Cystinuria is an inherited metabolic disease that is relatively common in dogs, but rare in cats. The condition is characterized by defective amino acid reabsorption, leading to the formation of cystine stones in the kidney, ureter and the bladder (cystine urolithiasis). This can lead to urinary obstruction.
The kidneys are involved in the filtration of the blood. Kidneys process the blood through filtration, reabsorption, secretions and excretion. In the first step, the blood is filtered. Cells, proteins and other large molecules are retained, while smaller molecules are filtered into an ultrafiltrate that will eventually become urine. Then, water (~55%) and electrolytes are reabsorbed from the ultrafiltrate back into the blood system and terminal metabolic products (waste products) are secreted from the blood into the ultrafiltrate. Finally, ultrafiltrate is excreted into the ureters, where it becomes urine. Through this process of filtration and excretion, kidneys help maintain homeostasis in the kitty’s body. In cystinuria, however, the amino acid cysteine is inadequately reabsorbed from the ultrafiltrate. This results in excessive cysteine concentrations in the urine and can lead to the formation of cystine uroliths.