Content area
Full Text
Development of uroliths
Urolithiasis occurs relatively infrequently in horses compared with many other veterinary species. In contrast to humans, uroliths of the kidney and ureter are relatively uncommon and most uroliths diagnosed in the horse are located within the bladder (see Box 1 ).
Box 1: Location of calculi diagnosed in the horse
Horses develop two basic forms of uroliths and both are composed primarily of calcium carbonate in various hydrated forms. More than 90 per cent are yellow, moderately friable, spiculated stones ( Fig 1 ). Less commonly, they are grey-white, smooth stones that are resistant to fragmentation. The latter often contain phosphate in addition to calcium carbonate. Other components include magnesium, ammonium, struvite, hydroxyapatite and uric acid, but these are seen infrequently. As crystals of calcium carbonate and mucoproteinaceous material are normal components of equine urine, it is difficult to formulate an explanation for calculus formation based solely on the presence of these materials. In general, two steps are required for calculus formation - nucleation and crystal growth.
Factors predisposing to crystal precipitation include supersaturation of urine, prolonged retention and promoters of crystal growth. Genetic variations in ion excretion rates, which are well documented in humans and dogs, have not been described in horses. Although poorly documented, inhibitors of crystal growth in equine urine, including its high mucus content, are likely to play an important protective role, particularly in the light of the substantial urinary excretion of calcium carbonate crystals by normal horses. Spontaneous nucleation rarely initiates calculus growth because normal urine of most species is supersaturated and is in balance between crystal precipitation and dissolution. Tissue damage from a number of causes is likely to be the most important factor for the development of uroliths in horses. Desquamated epithelial cells, leucocytes or necrotic debris may form a nidus for crystal growth. Stasis of urinary flow favours nucleation by increasing the chance of contact between crystalloid material and damaged uroepithelium. Once crystal growth commences around a nidus, the highly alkaline urine of horses favours crystallisation of most urolith components, especially calcium carbonate.
The role of urinary tract infection in the development of urolithiasis varies with species. Although positive urine culture in horses with urolithiasis is very low, in one series of cases,...