Equine Hepatic Disease and Biliary Disorders – NAVLE Study Guide

Overview and Clinical Importance

Hepatic disease in horses represents a significant diagnostic and therapeutic challenge in equine practice. The liver comprises approximately 1.5% of body weight and possesses remarkable regenerative capacity, which paradoxically means clinical signs often do not manifest until 70-80% of hepatic function is compromised. This delayed presentation makes early detection challenging and emphasizes the importance of understanding subtle clinical and laboratory indicators.

A unique anatomical feature of equine hepatobiliary anatomy is the absence of a gallbladder. Bile flows continuously from the liver directly into the duodenum via the common bile duct. This anatomical distinction affects the pathophysiology and clinical presentation of biliary disorders in horses compared to other species.

Hepatic Anatomy and Physiology

Functional Anatomy

The equine liver is positioned in the cranial abdomen, primarily on the right side, and can be imaged ultrasonographically from the 9th to 15th intercostal spaces on the right side. The liver receives dual blood supply: approximately 75% from the portal vein (nutrient-rich blood from GI tract) and 25% from the hepatic artery (oxygenated blood).

Key Hepatic Functions

Clinical Signs of Hepatic Disease

Clinical manifestations depend on the nature (acute vs chronic), severity, and primary site of injury (hepatocellular vs biliary). The onset of signs is often acute even when the underlying disease process is chronic, reflecting the liver's remarkable functional reserve.

Clinical Signs by Category

Hepatic Encephalopathy

Hepatic encephalopathy (HE) results from the accumulation of neurotoxins, primarily ammonia, due to impaired hepatic detoxification. The severity reflects the extent of hepatic failure and may occur with acute or chronic liver disease.

Pathophysiology

The liver normally converts ammonia (produced by intestinal bacteria and protein metabolism) to urea via the urea cycle. When hepatic function is severely compromised, ammonia crosses the blood-brain barrier and is metabolized by astrocytes to glutamine. This causes astrocyte swelling, cerebral edema, and altered neurotransmission leading to neurologic dysfunction.

Stages of Hepatic Encephalopathy

Diagnostic Approach

Hepatic Enzyme Profile

Understanding hepatic enzymes is crucial for both diagnosis and NAVLE success. Enzymes are classified as liver-specific (SDH, GLDH) versus non-specific (AST, ALP, LDH), and as hepatocellular versus biliary/cholestatic markers.

Liver Function Tests

Diagnostic Imaging

Ultrasonography is the imaging modality of choice for equine hepatic disease. The liver is best visualized on the right side from the 9th to 15th intercostal spaces. Normal hepatic parenchyma has homogeneous echotexture with sharp, crisp liver margins.

Ultrasound Findings in Hepatic Disease

Liver Biopsy

Liver biopsy provides definitive diagnosis and is performed percutaneously using ultrasound guidance at the 12th to 14th intercostal space on the right side. Contraindications include severe coagulopathy (check PT/PTT first), severe thrombocytopenia, and suspected hepatic abscess.

Specific Hepatic Diseases

Theiler's Disease (Idiopathic Acute Hepatic Disease)

Etiology and Pathogenesis

Theiler's disease, also known as serum hepatitis or idiopathic acute hepatic disease, is caused by Equine Pegivirus-H (EqPV-H), formerly known as non-primate hepacivirus. It remains one of the most common causes of acute hepatic failure in horses.

The disease classically occurs 4-10 weeks after administration of equine-origin biologics, particularly tetanus antitoxin (most common association), pregnant mare serum, and equine plasma products. Broodmares are frequently affected because they commonly receive tetanus antitoxin postpartum.

Clinical Presentation

• Acute onset of clinical signs (often within days)
• Severe icterus (prominent finding)
• Hepatic encephalopathy (majority of cases)
• Anorexia and depression
• History of biologic administration 4-10 weeks prior

Diagnosis

• History of equine-origin biologic administration
• Markedly elevated hepatocellular enzymes (SDH, GLDH, AST)
• Elevated bile acids and ammonia
• Histopathology: centrilobular to massive hepatic necrosis
• PCR for EqPV-H (definitive)

Prognosis

Mortality rate is 50-90%. Prognosis is poor once hepatic encephalopathy develops. Surviving horses may recover completely due to hepatic regeneration.

Cholangiohepatitis and Cholelithiasis

Etiology

Cholangiohepatitis is inflammation of the bile ducts and hepatic parenchyma, typically caused by ascending bacterial infection from enteric organisms including E. coli, Salmonella, Clostridium, Enterococcus, and other gram-negative bacteria. The absence of a gallbladder means bile flows continuously, which normally reduces bacterial colonization but does not eliminate the risk.

Cholelithiasis (biliary stones) occurs most commonly in middle-aged to older horses. Choleliths in horses are primarily composed of calcium bilirubinate and are associated with cholangitis. Obstruction of bile flow causes increased biliary pressure, abdominal pain, and secondary hepatocellular necrosis.

Clinical Signs

• Intermittent or persistent colic (from biliary distension)
• Fever (due to bacterial infection)
• Icterus (often pronounced)
• Weight loss and depression
• Signs may be intermittent unless common bile duct is completely obstructed

Diagnosis

• GGT markedly elevated (greater than 15x normal) - hallmark finding
• Markedly elevated bile acids
• Leukocytosis with neutrophilia
• Ultrasound: dilated bile ducts, hyperechoic foci with acoustic shadowing (stones)
• Liver biopsy: biliary hyperplasia, inflammation, fibrosis

Treatment

• Long-term antibiotics (median 51 days; often 2-6 months)
• Broad-spectrum coverage: trimethoprim-sulfa, metronidazole, fluoroquinolones
• Ursodiol (ursodeoxycholic acid) for choleresis
• Surgical removal may be attempted for accessible stones
• Supportive care

Pyrrolizidine Alkaloid Toxicosis

Etiology

Pyrrolizidine alkaloids (PAs) are hepatotoxic compounds found in numerous plants. The most clinically significant sources include Senecio species (ragwort, groundsel), Crotalaria (rattlebox), and Cynoglossum (hound's tongue). PAs cause cumulative, irreversible hepatic damage.

Pathophysiology

PAs are metabolized by hepatic cytochrome P450 enzymes to toxic pyrroles that alkylate DNA and inhibit mitosis. This produces megalocytosis - hepatocytes become enlarged with abnormal nuclei because they cannot divide. Damage is cumulative and irreversible, with clinical signs often appearing weeks to months after exposure when hepatic reserve is exhausted.

Clinical Signs

• Often delayed onset (weeks to months after exposure)
• Weight loss (chronic, progressive)
• Icterus
• Hepatic encephalopathy
• Photosensitization (hepatogenous/Type II)
• May affect multiple horses if shared pasture contaminated

Diagnosis

• History of potential plant exposure
• Persistently elevated GGT (due to periportal fibrosis)
• Elevated bile acids
• Liver biopsy: MEGALOCYTOSIS (pathognomonic), biliary hyperplasia, periportal fibrosis

Treatment and Prognosis

No specific treatment exists. Management is supportive and includes removing access to toxic plants, providing supportive care for hepatic encephalopathy, and nutritional support. Prognosis depends on degree of fibrosis; horses with extensive fibrosis have poor prognosis. The irreversible nature of PA-induced damage makes prevention (pasture management) critical.

Treatment of Hepatic Disease and Hepatic Encephalopathy

Treatment is largely supportive and aimed at reducing ammonia production, supporting hepatic function, and managing complications.

Disease Comparison Summary