BVSc (Hons) MMedVet (Med) PhD Dipl. ECVIM (Internal Medicine)
Bryanston Veterinary Hospital
PO Box 67092, Bryanston, 2021, South Africa
Often one is presented with an animal that shows elevated liver enzyme activity on routine biochemical profile with the question asked “does this animal have liver disease”. An important principle that should also be kept in mind when evaluating an animal with elevated liver enzyme activity is that the liver has a great reserve capacity and clinical signs of liver disease often do not appear until the disease is fairly advanced. Similarly, liver function tests only become abnormal when significant liver dysfunction is present. Because the liver is involved in many metabolic functions and there are variable hepatic responses to an insult, there is no ideal liver function test that will establish the extent of liver damage.
MECHANISMS OF LIVER INJURY
Oxidative injury involves cellular and molecular mechanisms that initiate and perpetuate damage that leads to liver injury and fibrosis. Oxidative injury and the subsequent production of reactive oxygen species (ROS) are central patho-mechanisms in most forms of acquired liver injury.
Toxins, endotoxins, and infectious agents
The central role of the liver in metabolism, detoxification processes, Kupffer cell population, and its sentinel position between the splanchnic and systemic circulatory systems puts the liver at high risk for toxic, infectious, endotoxin, and oxidant mediated injuries. Receiving 75% of its blood flow directly from the GI tract, acute and chronic enteritis can contribute to liver injury. Pancreatic inflammation imposes risks for obstructive cholestasis and hepatobiliary inflammation. A variety of toxins have been identified that specifically cause liver injury, including certain drugs (NSAIDs, phenobarbitone, primidone, diazepam), bacteria, entero- and endotoxins, toxins derived from moulds, fungi, algae, spoiled or contaminated food, and transition metals.
Cholestatic liver disease
A heterogeneous group of disorders that is associated with impaired bile flow. In severe hepatic insufficiency accumulation of membrano-cytolytic bile acids (BA) contributes to ongoing hepato-cellular injury. Noxious BA damage cell and organelle membranes, induce intracellular structural and functional change, result in inflammation, and compromise bile flow. A central mechanism of BA induced hepatotoxicity is reduction of mitochondrial glutathione (GSH) resulting in reduced production of cell energy. Additional contribution to ROS generation and oxidantant injury is the presence of neutrophils and the hepatocellular retention of copper and iron.
Immune-mediated mechanisms can perpetuate chronic inflammatory and cholestatic liver injury and augment injury instigated by infection, endotoxins, or obstructed bile flow. A variety of pathologic immunologic responses perpetuate inflammation that ultimately imposes oxidative insult. Phenomena thought to unite infection and immune responses include molecular mimicry (antigens of infectious agents that closely mimic self-antigens) or innocent bystander effects (exposure or mobilized self-antigens). These responses may culminate in a learned immune repertoire involving T- and B-cells ultimately targeting foci on normal cells. Infectious agents may initiate or aggravate responses through an adjuvant effect, providing co-stimulatory inflammatory signals or by functioning as super-antigens capable of broadly activating T-cells. Environmental factors and toxins also have been implicated in induction of chronic immune responses in humans.
Copper and iron
Although these metals function as important catalysts for enzymes and reactions essential to health, pathologic hepatic accumulation imposes oxidant injury. Mitochondria are a primary site of injury where impaired GSH concentrations disrupt cell energy production.
Iron accumulates in macrophages in many inflammatory disorders resulting in generation of free radical reactions as well as initiating and promoting fibrogenesis. Increased liver copper concentrations may derive from genetic disorders (transport or storage) but is more common secondary to cholestasis in dogs. Cholestasis from any cause will impede biliary copper excretion causing eventual lysosomal loading, leading to organelle damage secondary to cell membrane oxidation. Lysosomal rupture leads to hepatocyte death. Accumulation of copper storage “granules” can be identified on routine histopathology and are often over-interpreted as a copper storage disease.
Special stains for iron (Prussian blue) and copper (rhodanine or rubeanic acid) must be reconciled with quantitative metal analysis (ug/gm dry weight tissue) and the histological interpretation of a biopsy.
COMMON LIVER DISEASES
Reactive hepatopathy often occurs secondary to extrahepatic disease but can result in both serum biochemistry and histopathology liver abnormalities. Most of the reactive hepatopathies result in elevated liver enzyme with little change in bilirubin, albumin, glucose, and BA concentrations, which supports the concept that there is generally minimal hepatocellular dysfunction in the majority of these disease conditions.
A reactive hepatopathy is not a specific histological diagnosis rather grouped as a number of entities that are not associated as a primary liver disease. Findings are often characterized by non-specific hepatocellular degeneration or multifocal necrotic changes without evidence of chronic progressive inflammation. The reason the liver often undergoes these changes is that the liver is involved in many metabolic and detoxification functions and is very dependent on adequate oxygenation. Endogenous toxins, anoxia, metabolic changes, nutritional changes, and endogenous stress related glucocorticoid release could be responsible for the majority of these changes. These changes are usually very reversible and no specific hepatic therapy is required short of treating the primary disease and providing adequate liver support. The liver changes resolve once the primary aetiology is successfully treated.
A histological report of a vacuolar hepatopathy is often frustrating in determining the underlying aetiology. Hepatocellular vacuoles distending the cytosolic compartment may contain, fat, glycogen, intracellular water (oedema) or other metabolic wastes or intermediates. Glucocorticoid hepatopathy can occur in the dog secondary to exogenous or endogenous glucocorticoids. Concurrent illness or chronic stress can also cause a steroid hepatopathy and elevated ALP activity.
Idiopathic vacuolar hepatopathy is a frustrating diagnosis frequently observed in older dogs not associated with steroids or chronic stress related disease. For all intents they appear typical of steroid hepatopathy based on histology and elevated ALP activity but without clinical or laboratory evidence of Cushing’s disease, steroid therapy or other chronic disease. The liver of these dogs contains excess glycogen identical to the typical steroid hepatopathy. Animals having vacuolar hepatopathy and increased ALP activity without overt Cushing’s disease may have abnormal concentrations of other adrenal steroids (progesterone, oestradiol, and 17-hydroxy-progesterone).
Nodular hyperplasia is a relatively benign process that may cause abnormal hepatic test results and histopathology changes. Nodular hyperplasia is usually associated with variable elevations of liver enzyme activity. Ultrasound may be normal or may demonstrate nodules. Biopsy will confirm the diagnosis. A wedge section is preferred, as a needle biopsy may not demonstrate the nodules. No specific therapy is indicated.
Chronic hepatitis is the most common and most important liver disease diagnosed in the dog. The aetiology includes copper toxicity from abnormal metabolism, drugs, infectious agents, and possibly immune mechanisms. It is observed most often in middle-aged female dogs. Chronic hepatitis in Doberman pinschers and Cocker spaniels is considered to be an inherited liver disease. Other breeds that appear to have increased incidence include Labrador retrievers, Standard poodles and Scottish terriers.
Hepatic copper toxicity was first identified in Bedlington terriers as a genetic defect in copper metabolism. Liver disease with concurrent copper accumulation is also reported in the Doberman pinscher, Dalmatians, West Highland white terrier, Labrador retriever and Skye terrier.
The clinical signs parallel the extent of hepatic damage. Early in the disease there are usually no or only minimal clinical signs. Only after the disease progresses do the clinical signs of liver disease become evident such as ascites, icterus, and hepatic encephalopathy. With development of these late signs the long-term prognosis is generally poor. A presumptive diagnosis is made based on the clinical features and persistent elevated liver enzyme activity – mainly ALT with variable ALP activity. The diagnosis is supported by abnormal bile acid concentrations and ultrasound findings. A definitive diagnosis requires a hepatic biopsy showing characteristic morphological patterns. Anti-inflammatory therapy, copper reduction, and general liver support are included in the therapy.
Acute hepatic necrosis
Hepatocytes may be killed by various insults including hypoxia, toxins, drugs, micro-organisms, immunological events, and severe metabolic disturbances. When hepatic death is severe clinical signs of liver disease occur. The clinical course is acute, characterized by massive increases in liver enzyme activity. When the damage is severe liver function declines and clinical evidence of liver failure occurs. The prognosis for recovery depends on the degree of hepatic damage, ability of hepatic regeneration, and development of secondary complications. General liver support and antioxidant therapy is the mainstay of the management.
Many drugs have been reported to cause hepatic disease in animals with the most common ones being acetaminophen, anabolic steroids, anticonvulsants, chemotherapeutic drugs, azathioprine, carprofen, diazepam, furosamide, glucocorticoids, griseofulvin, halothane, itraconazole, ketoconazole, mebendazole, mitotane, sulphonamides, tetracycline, and trimethoprim.
Several types of hepatopathy are associated with drug administration, including hepatocellular necrosis, cholestasis, chronic active hepatitis, and vacuolar changes. Hepatotoxic drugs can be classified into those causing predictable hepatic damage (intrinsic toxicoses) and those that are idiosyncratic in their potential to cause hepatic damage. Drugs causing intrinsic hepatic damage have a high incidence of hepatotoxicity, are usually dose-dependent, predictable, and can be reproduced in experimental animals. On the other hand, idiosyncratic reactions occur in a small percentage of animals, occur randomly, are usually not dose or duration dependent, and are difficult to reproduce experimentally. Idiosyncratic toxicosis is the result of an unusual susceptibility of an affected animal to an adverse reaction resulting from metabolic aberration, hypersensitivity, or immune-mediated events. Specific mechanisms of injury are usually unknown when idiosyncratic toxicosis occurs. In general, treatment of drug-induced hepatic disease involves withdrawal of the drug and supportive care.
Feline hepatic lipidosis is characterized by intracellular accumulation of lipid with clinicopathologic findings consistent with intrahepatic cholestasis. The triglyceride content in the liver of cats with lipidosis averages 43% compared to 1% in the liver of healthy cats. While some cases of hepatic lipidosis result from diabetes mellitus, the majority of cases are attributed to the nutritional and biochemical peculiarities of the cat as the cat does not appear very capable of regulating intermediary metabolism during starvation. Histological evidence of hepatic lipidosis was found to develop within two weeks of the onset of fasting in a feline experimental model. While many cats develop lipidosis during periods of anorexia related to another underlying disease, otherwise healthy cats can also develop lipidosis due to inadequate intake during periods of enforced weight loss, unintentional food deprivation, or stress. This understanding has emphasized the importance of maintaining food, especially protein, intake in cats that become anorexic for any reason for longer than a few days.
Chronic cholangitis usually affects middle-aged and older cats and is characterized by a mixed inflammatory response (lymphocytes, plasma cells and neutrophils) within the portal areas and bile ducts. Other features include marked bile duct proliferation, bridging fibrosis, and pseudo-lobule formation. Chronic cholangitis may represent a persistent bacterial infection or an immune-mediated response that results in a chronic self-perpetuating disorder and is often associated with inflammatory bowel disease or pancreatitis. Clinical signs are usually of a chronic, intermittent or persistent nature. Vomiting, icterus, and hepatomegaly are common findings, and ascites may be present. Hepatic encephalopathy and a haemorrhagic diathesis are uncommon unless severe end-stage liver disease is present.
Lymphocytic portal hepatitis
As opposed to cats with cholangitis, there is a lack of neutrophilic inflammation, bile duct involvement, infiltration of inflammatory cells into the hepatic parenchyma, or periportal necrosis, and is not associated with inflammatory bowel disease or pancreatitis. Lymphocytic portal hepatitis is a common finding in liver biopsies of older cats. In one study, 82% of cats greater than 10 years old had histopathology changes consistent with lymphocytic portal hepatitis, whereas only 10% of cats younger than 10 had these histopathological changes. This is suggestive that it is either a common aging change or that a sub-clinical form of disease is prevalent. Lymphocytic portal hepatitis appears to progress slowly with varying degrees of portal fibrosis and bile duct proliferation but no pseudo-lobule formation. Concurrent hepatic lipidosis is less likely than with cholangitis.
Neutrophilic or acute cholangitis
This disorder is primarily in young to middle-aged male cats with clinical signs of acute vomiting, diarrhoea, anorexia, and lethargy. Clinical examination findings include pyrexia, dehydration, icterus, abdominal pain, and hepatomegaly. Laboratory findings frequently reveal mild to moderate leukocytosis with mild to moderate elevations in liver enzyme activity. Cats affected with this form of cholangitis often have concurrent pancreatitis and inflammatory bowel disease. Acute cholangitis may begin as an ascending bacterial infection within the biliary tract as bacterial isolates in affected cases often include E. coli, Enterococcus, Bacteroides, and Clostridia.
These include chronic hepatitis, cholangiohepatitis, extra-hepatic bile duct obstruction, lobular dissecting hepatitis, and repeated toxin-induced injury. Treatment options include immune-modulation, antioxidants, ursodeoxycholic acid, and anti-fibrotics.
These include parenchymal disorders with hyperbilirubinaemia or elevated bile acids, cholangiohepatitis, cholangitis, extra-hepatic bile duct obstruction, hepatic lipidosis, and severe vacuolar hepatopathy. Treatment options include correction of any mechanical obstruction, antioxidants, ursodeoxycholic acid, and taurine in the cat.
These are the inflammatory and/or cholestatic diseases associated with high copper or iron concentrations with possible zinc depletion. Treatment involves restricting copper intake in the food and water, zinc supplementation, chelation therapy, and anti-oxidants.
Hepatic fibrosis associated with chronic hepatitis, cholangiohepatitis, extra-hepatic bile duct obstruction, and juvenile fibrosing hepatitis. Treatment options include immune-modulation, antioxidant, vitamin E, and colchicine.
Multiple toxins and drugs can affect the liver. Treatment options include eliminating the toxin, enteric removal, and antioxidants.
Portosystemic vascular anomaly
This can either be congenital macroscopic portal shunting or micro-vascular dysplasia with no inflammatory or cholestatic patho-mechanisms involved. Ideal therapy is surgical attenuation of the shunt or medical management of hepatic encephalopathy. In most cases ursodeoxycholic acid and antioxidants not required.
Can be a primary hepatopathy or associated with the chronic release of inflammatory cytokines (dental disease, inflammatory bowel disease, skin infections, neoplasia) or hypercortisolaemia. Treatment is aimed at correcting the underlying cause.
The underlying aetiology is triglyceride distension of the hepatocytes, which may be a primary disorder but often secondary to anorexia. General therapy is to identify and treat the underlying cause of the anorexia and to provide adequate nutrition. Specific treatment is acetyl cysteine, correct hypokalaemia and hypophosphataemia, and supplementation with taurine, L-carnitine, vitamin E, and water soluble vitamins.
This a term applied to liver biopsies that lack distinct pattern but showing multifocal lipogranulomas, minor lympho-plasmacytic portal infiltrate, but lacking overt necrosis, fibrosis, or architectural remodelling. Reactive hepatitis is not a disease but merely represents hepatic sentinel function and often does nor warrant anti-inflammatory and/or immune modulatory therapy.
SPECIFIC CONSIDERATIONS INTERVENTIONAL STRATEGIES
Balanced nutritional support is critical including vitamin supplements but avoiding copper supplement if there is a possibility of high tissue copper levels. Protein restriction should only be done in patients showing signs of hepatic encephalopathy as most animals with acquired hepatobiliary disease do not require protein restriction, especially cats. Cats with lipidosis may succumb subsequent to dietary protein restriction.
Approximately 65% of dogs and cats with inflammatory liver disorders have low liver GSH concentrations. Since oxidant injury is better inhibited than reversed, early pre-emptive therapy in inflammatory and cholestatic liver disease may be most effective. Antioxidant therapy should be combined with immune-modulatory, anti-inflammatory, and anti-fibrotic drugs to achieve a synergistic effect. Antioxidant drugs include acetylcysteine, S-adenosylmethionine (SAMe), vitamin E, silymarin, ursodeoxycholic acid, and zinc.
S-adenosylmethionine (Denosyl®) acts a GSH donor, and has been shown to increase hepatic GSH in healthy cats, cats with portal triad inflammation, and dogs treated with high dose glucocorticoids. Although silymarin (milk thistle) has been proven for the prevention and recovery from certain toxins (amanita mushroom, carbon tetrachloride, ethanol), clinical benefit in chronic liver disease remains controversial, despite numerous studies in humans. Ursodeoxycholic acid (Urostan®) protects against membranocytolytic bile acids in the liver, bile, and systemic circulation providing direct cytoprotection. Additional actions are immune modulation, hydrocholeresis that may aid in biliary toxin elimination, and may blunt peribiliary inflammation and fibrosis. Zinc is an essential trace element that is required for many homeostatic functions with central importance to the liver such as normal protein metabolism, function of metallo-enzymes, and membrane integrity. Zinc assists in immune functions, detoxification pathways, and has an antioxidant effect in ROS mediated injury.
Drugs that can be used are prednisolone, azathioprine, mycophenolate, metronidazole, methotrexate, chlorambucil, and cyclosporine.
Metronidazole has bactericidal, trichomonacidal, cytotoxic, immune modulatory, and antioxidant effects. Azathioprine is used for canine chronic hepatitis and lobular dissecting hepatitis when an infectious cause has been eliminated. Mycophenolate is an alternative for dogs intolerant of azathioprine. Methotrexate has been shown to be effective in some cats with chronic cholangitis when infection has been ruled out as these typically do not respond to prednisolone as single agent immune modulatory, and appear predisposed to diabetes mellitus with prednisolone therapy.
A number of drugs have anti-fibrotic properties including SAMe, vitamin E, and ursodeoxycholic acid. The major anti-fibrotic drugs are colchicine, D-penicillamine, and glucocorticoids. Colchicine inhibits hepatic fibrogenesis and fibroblast proliferation and has some anti-inflammatory effects.