Nutritional management of struvite urolithiasis in dogs

The most common canine uroliths are composed of magnesium ammonium phosphate (struvite) or calcium oxalate¹. The relative proportion of these two types of urolith has varied between different countries and over time but they account for more than 80% of all canine uroliths².

The aetiopathogenesis of urolithiasis remains incompletely understood – for uroliths to form and grow, the urine must be supersaturated with the relevant crystalloid materials. However, the fact that urine is commonly supersaturated in animals that never form uroliths illustrates that other factors are also important.

VARIOUS FACTORS AFFECT THE RISK OF UROLITHIASIS

Certain risk factors are well characterised for canine struvite urolithiasis. Some of these may be inter-linked, but struvite uroliths occur more commonly in^1-4:

• Middle-age adult dogs – Most commonly between 2-9 years of age (average ~6yrs).
• Certain breeds – e.g., Miniature Schnauzer, Welsh Corgi, Dachshund, Shih Tzu, Bichon Frisé, Miniature Poodle, Cocker Spaniel, Lhasa Apso and Labradors.
• Alkaline urine – As struvite is very much less soluble in alkaline urine.
• Female dogs – Approximately 70-90% of struvite uroliths occur in females.
• Dogs with urinary tract infections –70% or more of all canine struvite uroliths are associated with a urease-producing urinary tract infection.
  - The most common bacteria are Staphylococcus spp, Proteus spp. and occasionally Klebsiella spp or Psuedomonas spp.
  - Hydrolysis of urea by urease liberates NH₃ that is a component of struvite (increasing urine saturation) and also raises urine pH (decreasing struvite solubility).
  - Even when urine is culture-negative, viable bacteria are often found within uroliths.
• Anatomical and functional abnormalities of the urinary tract.
• Abnormalities of metabolism e.g., hyperadrenocorticism.

Struvite uroliths tend to be radiodense (opaque on radiographs) and are usually quite smooth. Most larger uroliths in dogs are struvite – in general >90% of uroliths >10mm in diameter will be struvite.

MANAGEMENT OF STRUVITE UROLITHIASIS - ANTIBIOTICS

More than 70% of dogs with struvite uroliths have an associated urease-producing bacterial urinary tract infections (UTI). Eradication of the UTI is essential for adequate control of the disease:

• Quantitative urine culture should be performed, ideally on urine obtained by cystocentesis.
• Antibiotics should be chosen on the basis of sensitivity testing and be combined with dietary management for dissolution.
• Therapy should be maintained for at least one month beyond radiographic resolution of the urolith (as viable bacteria may be contained in the nidus).
• Follow-up urine cultures to ensure eradication of the UTI are advised.

CLINICAL ADVANTAGES WITH THE USE OF CANINE UR

Where a UTI is present, dietary therapy should always be combined with appropriate antibiotic therapy. The diet is specifically formulated to provide:

• Urine with a target pH of 6.0 – this will help prevent the formation of new struvite crystals and uroliths and help dissolve existing uroliths and crystals.
• Undersaturation of ammonium – by using a low quantity of high quality protein, there is reduced production of urea, the substrate for bacterial NH₃ production in the urine.
• Undersaturation of phosphate and magnesium – by carefully controlling the content of the diet, Canine UR is designed to also undersaturate the urine for these two constituents.