Renal Dysplasia and Hypoplasia:
These defects are most common in dogs and have been reported in many breeds, including Alaskan Malamutes, Bedlington Terriers, Chow Chows, Cocker Spaniels, Doberman Pinschers, Keeshonden, Lhasa Apsos, Miniature Schnauzers, Norwegian Elkhounds, Samoyeds, Shih Tzus, Soft-coated Wheaten Terriers, and Standard Poodles. Renal dysplasia is rare in cats, lambs, and horses. In pigs, renal dysplasia may be idiopathic or associated with nutritional avitaminosis A.
Renal dysplasia may be unilateral or bilateral. Animals affected bilaterally generally die in the early neonatal period, whereas animals affected unilaterally typically develop hypertrophy of the contralateral kidney. The kidneys are usually small, firm, and pale; they may have a uniformly diminished renal cortex. Histologic examination reveals immature glomeruli, primitive tubules, and secondary inflammatory lesions, especially interstitial fibrous connective tissue.
Polydipsia and polyuria usually precede signs of uremia. Dwarfing may be noticed if the onset of renal failure occurs within the first few months of life. Urinalysis, hemogram, and blood chemistry changes are the same as in other chronic, progressive renal diseases. Uremia is usually identified between 6 mo and 2 yr of age. The diagnosis is suspected based on the breed and age of disease onset and is confirmed by renal biopsy. Treatment is aimed at managing the associated chronic renal failure.
Renal agenesis, the complete absence of one or both kidneys, is always accompanied by ureteral aplasia and may be associated with aplastic reproductive tissues on the same side. The condition is typically an incidental finding so long as the other kidney functions normally; frequently, the contralateral kidney undergoes compensatory hypertrophy. Renal agenesis is believed to have familial predisposition in Beagles, Shetland Sheepdogs, and Doberman Pinschers. Unilateral renal agenesis is the most common congenital kidney condition in pigs; it is less common in sheep, cattle, and goats. Bilateral agenesis results in early perinatal death.
Multiple cysts form within the renal parenchyma, and such kidneys are usually grossly enlarged on palpation. This condition may be associated with hepatic biliary cysts in both dogs and cats. Polycystic kidneys may cause no clinical signs or lead to progressive renal failure. This condition is familial in Beagles, Bull Terriers, Cairn Terriers, and West Highland White Terriers. Polycystic kidneys are an autosomal dominant inherited trait in Persian cats and domestic long-haired cats. Polycystic kidneys are also hereditary in pigs and may alternatively be associated with avitaminosis A. Polycystic kidneys are rare in cattle and horses and very rare in sheep. Diagnosis is based on physical and radiographic findings, ultrasonic examination, or exploratory laparotomy. Pyelonephritis may be seen concurrently and precipitate renal insufficiency.
Simple Renal Cysts:
These solitary, unilocular cysts generally do not communicate with the renal collecting system; the rest of the kidney is normal. The origin of these cysts is uncertain. They are usually an incidental finding in all domestic species.
Perirenal pseudocysts are accumulations of fluid that develop external to the renal parenchyma. They have been identified in cats. Their origin is unknown. The accumulated fluid may be located between the renal parenchyma and the renal capsule or between the renal capsule and a thin-walled fibrous sac attached to the capsule. They are termed pseudocysts rather than cysts because they are not lined by epithelium. Because only a limited number have been examined histologically, it is unknown whether all perirenal fluid-filled structures are pseudocysts. The fluid contained within these structures is not urine or lymph but is described as a transudate. Clinical signs are characterized by progressive abdominal enlargement. Abdominal palpation reveals a large, firm, nonpainful mass located in the area of the kidneys. Renal function tests and urinalysis are typically normal; however, a mild azotemia may occur. Diagnosis is made by excretory urography or ultrasonography. Treatment involves exploratory surgery to confirm the diagnosis, drainage of the pseudocyst fluid, and resection of as much of the pseudocyst wall as possible. The prognosis appears to be good; however, only a limited number of cases have been evaluated.
Miscellaneous Renal Anomalies:
Double or multiple renal arteries are seen in ~5% of dogs. Other congenital defects include renal malpositioning, renal fusion, and nephroblastoma. Nephroblastoma is an embryonal tumor and is rare in domestic animals except pigs. It may cause no problems but may be very large and cause abdominal distention.