Noninfectious Diseases of the Urinary System in Horses
Not every disease is caused by infection with bacteria, viruses, or other outside agents. There are a variety of noninfectious disorders that can impair the urinary system. All of these diseases and conditions can be serious threats to the health of your horse.
The kidneys’ most important function is to filter waste from the blood. When this does not happen properly, waste products can build to dangerous levels in the blood. This is called azotemia. Azotemia can also occur as a result of urine not being able to flow properly through the urinary tract. This process damages the kidneys and, over time, leads to kidney failure.
Longterm (chronic) disease can damage the kidney so severely that it is not able to function properly. This happens slowly. Chronic kidney disease often continues for many months or years before any signs appear. There is rarely anything that a veterinarian can do to treat existing damage or prevent further damage once the process has started. Occasionally, chronic kidney disease results from a problem that is inherited or an abnormality present at birth; however, most of the time it is a problem related to old age.
Determining the cause of chronic kidney disease, especially in the early stages, will help determine the appropriate treatment and allow your veterinarian to determine the outlook for your horse. Some of the common causes include diseases of the circulatory system (such as high blood pressure, problems with blood clotting, and not having enough oxygen in the blood) or other diseases of the kidneys such as pyelonephritis or tumors. Whatever the cause, chronic kidney disease usually results in scarring of the kidneys, which worsens over time.
Veterinarians may be able to detect a problem in a blood test or on physical examination even before the horse develops signs of kidney failure. Usually, the earliest signs are unexplained weight loss, excessive thirst, and excessive urination. However, these signs may signal other diseases as well. To diagnose chronic kidney disease, veterinarians generally use a combination of x-rays, ultrasonography, urine and blood tests, and physical examination. These tests are also used to check the response to treatment and monitor complications related to the kidney disease.
Treatment involves identifying the underlying cause of kidney disease and treating it, if possible, along with any complications. The recommended treatment depends on the stage of disease. If kidney failure has occurred, supportive treatment such as intravenous fluids and feeding tubes may be recommended. In most cases, the outlook for longterm survival in horses with chronic kidney failure is poor.
Acute kidney disease is the result of sudden, major damage to the kidneys. This damage is usually caused by toxic chemicals either consumed by your horse or built up by an abnormal condition in your horse’s body. Kidney function can also be affected when the kidneys do not receive sufficient oxygen, such as when a blood clot blocks the flow of blood to the kidneys. Other causes of acute kidney disease include severe dehydration, bleeding, infections involving the entire body, and general anesthesia.
Mild kidney disease often goes unnoticed. However, repeated occurrences can lead to chronic kidney disease (see above). It is important to determine whether the kidney disease is acute or chronic, as well as the cause of the disease. This information will help your veterinarian determine the most appropriate treatment.
The glomerulus is one of the structures that are essential for kidney function (see figure). It is made up of special blood vessels that help filter blood. Each kidney contains hundreds of thousands of these structures, called glomeruli. Glomerular disease sometimes causes kidney disease.
Glomerular disease can occur due to the effects of high blood pressure, hyperadrenocorticism (an excess of cortisol), or amyloidosis (a disorder in which abnormally folded protein is deposited in the glomeruli). Some glomerular disease is immune-mediated, that is, caused by the horse’s immune system attacking parts of its own body.
Disease in the glomerulus often leads to protein in the urine, low levels of protein in the blood, a buildup of fluid in the abdomen (which can cause visible swelling), shortness of breath, and swelling in the legs. The loss of protein through the urine can cause loss of muscle tissue. Most horses with glomerular disease eventually develop chronic kidney disease and kidney failure.
A biopsy of the kidneys is often required to determine the cause of the glomerular disease. Additional tests may be necessary in some cases, including x-rays, ultrasonography, and special blood tests. Treatment for glomerular disease varies according to the underlying cause. In addition, kidney failure should be treated with appropriate medications.
Renal tubules are structures in the kidneys that help filter blood. Healthy kidneys help the body to get rid of acid by producing urine that is acidic. Diseased kidneys cannot get rid of acid properly, and instead of being eliminated in the urine, this acid builds up in the blood, leading to a condition called uremic acidosis. This condition can also occur when there are defects in the function of renal tubules, in which case it is called renal tubular acidosis. These defects are rare in horses. Treatment may involve medications to rebalance the amount of acid in the blood. Some horses may recover with several days of treatment, but others may require continued supplementation with sodium bicarbonate to maintain proper acid balance.
Tumors that originate in the urinary tract are rare in horses. In the kidney, renal cell carcinoma occurs in older horses. Usually only one kidney is involved. If the tumor spreads to the lungs or lymph nodes, the outlook is poor. Tumors that may develop in the bladder include squamous cell carcinoma and transitional cell carcinoma. Bladder tumors are usually malignant, and there is no effective treatment. Signs of tumors in the urinary tract include blood in the urine, abdominal pain, and, for tumors of the kidney, signs of kidney disease or failure.
Uroliths are stones (also known as calculi) formed when minerals that naturally occur in urine clump together to form tiny crystals. These stones can develop anywhere in the urinary tract, including the kidney, ureter, bladder, or urethra. The formation of uroliths is not common in horses. It can affect young horses but is seen most frequently to adults. There is no one breed that is more prone to urolithiasis than another; however, it is seen more frequently in males than females.
Uroliths in horses can be anywhere from 0.2 to 8 inches (0.5 to 20 centimeters) in size, weigh as much as 14 pounds (6.5 kilograms), and are found most often in the bladder. There are many different types of stones, each formed from a complex mixture of various minerals. Veterinarians are not sure how or why stones form in horses, although they suspect that feeding your horse food or water that is high in mineral content may play a role. Kidney stones may develop in horses with renal papillary necrosis, a disorder of the kidney in which areas of the kidney die off.
Signs vary depending on where the stone is located. Most stones are located in the bladder and cause blood in the urine and increased thirst and urination. Occasionally, stones are found in other parts of the urinary system, including the kidneys.
Blood in the urine is most obvious after exercise and toward the end of a urine stream. Horses with this condition usually stretch out in order to urinate and may maintain this pose for some time before and after urinating. Additional signs include scalding of the area around the urethra (the area between the vulva and the anus in females, and the inside of the hind legs in males). The scalding occurs because urine is acidic and constant contact with urine can burn the skin over time. Geldings and stallions may protrude the penis for prolonged periods of time while dribbling urine on and off. Horses with this condition may occasionally have recurring bouts of colic or an altered gait. The urethra may become blocked as a result of a trapped stone; this usually causes restlessness, sweating, colic, and frequent attempts to urinate. On doing a rectal examination, your veterinarian will be able to tell that the bladder is overly full.
Kidney stones in both kidneys occasionally occur in adult horses that have been used for performance. Over time, these stones may repeatedly block the ureter and cause kidney failure.
Your veterinarian can usually detect the stones by the signs listed above in addition to performing a thorough physical examination. Sometimes x-rays or ultrasonography are used to confirm the diagnosis. Your veterinarian may want to insert a catheter (a narrow, flexible tube) through the urethra and into the bladder in order to empty the bladder and make sure there is no stone or other growth blocking the urethra. An analysis of the urine may also be done to provide more information.
Uroperitoneum is the leakage of urine into the abdominal cavity. In foals, this most commonly results from tearing of the bladder during birth or rupture of the urachus after an infection. Bladder rupture may be more common in male foals.
Foals with this condition usually appear normal at birth, but gradually become ill over 1 to 2 days. They are weak, lack energy and interest in their surroundings, and have an increased heart rate. Gradually, the abdomen will enlarge. Most of these foals attempt to urinate often, but only produce a small amount of urine.
Tests of blood and fluid from the abdomen can help your veterinarian make the diagnosis. Other diseases with signs that could be confused with uroperitoneum include blood infection (septicemia), nervous system disorders, and colic.
Surgery is generally required to correct the defect and is successful in most uncomplicated cases. The foal should be stabilized before surgery to prevent complications due to poor lung function (a consequence of distension of the abdomen) or electrolyte imbalances (especially high potassium, which can affect the heart). If the problem is recognized early, the outlook for correction is excellent. If the foal becomes infected or is born prematurely, other complications may decrease the chances of recovery.
Also see professional content regarding noninfectious diseases of the urinary system in large animals.