Certain urinary tract abnormalities are inherited or congenital (present at birth). These abnormalities are caused by abnormal genes or produced by injury, disease, or exposure to toxic substances in the womb. They may or may not cause health problems for your horse later in life. These types of abnormalities are very rare in horses, but they are important to consider if your horse has urinary tract problems.

Kidney Malformations

Kidney malformations, called dysplasias, occur when a horse's kidneys do not develop properly before birth. When the kidneys are unusually small, the condition is called hypoplasia. Dysplasia can occur in either one or both kidneys. When these conditions occur, the kidneys are usually small, firm, and pale. The outer portion of the kidney that contains glomeruli (microscopic structures that are critical for filtering blood) may be smaller than normal in size.

Horses with these conditions typically have a buildup in the blood of the toxic waste products that are normally excreted in the urine. An affected horse usually has ongoing, excessive thirst (called polydipsia) and corresponding excessive urination (polyuria), as well as other signs of kidney disease such as weakness, lack of appetite, abdominal pain, fever, or swelling of the legs.

Your veterinarian can usually diagnose dysplasia or hypoplasia based on these signs and appropriate tests. The diagnosis can be confirmed with a biopsy of the kidneys.

Failure to Develop

Rarely, one or both kidneys fail to develop. This condition is always accompanied by a lack of the tube connecting the kidney to the bladder (ureter). The reproductive organs may also be under-developed. A foal in which both kidneys have failed to develop will die shortly after birth. However, a horse with one functioning kidney can usually live a full and healthy life. In this case, the failure of one kidney to develop is usually dis-covered by accident.

Polycystic Kidneys

Polycystic kidneys have multiple cysts inside the functional part of the organ. The kidneys are greatly enlarged, which a veterinarian may be able to feel during a physical examination. Horses with this condition sometimes also have cysts in the bile ducts of the liver. Problems caused by this condition can range from none at all to progressive kidney failure. Your veterinarian can diagnose polycystic kidneys based on examining your horse, using x-rays or ultrasonography, or performing exploratory abdominal surgery.

Other Kidney Disorders

Other congenital problems include kidneys that are not positioned correctly, kidneys that are fused together, and nephroblastoma, a cancer that develops in the kidneys during embryonic development (before birth).

Disorders of the Ureters

Several abnormalities can affect the ureter, the tube that connects the kidneys to the bladder. Normal, healthy horses have 2 ureters, 1 for each kidney.

An ectopic ureter is one that opens somewhere other than into the bladder. Ectopic ureters can connect to the urethra (the tube used for urination), or in females, the uterus or vagina. This defect is rare in horses.

Other abnormalities of the ureter include failure to develop or abnormal development, the presence of more than the usual 2 ureters, and ureterocele (enlargement of the portion of the ureter that connects to the bladder). Ureterocele can usually be successfully treated with surgery.

Urachal Remnants

The urachus is a cord of fibrous tissue that normally extends from the bladder to the navel. Before birth, the urachus is a tube that connects the bladder to the umbilical cord so that wastes can be removed. After birth, it normally closes and becomes a solid cord. In some animals, however, the urachus does not close properly after birth. Depending on which portion of the urachus remains open, these abnormalities are called a patent urachus or an umbilical urachal sinus. Other problems include urachal diverticula (small sac-like structures attached to the urachus) and urachal cysts. Signs include an inability to control urination, urine scalding (due to the fact that urine is acidic) of the skin near the navel, and urinary tract infections. Your veterinarian can diagnose these problems using x-rays taken after a special dye is given intravenously. Treatment usually includes surgery and, sometimes, antibiotics.

Disorders of the Bladder

The bladder is a muscular sac that stores urine produced by the kidneys. Several congenital and inherited problems can affect the bladder. These include the presence of more than one bladder, an abnormally developed or underdeveloped bladder, failure of the bladder to develop, and a bladder that is turned inside-out. Usually these problems occur along with other abnormalities in the urinary tract. Your veterinarian can diagnose these problems based on physical examination, observation of your horse while it urinates, and contrast x-rays. Treatment varies depending on the type of problem.

Disorders of the Urethra

The urethra connects the bladder to the exterior of the body. It is the tube through which urine passes when a horse urinates. Congenital urethral problems are rare in horses. Some of the conditions that do occur include failure of the urethra to develop, a urethra that does not open all the way or does not open at all, openings of the urethra that are on the underside or on top of the penis rather than on the tip in males, multiple urethras, urethral diverticula (small pouches that form in the urethra that become inflamed and painful), an abnormal opening between the urethra and the rectum, and an unusually narrow urethra.

Last full review/revision July 2011 by Scott D. Fitzgerald, DVM, PhD, DACVP, DACPV; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Thomas J. Divers, DVM, DACVIM, DACVECC; Sherry Lynn Sanderson, BS, DVM, PhD, DACVIM, DACVN