Feline lower urinary tract disease (FLUTD) encompasses several disorders: feline idiopathic cystitis (FIC), urolithiasis, bacterial urinary tract infections (UTIs), anatomic malformations, neoplasia, behavioural disorders and neurological problems. Regardless of the underlying aetiology, the resultant clinical signs are similar and include dysuria, stranguria, haematuria, pollakiuria and periuria.
Urolithiasis accounts for approximately 20 percent of FLUTD cases (Gerber et al., 2005) and may be treated by surgical removal or by dietary dissolution, dependent on the type of stone present. Dietary support can play a key role both in the initial management of cats with uroliths and in reducing the risk of recurrence longer term, alongside other strategies such as environmental enrichment to reduce any contributions of stress. Dietary factors also make important contributions to the risk factors for development of urolithiasis.
What is a urolith?
BOX (1) General risk factors associated with urolithiasis; these are thought to interfere in the elimination of metabolites, predisposing to a greater occurrence in certain individuals. It should be noted that the existence of one or more factors is not determinant for the development of uroliths and each factor alone plays a limited role in the disease (Gomes et al., 2018)
Uroliths are composed primarily of crystalloid material with a small amount of organic matrix, formed by non-crystalloid proteins. They may form anywhere along the urinary tract, including the renal pelvis, ureters and urethra; however, the most common location is the bladder lumen. Box 1 details general risk factors associated with urolithiasis in cats.
In cats, the two most common uroliths are struvite (magnesium ammonium phosphate) and calcium oxalate, each accounting for over 40 percent of uroliths analysed (Osborne et al., 2009). These two uroliths have each been predominant over the last few decades, with the leading urolith type switching back and forth between the two. Fluctuations are primarily driven by dietary changes, including modifications of maintenance and preventative diets to try to minimise the occurrence of different crystals and uroliths, and the use of calculolytic diets designed to dissolve struvite uroliths (Osborne et al., 2009). Risk factors for struvite and calcium oxalate uroliths are detailed in Table 1.
Other rarer uroliths include urate, cysteine and calcium phosphate stones, but discussion of these is beyond the scope of this article.
|Struvite urolithiasis||Calcium oxalate urolithiasis|
|Age: younger than 7 years (particularly 4 to 7 years)||Age: older than 7 years (particularly 7 to 10 years)|
|Breed: Himalayan, Persian, Siamese||Breed: British Shorthair, Himalayan, Persian|
|Alkaline urine||Acidifying diets|
|Dietary factors including diets rich in magnesium, phosphorus, calcium, chlorine and fibre||Dietary factors resulting in hyperoxaluria, including increased intake of oxalates and vitamin C, or hypercalciuria|
The formation of uroliths depends on urine “supersaturation” with the precursor calculogenic crystalloids, which enables crystals to aggregate and grow into calculi. Other factors, such as urine pH, available organic matrix and the presence of crystallisation promoters or inhibitors, also influence crystal formation and may be of variable importance with different urolith types. The urinary environment can be influenced by dietary manipulations to increase or decrease the likelihood of crystals and uroliths forming.
Increased urolith precursor concentrations, increased urine pH, decreased urination frequency and other unknown predisposing factors are involved in the formation of struvite uroliths as they lead to the promotion of crystal formation and aggregation. Unlike in dogs, most struvite uroliths in cats form in sterile urine; however, a urine culture is still worthwhile, especially in older cats. Feline struvite uroliths are only associated with infection in 5 percent of cases (Gomes et al., 2018), and usually with urease-producing species such as Staphylococcus and Proteus species (Figure 1).
The American College of Veterinary Internal Medicine (ACVIM) recommends that all struvite uroliths should be medically dissolved unless: appropriate foods or medications are contraindicated (or the cat will not eat them); the stone cannot be adequately bathed in modified urine (due to, for example, a very large urolith occupying nearly all of the bladder); or there is an uncontrollable infection (Lulich et al., 2016).
A risk of urethral obstruction by using medical dissolution rather than surgery has not been reported to the author’s knowledge. Disadvantages of surgical cystotomy include the risks associated with anaesthesia and bladder suture-induced urolith recurrence, and failure to remove all uroliths in up to 25 percent of cats (Lulich et al., 2013, 2016).
Nutritional dissolution is well established as a successful strategy for struvite urolithiasis and clinical studies have demonstrated that feeding a specially designed functional diet can be very effective to achieve full struvite dissolution (Houston et al., 2011; Lulich et al., 2013; Torres-Henderson et al., 2017).
Characteristics of a struvite dissolution diet include low magnesium content and urinary acidification. Several commercially available “maintenance” urinary diets are now available with clinical studies to demonstrate their efficacy and which can be fed both during struvite dissolution and longer term to minimise the risk of recurrence.
Most struvite uroliths can be dissolved in less than two to five weeks and dietary dissolution is considered both safe and effective
Most struvite uroliths can be dissolved in less than two to five weeks and dietary dissolution is considered both safe and effective. Generally, the percentage of dissolution should be evaluated by radiographs two weeks after starting an appropriate diet. If the stone is not completely dissolved it should have decreased in size by at least 35 percent (Lulich et al., 2013). Minimal or no reduction in urolith size suggests either non-compliance or a misdiagnosis, with calcium oxalate being the most likely alternative urolith or a compound urolith containing multiple mineral types. In such cases, a different management approach is required, including removal by the most minimally invasive procedure possible (Lulich et al., 2016).
When appropriate, antibiotic therapy should also be used alongside an appropriate diet, with follow-up urinalysis including a culture to ensure any infection diagnosed is successfully resolved.
Care should be taken to ensure the cat is fed the calculolytic diet exclusively, and pH should be monitored to ensure the desired urinary pH is being achieved to maximise the likelihood of success. In one study comparing two feline dissolution diets, the diet which produced the lower mean urine pH (6.12 ± 0.1 vs 6.34 ± 0.1) resulted in a shorter time to complete urolith dissolution (Lulich et al., 2013).
Care should be taken to ensure the cat is fed the calculolytic diet exclusively, and pH should be monitored to ensure the desired urinary pH is being achieved to maximise the likelihood of success
Urinary pH should be measured using a pH meter to maximise accuracy, on urine samples taken at least two hours after a meal to minimise the likelihood of an inaccurate pH measurement due to the post-prandial alkaline tide. Where possible, the diet should be fed in multiple small meals daily to minimise urinary pH fluctuations. Stress should be accounted for when analysing urine: stress can increase urinary pH, so obtaining urine samples from a cat at the clinic, when they potentially feel stressed, may raise the urinary pH; therefore, sampling should be done in the home environment where possible.
Oral urinary acidifiers such as methionine should only be considered if the target urinary pH cannot be achieved by diet alone. Maximising fluid intake to reduce urine specific gravity (USG) and the saturation of the urine is also crucial (see below).
Prevention of recurrence of struvite uroliths
It is recommended that a functional maintenance diet with low magnesium and phosphorus and which acidifies urine is fed to prevent recurrence of struvite uroliths (Lulich et al., 2016). Target urinary pH should be less than 6.5 (Houston et al., 2011; Lulich et al., 2013) and regular urinalysis is encouraged. Maximising fluid intake is a cornerstone of urolithiasis management (Lulich et al., 2016). Increased water intake and decreased urine concentration decreases the risk of urolithiasis, as formation of more dilute urine lowers the risk of supersaturation. A USG of below 1.030 to 1.035 should be aimed for in cats with a history of urolithiasis (Villaverde, 2020). Ideally, this should be achieved by feeding a high moisture diet; feeding a 100 percent wet diet has been shown to increase hydration parameters effectively in cats (Greco et al., 2014). Feeding high sodium dry foods (greater than 375mg/100kcal) should not be recommended as a substitute for high moisture foods, but may be necessary if the cat will not eat dry food (Lulich et al., 2016).
It is recommended that a functional maintenance diet with low magnesium and phosphorus and which acidifies urine is fed to prevent recurrence of struvite uroliths (Lulich et al., 2016)
Different risk factors influence the likelihood of formation of struvite and calcium oxalate uroliths. Nutritional management has a crucial role both in the dietary dissolution of struvite stones, where it can rapidly and effectively eradicate sterile struvite uroliths in cats, and in the longer term to prevent such stones from forming.
Regular urinalysis in patients should be conducted to ensure the efficacy of management strategies employed and maximise the likelihood of long-term success, and owners should be educated to ensure they are aware of the importance of dietary management and the signs of recurrence they should monitor for.