Review of Pasture-Associated Liver Disease

Diseases of the liver can be a diagnostic and therapeutic challenge for equine practitioners. Clinical signs of liver dysfunction are often nonspecific and result from abnormalities of metabolism and excretion. The most common etiologies of pasture-associated liver disease are pyrrolizidine alkaloids, panicum grasses, and clover (alsike and red clover). Diagnostic testing is valuable for determining etiology and prognosis as well as for directing appropriate therapy. Authors’ addresses: Texas A&M University, Department of Large Animal Clinical Sciences, 4475 TAMU, College Station, TX 77843 (Elfenbein); and University of Florida, Department of Large Animal Clinical Sciences, PO Box 100136, Gainesville, FL 32610 (House); e-mail: jelfenbein@ cvm.tamu.edu.

1. Introduction

The liver regulates energy metabolism and biotransforms and eliminates foreign substances. Clinical signs of liver dysfunction are often nonspecific and result from abnormalities of metabolism and excretion. Common historical and physical examination abnormalities include icterus, poor body condition, anorexia, lethargy, and intermittent mild colic. Phylloerythrin, a byproduct of bacterial degradation of chlorophyll, may accumulate in blood and tissues, causing photosensitization. Cerebral dysfunction, also known as hepatic encephalopathy, may range from subtle behavioral changes to stupor and coma. Other signs of liver disease include diarrhea, ventral edema, pruritus, polydipsia, bilateral laryngeal paralysis, and injected mucous membranes. Because loss of more than 80% of liver function is necessary for clinical signs to appear, liver disease is often severe and widespread prior to diagnosis.¹  The purpose of this review is to review the most common etiologies of pasture-associated liver disease, diagnostic testing options, and treatment.

2. Etiologies

Chronic megalocytic hepatopathy is the most common cause of chronic hepatic disease in horses in the United States.² It is caused by ingestion of pyrrolizidine alkaloid (PA)-containing plants including Crotalaria spp., Senecio spp., Amsinckia spp., Heliotropium europaeum, Echium plantagineum, and Cynoglossum officinal.^3–8 Toxicity is cumulative over the course of a lifetime; with consumption of 2% to 5% body weight required for induction of liver disease.³ These plants are generally unpalatable to horses, but palatability increases with weather extremes such as drought or frost. The alkaloid remains toxic despite drying, so horses may become intoxicated by exposure to plants baled into hay. In addition, pastures may become overgrown with the plant, increasing the likelihood of consumption. Toxicity occurs in horses of all ages and breeds, but not all horses on a farm will develop signs of hepatic dysfunction.⁹

Clinical signs of PA toxicity typically develop weeks to months after consumption of sufficient quantity of the toxic plant and are consistent with chronic hepatic disease.^3,5 Clinicopathologic abnormalities at the time of diagnosis include but are not limited to increases in γ-glutamyl transferase (GGT), alkaline phosphatase (ALP), and serum bile acids.⁶ Serum bile acids concentrations greater than 50 µmol/L have been associated with an increased risk of nonsurvival.⁵ Horses with subclinical PA toxicity may have increased activities of GGT¹⁰; it is therefore recommended to serially monitor GGT in horses on the same property as a known case.

The alkaloid causes cross-linking of DNA, formation of DNA adducts, and DNA strand breakage.¹¹ By cross-linking DNA, hepatocytes are incapable of undergoing nuclear division, thus preventing regeneration of parenchyma in the face of cellular damage. This leads to fibrosis of the hepatic parenchyma. The hallmark histologic findings in PA toxicity are therefore megalocytosis (presence of cells with very large cytoplasm and a single nucleus), bridging portal fibrosis, and biliary hyperplasia. Although reports exist of horses surviving PA intoxication, the prognosis is generally poor especially when clinical signs are apparent. No specific treatment exists, but life may be prolonged by appropriate attention to supportive care and nutrition. The disease may be prevented by removal of all PA-containing plants from pasture and ensuring that hay sources are free from PA-containing plants.

Toxicity has been reported from ingestion of alsike clover (Trifolium hybridum) or red clover (Trifolium pratense). These legumes grow in moist soils and have been cultivated as a forage source, often alongside alfalfa.¹² Horses become exposed either through pasture grazing or consuming hay containing the plant. The toxic principle is unknown,¹² but ingestion of a diet of 20% alsike clover for 2 weeks will induce signs of liver disease.¹³ Clinical disease may manifest as acute or chronic hepatic dysfunction. Clinical signs of acute disease are those of hepatic encephalopathy such as lethargy, depression, altered mental status, head pressing, and behavioral changes. Signs of chronic disease include ill thrift, anorexia, icterus, and lethargy.¹⁴ Photosensitization has been reported frequently with the disease but may represent a separate disease process, as many horses with photosensitization lack liver disease.¹⁴ This disease is distinct from slaframine toxicosis (aka black patch disease or “slobbers”), which is caused by ingestion of red clover infected with the fungus Rhizoctonia leguminocola and characterized by hypersalivation without effects on the liver. It is also important to note that alsike clover and red clover are a different genus from sweet clover (Melilotus sp.). Moldy sweet clover consumption would result in clinical signs of coagulopathy.

Laboratory abnormalities associated with clover toxicity include hyperbilirubinemia, increases in liver enzymes, and increases in serum bile acids. Gross postmortem abnormalities include an enlarged liver, and histopathology reveals biliary hyperplasia and periportal fibrosis that may cross the limiting plate.¹² Inflammatory changes, megalocytosis, and bile retention are not features of the disease, and hepatocyte regeneration may be present. Treatment is focused on supportive care and removal of the plant from the premises and from hay. The prognosis is fair, depending on the degree of fibrosis. Regeneration of the parenchyma remains possible, and the liver may regain function.

Hepatic toxicity has also been documented from ingestion of hay containing fall panicum (Panicum dichotomiflorum) in multiple horses in a boarding stable¹⁵ and has been reported in horses grazing the grass.¹⁶ Horses naturally intoxicated as well as two horses experimentally intoxicated developed signs of liver disease including icterus, lethargy, anorexia, and weight loss after 2 to 3 weeks of eating the contaminated hay. The most notable laboratory abnormalities included marked increases in the activities of GGT, sorbital dehydrogenase (SDH), ALP, aspartate aminotransferase (AST), and mild to moderate increases in creatine kinase. Marked increases in concentrations of total and unconjugated bilirubin and serum bile acids may also occur. Histopathologic abnormalities were predominately hepatic necrosis with mild hepatocyte vacuolation, fibrosis, and bile duct proliferation. The prognosis is fair to poor, with 5 in 14 horses euthanized because of clinical deterioration.¹⁵

3. Diagnostic Testing

Laboratory testing is required for diagnosis of liver disease. Clinicopathologic data, ultrasound examination, and liver biopsy provide the most comprehensive diagnostic utility. The enzymes that are specific for liver in the horse are GGT and SDH. GGT is a microsomal protein present on the surface of epithelial cells of the biliary tract, renal tubules, mammary gland, and pancreatic tissue. An increase in the activity of GGT in serum or plasma is specific for cholestatic disease. Other causes of increases in plasma GGT include pancreatic disease (extremely uncommon) and large colon displacement. The half-life of plasma GGT is 3 days; therefore GGT may remain increased for days to weeks after an acute insult.¹⁷ SDH is a cytosolic enzyme that is released with hepatocellular necrosis or changes in hepatocyte membrane permeability. It is not an inducible enzyme, and increases are specific for hepatocellular disease. The half-life of plasma SDH is 12 to 24 hours, with values returning to normal within days of an acute insult. The stability of SDH at room temperature is poor, with activities remaining stable for only 4 hours. It loses 3.5% activity per day when refrigerated and 1% activity per day when frozen.¹⁸ It is therefore important to measure SDH within hours of blood collection or refrigerate or freeze samples as soon as possible after blood collection. [...]