Nutritional Support of Horses with Liver Disease

The power of targeted equine nutrition programs is unquestionable. Take, for instance, the feeding management practices that almost entirely relieve horses of the crippling signs associated with tying-up. By reducing starch intake and filling the caloric void with fat and fiber, many racehorses genetically predisposed to the disease have become successful athletes. In these cases, the entire horse benefits from dietary manipulation. Even more precisely targeted diets can benefit horses with certain compromised organs such as horses with liver disease.

The liver accounts for approximately 1% of an adult horse’s body weight. The organ lies almost entirely to the right of median, and is encased completely by the rib cage. Of greatest significance anatomically is the absence of a gall bladder. Because of this, bile is emptied directly from the liver into the duodenum. Horses are continuous grazers and bile is constantly secreted, unlike humans and other animals with gall bladders that store and release bile when a meal is eaten.

The liver maintains a crucial role in digestive processes, and is chiefly involved in regulating nutrient distribution. Nutrients absorbed in the gastrointestinal tract must pass through the liver via portal circulation. Serving as a clearinghouse of sorts, the liver assigns nutrients to essential roles. Some are metabolized for energy, transformed to other nutrient classes, or sent to peripheral tissues, and others are stored by the liver for future use. The liver plays a vital role in maintaining consistent blood levels of nutrients between feedings.

While the liver is resistant to damage and can usually function with as little as two-thirds of its normal mass, whole-organ disease is possible, and is life-threatening when it occurs. The Kentucky Livestock Disease Diagnostic Laboratory reported hepatitis (inflammation with varying degrees of degeneration of the liver) to be the most commonly diagnosed liver problem (6-10 cases/year). Liver disease in mature horses is most often caused by exposure to toxic plants or feed toxins, infection, or bile stones. Horses can occasionally develop serious liver disease after treatment with products produced from horse blood, such as plasma or tetanus antitoxin. Miniature horses, ponies, and donkeys that stop eating, usually due to serious illness, can develop a potentially fatal accumulation of fat in the liver.

Bloodwork will usually reveal elevations in liver enzyme activity (gamma-glutamyl transferase [GGT], aspartate aminotransferase [AST], lactate dehydrogenase [LDH], and serum alkaline phosphatase [ALP]). Measurement of serum bile acids is a useful blood test that evaluates liver function. Because the liver produces many of the proteins in blood, horses with liver disease may have low blood protein concentrations, especially albumin. Bilirubin can also be elevated, but another important and common cause of elevated bilirubin is anorexia.

Biopsy is the most definitive diagnostic test for horses with suspected liver disease. Liver biopsies are generally safe, and veterinarians can perform them in standing horses with ultrasound guidance. Liver biopsy, bacterial culture, and microscopic examination can usually determine the cause of liver disease, as well as the prognosis.

Clinical signs
Clinical signs of liver disease can be vague. Affected horses may have fever (temperature greater than 101.5° F [38.6° C]), jaundice, colic, lethargy, or abnormal behavior. If blood protein concentration is low, there may be edema of the legs and bottom of the chest and abdomen.

Some horses with hepatic disease will develop photosensitization and appear to have severe sunburn on the white areas of their bodies such as on face and leg markings.

Photosensitization occurs when the liver is unable to process chlorophyll (the green pigment in plants). An intermediate metabolite of this process called phylloerythrin reacts with ultraviolet light in sunlight to produce toxic-free radicals that cause severe tissue damage.

Photosensitization looks like serious sunburn, often oozes serum, and should only be seen on the white areas of horses.

Bile stones in horses can be incidental findings, but they occasionally cause disease if they block bile ducts.

Many toxins and metabolites normally produced by gut bacteria and removed by the liver can act on the nervous system if they circulate in the bloodstream, causing some horses with severely damaged livers to develop abnormal behavior, a manifestation called hepatoencephalopathy. These metabolites are shaped like normal neurotransmitters and actually damage the nervous system (as is the case with ammonia), thus resulting in abnormal behavior. Affected horses may appear blind, head-press, seizure, or become severely depressed or maniacal.

Some horses with liver disease might have bleeding tendencies because the liver produces many of the clotting factors.

Treatment
Specific treatment of liver disease depends on its cause. Infections can be treated with antibiotics and anti-inflammatory drugs (corticosteroids or nonsteroidal anti-inflammatory drugs). Milk thistle supplements contain silymarin, which has antioxidant and other liver-protecting actions. There are several commercially available silymarin products or milk thistle seed can be ground and fed with concentrate. Colchicine and pentoxifylline are sometimes used to help prevent scar tissue or fibrosis of the liver.

Nutritional management
Nutritional management of liver disease is aimed at reducing dietary protein and the amount of ammonia and other gut-derived toxins that affect the nervous system. Lower dietary protein will result in less intestinal ammonia production. Dietary protein should be restricted as much as possible if the horse has signs of hepatoencephalopathy. Mature adult horses in light work require only about 8% protein in their diet. This requirement can be met easily with good-quality grass hay or pasture. Feeding legumes such as alfalfa and clover, which are generally high in protein, should be avoided. Some clinicians recommend oat hay. It is advisable to avoid high-fat diets in horses with liver disease to reduce the possibility of fat deposition in the liver, which can further impair its function. Horses with photosensitization can still graze or be turned out at night to avoid sunlight.

Dividing up the ration into several small meals will prevent large amounts of ammonia from leaving the gut and reduce the load of ammonia that the liver must detoxify.

Feeding protein with an increased ratio of branched chain to aromatic amino acids should improve clinical signs of hepatoencephalopathy. Branched chain amino acids can be used for energy and protein production. Aromatic amino acids (tyrosine, tryptophan, phenylalanine) are more likely to act as false neurotransmitters and produce neurologic signs. There are commercially available branched chain amino acid supplement pastes for horses that can be used to increase their proportion in the diet.

Beet pulp, corn, sorghum, wheat bran, and milo all have a favorable branched chain to aromatic amino acid ratio and are recommended to make up the majority of the diet for horses with liver disease. Although its efficacy is questionable, vinegar can be added at a rate of two cups twice per day to acidify the gut and reduce ammonia production. Antibiotics (metronidazole or neomycin) are sometimes administered to change the bacterial population of the gut and reduce gut ammonia production.

Folic acid, B vitamins, and especially fat-soluble vitamins (A, D, E, and K) should be supplemented to meet requirements. Fat-soluble vitamins should not be supplemented in excess because they can accumulate and have adverse effects. Vitamin K is important for blood clotting and will help reduce abnormal bleeding. It is more important to keep horses eating to maintain body condition, rather than make dietary adjustments that cause them to stop eating. If the horse will only eat legume forages, then they can be fed in moderation and divided into several small meals.

Hepatic disease can be a serious illness in horses and at its worst can be fatal. Biopsy of the liver is usually the only diagnostic test that can give a definite cause and prognosis. Horses with severe clinical signs and profound changes in bloodwork and biopsy often have a poor prognosis for recovery. The liver has an amazing ability to regenerate, and nutritional management and medical therapy can help affected horses regain or maintain adequate liver function.