Cystinuria is an inherited disorder affecting the kidney tubules. The defect is inherited in an autosomal recessive manner, so two copies of the abnormal gene must be present (one from each parent) for disease to occur. When this happens the kidneys are unable to properly resorb the amino acid cysteine from the fluid that will become urine as they usually would (along with several other amino acids). Cysteine therefore accumulates in the urine, and in acidic urine it can form crystals, which can then clump together and eventually form solid stones (called calculi, or uroliths).
Stones in the bladder irritate the mucosal lining, and cause inflammation, bleeding and pain. This may lead to blood in the urine, pain urinating (causing small frequent attempts to urinate) or sometimes no clinical signs at all that an owner would notice. It is more commonly seen in males than females, except in the Newfoundland, where both sexes are affected equally. The Newfoundland suffers from a more severe form of the disease than other breeds, and stones may form by the time the dog is 5-6 months old. (In most predisposed breeds signs are seen around 4-5 years of age on average.) The Newfoundland also does not respond to treatment as well as most other breeds, and stones tend to recur fairly quickly. Stones in the bladder can make dogs prone to urinary tract infections. In males stones can lodge in the urethra, causing a blockage to urine flow. This is because males have a longer and narrower urethra than females. When no urine can be passed at all this is a medical emergency - kidney failure and coma follow, and then death is the ultimate result.
Cystinuria is diagnosed via specialised tests on the urine, such as the nitroprusside spot test. This may be done when signs of lower urinary tract infection and/or irritation are recurring. Cystine stones are not easy to see on x-rays, as there is not much mineral in them. Commonly an ultrasound of the bladder or a contrast study of the bladder is required to detect cystine stones. There are drugs that can be used to try to dissolve cystine stones, but they are associated with a number of side effects. Surgical removal may be required in a number of cases. Recurrence will occur unless management is undertaken to try to prevent cysteine accumulating in the urine. This includes feeding a low protein diet, using drugs to help manage cysteine levels in the urine, and increasing fluid intake to help induce a flushing action of the bladder.