Renal gout in reptiles
In reptiles uric acid is the main end product of protein metabolism and in most reptiles’ uric acid is the main component of the urine . It is primarily excreted by the proximal convoluted tubules with minimal excretion by glomeruli. Risk factors for the development of gout in reptiles include hyperuricaemia caused by renal dysfunction (particularly tubular disease), dehydration and /or intake of purine rich diets. This latter condition being common in herbivorous animals fed a diet high in animal protein. (Gross image of leguaan kidney showing chalky white urate accumulates).
In early renal gout deposition of uric acid crystals is in proximal tubular epithelial cells (usually seen as crystal ghosts in histological sections). These crystals act as the nidus / crystallization centers for further deposition, breaking through the tubular epithelial basement membrane initiating an inflammatory response resulting in the characteristic urate tophus formation. Urate tophi consist of central radially arranged urate crystals surrounded by macrophages, giant cells and variable numbers of heterophils and lymphocytes. Tophi maybe found throughout the kidney often occluding tubules, in the interstitium and rarely mesangium of glomeruli. (See histopathology image right)