium phosphate, are sometimes referred to as urease, infection-induced, phosphatic and triple phosphate stones. Struvite grow in an alkaline environment and it is possible to dissolve them by feeding a specially designed diet (Krawiec et al., 1984; Smith et al., 2001).
Struvite is usually associated with an infection so it’s imperative to correctly diagnose and treat with appropriate antibiotics alongside appropriate dietary management (Rinkardt and Houston, 2004). Since it can take up to six weeks to dissolve a struvite stone, antibiotics should be prescribed for one month after the stone is dissolved to prevent recurrent infections, as bacteria are continually released into the bladder during the dissolution process. If stones are surgically removed, then antibiotics are recommended for at least three weeks post-operatively. According to Lulich et al. (2016), parameters for measurement are:
- Urine pH of <6.5
- Urine specific gravity (SG) <1.025 (or <1.020 for recurrent cases)
Also important in the management of struvite stones is to increase urine turnover, thereby increasing urine dilution. This can be achieved by feeding a diet with elevated sodium levels (Buckley et al., 2011). Struvite stones can be prevented from returning after removal or dissolution by feeding a diet which creates a high water turnover of slightly acidic urine and one in which mineral levels have been manipulated to reduce the risk of recurrence.
Uroliths can comprise several different types and it is not uncommon to find struvite mixed with calcium oxalate. Fortunately, it is possible to manage struvite and oxalate with one diet because, although pH does affect calcium excretion, it has no impact on the relative saturation levels for calcium oxalate in urine (Queau et al., 2013).
Calcium oxalate
In contrast to struvite, pure calcium oxalate uroliths (Figure 3) are rarely associated with infection, so tend not to require antibiotics. Also, unlike struvite, calcium oxalate uroliths are more commonly found in male dogs and the most commonly affected breeds are the Miniature Schnauzer and other small breeds such as the Lhasa Apso, Bichon Frise and Yorkshire Terrier (Case et al., 2011).
Although uncommon, hypercalcaemia is seen in animals with primary hyperparathyroidism, pseudohyperparathyroidism, malignant lymphoma and secondary hyperparathyroidism (Lulich et al., 1992); hypercalcaemia is also associated with increased urine calcium excretion.
There is a 10-fold increased risk of dogs developing calcium oxalate if they have hyperadrenocorticism compared with dogs without the condition (Hess et al., 1998) and it is surmised that prompt diagnosis and treatment of hyperadrenocorticism may decrease prevalence of calcium-containing uroliths in dogs.
Once calcium oxalate crystals or stones have formed, a diet should be fed that has adapted calcium levels and maintains the appropriate Ca:P ratio and controlled levels of vitamin D. It would also be desirable to control the levels of magnesium and phosphorus, which are oxalate precursors. As with struvite, providing a diet which increases water turnover through the bladder is important and this can be done by feeding either a wet diet or a dry diet with adapted sodium levels to encourage increased water intake.
Measurement parameters are (according to ACVIM 2016 recommendations): urine pH >6.5 and urine SG <1.025 (<1.020 in recurrent cases). Serum calcium levels should also be monitored. It may also be worth noting that human foods such as chocolate, nuts, beans, sweet potato, wheat germ, spinach and rhubarb are high in oxalic acid, although absorption does vary depending on the food and which other foods they are fed in combination with (Liebman and Al-Wahsh, 2011), so for dogs fed with home-prepared diets, a conversation around this is warranted.
Ammonium urate
Dalmatians are homozygous for the recessive gene leading to defective uric acid metabolism, so only 30 to 40 percent of uric acid is metabolised to soluble allantoin, which is the end product of purine catabolism; this results in a build-up of uric acid and leads to the formation of ammonium urate stones (Bannasch et al., 2008). In unaffected dogs, 90 percent of uric acid is metabolised.
Dalmatians are the most common breed affected by the metabolic stone, ammonium urate. Other breeds can also be affected; these include, most commonly, American and English Bulldogs, Black Russian Terriers and Giant Schnauzers, all of which have a 37 percent increased risk of developing urate compared with mixed breeds (Houston et al., 2017).
Since ammonium urate formation is a genetic problem, it cannot be cured and there is always a risk of recurrence. A 2017 study by Westropp et al. found that feeding a diet with moderate levels of protein (18 to 20 percent as fed) with low-purine protein sources can reduce the risk of recurrence. This is best achieved using non-meat protein sources, such as egg or wheat, and particularly avoiding beef protein – with added water for urine dilution, since a diet should also be lowered in sodium. It is also useful for a diet to include an alkalising agent. Interestingly, Albasan et al. (2005) found that there was a significantly higher risk in male Dalmatians and concluded that long-term protocols to reduce the risk of recurrence might not actually be necessary in females.
Measurement parameters:
- Urine pH: >6.8 to 7
- Urine SG: <1.035 (<1.025 in recurrent cases)
As noted by Lulich et al. (2016), it is possible to dissolve urate stones within four weeks if fed the appropriate diet alongside administration of a xanthine oxidase inhibitor (ie allopurinol). They also state that where the urate is related to a liver condition, such as an uncorrected portosystemic shunt, dissolution is not possible because allopurinol is contraindicated in these cases.
Case of a six-year-old male (entire) dalmatian with urate stones
This dog presented with a history of occasional whimpering followed by nibbling at his prepuce. Urination was normal. His prepuce showed saliva staining and mild dermatitis but otherwise there were no abnormalities except that he was slightly underweight. Two months later, bloody urine was found in the owner’s kitchen but still no history of straining or discomfort noted during micturition. A culture of a discharge from the penis was done and balanoposthitis was treated with antibiotics. After another eight weeks, the dog presented with the same symptoms but now urine wasn’t flowing normally. Urinalysis showed a specific gravity of 1.031, pH 5.5 and frequent ammonium urate crystals. A contrast urethrogram showed narrowing of the urethra, consistent with chronic urate stone lodgement. According to a study by Bartges et al. (1994), Dalmatians with uroliths are over 200 times more likely to be diagnosed with ammonium urate (Figure 4) than any other stone. Management of the condition was first the removal of the stones, then the dog was placed on a low-purine diet for life.
Other canine uroliths
Other canine uroliths include cysteine, which most commonly affect Deerhounds (Houston et al., 2017), and xanthine, silica and calcium phosphate carbonate, which are all fairly rare. Dogs can also suffer with bacterial urinary tract infections, very often concurrent with diabetes mellitus or hyperadrenocorticism (Forrester et al., 1999), and as already mentioned, this can lead to urolith formation, particularly struvite.
Summary
The causes of urolith formation are varied and complex or idiopathic and, in all cases, specific nutritional management is valuable and recommended. In the case of ammonium urate, dietary changes should be lifelong. Many patients will have a recurrence and the most effective way to avoid this is by feeding a diet with a low RSS level for both struvite and oxalate for the rest of their lives. Dogs of different genders, lifestyles and breeds may be more or less predisposed; understanding this gives one a better chance of choosing an appropriate treatment and feeding protocol for ongoing patient management.
