Nutritional Support of Kidney Disease in HorsesBy Dr. Bryan Waldridge · January 24, 2010
Serious kidney (renal) disease in horses is fairly uncommon. Clinical signs of kidney disease can be difficult to differentiate from other conditions but include lethargy, depression, inappetence, ulcers on the mouth or tongue, and edema or swelling of the legs and lower abdomen. Urination can be normal, decreased, or increased. Excessive dental tartar on the teeth, especially the canine teeth, has commonly been associated with kidney disease, but it is usually not the result of kidney problems. Often, some other serious illness such as diarrhea, tying-up, or destruction of red blood cells (hemolysis) leads to renal failure, and the kidney disease is initially masked by the severe clinical signs of the predisposing disease.
Some drugs (aminoglycoside antibiotics, tetracyclines, nonsteroidal anti-inflammatory drugs such as phenylbutazone and flunixin) used by veterinarians to treat colic or diarrhea can be toxic to the kidney, especially if the horse is dehydrated. This creates a huge problem because veterinarians have to balance the necessary treatment with kidney health. For a horse to maintain elevated kidney values in its bloodwork, at least 75% of the nephrons, the microscopic filter-like units that produce urine, must be damaged. Horses with kidney failure are often treated with intravenous fluids to flush renal toxins and excess electrolytes from the blood, through the kidneys, and into the urine.
Bloodwork will usually reveal dehydration, elevations in blood urea nitrogen (BUN) and creatinine (called azotemia), changes in protein concentration, elevated calcium, and other electrolyte disturbances. Some horses can become anemic because the kidneys produce erythropoietin, a hormone that signals the bone marrow to produce red blood cells. When the kidneys are severely damaged, they become leaky, and protein seeps from the blood and into the urine. Low blood protein causes edema because protein holds water in the blood, and when it is low water can settle out into the tissues. Horses with kidney disease can develop elevated calcium because the kidneys excrete excess calcium into the urine. Blood sodium, chloride, and potassium can be low because the renal tubules normally maintain the correct concentrations of these electrolytes between the urine and blood.
Urinalysis detects protein or blood lost into urine, possible infection, and other abnormalities. Urinalysis also determines how well the horse is concentrating its urine and maintaining water balance (specific gravity). Other laboratory tests are often necessary, such as ultrasound examination of the kidneys, urine culture, and endoscopy of the urinary tract.
Most horses with acute renal failure recover with appropriate treatment. For these horses, it is important to keep them eating and drinking normally; specific dietary management is less essential. Dietary management of chronic renal failure is aimed at reducing calcium intake and avoiding excessive dietary protein. Mature adult horses (1000 lb) in light work require approximately 700 g of protein per day. Assuming a total daily intake (for feed and forage combined) of 1.75% of the horse's body weight, this can easily be achieved with a diet containing 8-9% protein. This amount of protein can easily be met with good-quality grass hay or pasture. The low blood protein in horses with chronic kidney disease is due to losses into the urine secondary to renal damage. Unfortunately, undue protein supplementation in these cases provides no real benefit to the horse. Excessive dietary protein will make the horse urinate more and may overwork already badly damaged kidneys. Legumes like alfalfa and clover are high in both protein and calcium. Therefore, legumes should be avoided in most cases.
The only effective way to reduce blood calcium levels is to reduce dietary calcium and the amount of calcium that the kidneys have to excrete into urine. Salt supplementation (1-2 ounces, 1-2 times per day) may encourage horses to drink and urinate more. However, studies in several other species have shown that salt supplementation can worsen kidney disease. Regardless, horses with renal disease should have free-choice access to trace mineralized salt.
Some clinicians have recommended supplementing omega-3 fatty acids to horses with kidney disease. Omega-3 fatty acids have beneficial anti-inflammatory activity and may reduce renal inflammation without the possible adverse effects of nonsteroidal anti-inflammatory drugs. Feeding horses with chronic kidney disease to maintain body condition and quality of life is vital. It is more important to keep horses eating, rather than make dietary adjustments that cause them to stop eating. If a horse will only eat legumes such as alfalfa, then it can be fed in moderation.
Horses with chronic kidney disease have a guarded to poor prognosis. Those that recover from mild to moderate kidney disease can live normal lives but may maintain some degree of abnormal kidney function when their blood is analyzed. Any illness or stress that causes these horses to become dehydrated can severely impact their kidneys. Bloodwork and possibly urinalysis should be rechecked about every 6 months to monitor kidney function.