Inflammatory liver diseases in the dog (Proceedings)

The liver plays an important role in carbohydrate, lipid and protein metabolism as well as vitamin and mineral storage. The liver is also vital in detoxification of metabolic products (ammonia, uric acid), hormones and drugs. There are several diseases of the liver in the dog that can result in inflammation. The two most common, acute and chronic hepatitis, will be the focus of this discussion.

Signalment

There is no age, sex or breed predisposition to acute hepatitis. Breeds with an increased incidence of chronic hepatitis include the beagle, Bedlington terrier, cocker spaniel, dalmatian, Doberman pinscher, German shepherd dog, Labrador retriever, Scottish terrier, Skye terrier, standard poodle and West Highland white terrier. Most of these breeds have also been shown to have increased copper in their liver. Middle aged dogs are more commonly affected with chronic hepatitis and in certain breeds there is a sex predilection (i.e. female Doberman pinschers, male cocker spaniels).

Etiology

Acute and chronic hepatitis is often idiopathic but can also occur because of exposure to chemicals, drugs, mycotoxins and infectious agents. The liver is particularly susceptible to toxic injury because of its rich blood supply that will bring large quantities of potentially toxic substances to the liver. Infectious diseases such as infectious canine hepatitis and leptospirosis have also been associated with acute hepatitis and proposed as an etiologic agent for more chronic forms of hepatitis. Metals such as copper and iron are sometimes increased with more chronic forms of hepatitis. Copper accumulation as a primary defect is inherited as an autosomal recessive trait in the Bedlington terrier where there is a defect in copper metabolism in hepatocytes so it is not excreted normally into the bile. As a result, copper gradually accumulates in the hepatocytes resulting in inflammation. In other breeds reported to have high quantities of copper in their livers (Dalmatian, West Highland white terrier, Skye terrier, cocker spaniel, Doberman pinscher, Labrador retriever) it is not known whether it is the primary cause of hepatitis (i.e. some sort of defect leading to accumulation) or the result of secondary damage due to cholestasis (impaired excretion secondary to underlying liver disease). Iron accumulation is believed to be secondary in hepatitis, but may contribute to hepatic injury and inflammation. In chronic hepatitis immune dysregulation is suspected to play a role but whether it is the cause of disease or a response to the inciting event is unknown. The presence of lymphocytes and plasma cells is fairly common with chronic hepatitis and CD3+ T lymphocytes were the most common lymphoid cell in the liver of a group of dogs with chronic hepatitis. Antinuclear antibodies and antibodies to hepatocytes have also been found in some dogs supporting the role of an immune response. There are other systemic inflammatory or infectious diseases that can cause chronic hepatitis but in most cases the cause is unknown and the term idiopathic is used.

Clinical Signs

In acute hepatitis the signs tend to be more acute and severe. These animals may present with signs of hepatic encephalopathy and may be depressed, moribund or comatose and occasionally seizure. Lethargy, anorexia, vomiting and icterus are common. Fever may occur. Dogs may be dehydrated so mucous membranes may be tacky. Disseminated intravascular collapse (DIC) with petecchia and other signs of bleeding (hematemesis, melena) may be seen in severely affected cases. There might be pain and hepatomegaly on abdominal palpation.

In chronic hepatitis, dogs may be normal or present with weight loss, polydipsia, polyuria, intermittent vomiting, diarrhea, decreased appetite and abdominal distention. Hepatic encephalopathy, icterus and bleeding tendencies are occasionally seen in more advanced stages but seizures are rare. The liver may be normal or decreased in size in these dogs.

Laboratory Findings

In acute hepatitis neutrophilia is common. A regenerative anemia and thrombocytopenia secondary to DIC are less common. With chronic hepatitis, the anemia may be non-regenerative due to chronic disease, decreased red cell survival or intestinal hemorrhage. Rarely, dogs with copper-associated hepatitis develop an acute hemolytic anemia.

Liver enzymes are usually elevated with acute and chronic hepatitis but may also be elevated with the administration of many drugs and in non-hepatic diseases (e.g. pancreatitis, peritonitis, sepsis, hyperadrenocorticism, diabetes mellitus). The degree of elevation does not correlate with a particular disease process or prognosis. In some cases of advanced chronic liver disease (e.g. extensive cirrhosis) the liver enzymes may be normal due to significant loss of functional hepatocytes. Alanine aminotransferase (ALT) is a hepatocellular leakage enzyme that increases as a direct result of hepatocellular damage. Muscle and red blood cells (RBC) also contain small amounts of ALT so extensive RBC destruction or muscle damage may contribute to mild increases in this enzyme. Aspartate transferase (AST) is similar to ALT except it is located in the mitochondria (vs. cytosol with ALT) of the hepatocyte so elevations in liver disease are indicative of more severe damage. Increases in AST are also seen with muscle and red blood cell damage and this enzyme is less specific for liver disease than ALT. In the dog, alkaline phosphatase (ALP) is composed of bone, corticosteroid-inducible and hepatocellular isoenzymes. The bone isoenzyme may be moderately increased in young dogs less than 8 months of age or mildly increased with certain boney diseases (osteomyelitis, osteosarcoma). Historically a corticosteroid inducible ALP (c-ALP) in the dog was identified that increases with endogenous and exogenous corticosteroids. This isoenzyme was investigated in dogs to help differentiate causes of elevation in ALP (e.g. naturally occurring hyperadrenocorticism vs. chronic hepatitis). Unfortunately there has been overlap in diseases that cause increase in the c-ALP so its utility is limited. There is a hepatobiliary isoenzyme that increases with many non-hepatic diseases, as a result of administration of many drugs (via enzyme induction or cholestasis) and with numerous hepatobiliary diseases. Gamma-glutamyl transpeptidase (GGT) is more sensitive than ALP for hepatobiliary disease in dogs. Increases in GGT occur for the same reasons as ALP but there is no contribution from the bone.

Bilirubin is a normal product of hemoglobin and, to a lesser extent, myoglobin metabolism. Bilirubin is conjugated in the hepatocytes and excreted in the bile. There is no merit in determining conjugated (direct) vs. unconjugated (indirect) bilirubin to help determine the cause (prehepatic, hepatic, posthepatic) of the increased bilirubin because of overlap with various disease processes. In acute and chronic hepatitis, bilirubin may be increased due to hepatocellular damage and cholestasis. Increases in bilirubin are often more marked in acute than chronic hepatitis. Bilirubinemia results in bilirubinuria.

Bile acids are formed in the liver from cholesterol and excreted in the bile to aid digestion of fats. Bile acids are then reabsorbed in the ileum and extracted by the liver for use. Bile acids will be increased with loss of functional hepatocytes (impaired extraction) or cholestasis (impaired excretion). This test does not differentiate between types of hepatobiliary disease. Typically paired serum samples are collected due to the increased sensitivity of the post prandial sample in detecting certain liver diseases. Serum bile acids can be affected by intestinal motility (increase or decrease), infiltrative intestinal disease (decrease), intestinal bacteria (increase) and possibly ursodeoxycholic acid (increase). More recently a urine bile acids test has been utilized in dogs and cats with the advantages of requiring only a single sample and no venipuncture. The sample is collected 4 to 8 hours after a meal is administered.

Glucose may be decreased with fulminant hepatic failure. This is more common in severe forms of acute hepatitis than chronic hepatitis. If found with chronic hepatitis, hypoglycemia is considered a poor prognostic indicator.

Blood urea nitrogen is commonly decreased in dogs with liver disease. This may be due to medullary washout (secondary to polydipsia), decreased protein intake (inappetence, dietary restriction) or decreased synthesis.

Albumin may be decreased due to inflammation (acute hepatitis) or severe diffuse parenchymal dysfunction (acute and chronic hepatitis). Decreased protein intake and sequestration in ascetic fluid may contribute to decreases in albumin.

Cholesterol may be increased with hepatobiliary disease secondary to cholestasis. Rarely, cholesterol is decreased in hepatitis as a result of decreased functional mass.

Ammonia may be increased with hepatitis. The liver normally converts ammonia to urea. A decrease in functional hepatocytes decreases urea synthesis and more ammonia is left in circulation. Increases are more common with acute fulminant hepatitis because of the large reserve capacity of the liver that compensates in most cases of chronic hepatitis. Ammonia tolerance tests may be abnormal in dogs with normal fasting blood ammonia. A pre-administration blood sample is taken followed by ammonium chloride (oral or rectal) administration then a second sample at 30 to 45 minutes. Alternatively a protein meal can be fed, following a fasted ammonia, and a sample taken at 6 hours. Ammonia tolerance tests should not be run on dogs with hepatic encephalopathy or it may exacerbate signs. Ammonia is a metabolic product of cells including RBC's so blood should be collected and immediately placed in a cooled lithium heparin tube then placed in an ice bath and run within 20 minutes.

The liver synthesizes many coagulation proteins including the procoagulants factor II, VII, IX and X and the anticoagulants antithrombin III, protein S and protein C. In acute or advanced chronic hepatitis prothrombin time (PT), activated partial thromboplastin time (APTT) and proteins inactivated by vitamin K (PIVKA) may be increased. This may be due to decreased synthesis of coagulant or anticoagulant proteins or from failure of vitamin K to activate these proteins. For this reason, abnormalities in coagulation can occur with severe parenchymal or cholestatic diseases. In the presence of DIC, prolonged PT and APTT (along with low fibrinogen, low platelets and increased fibrin degradation products) may also be seen due to consumption of clotting factors.

Diagnostic Imaging

Radiographically, the liver may appear normal, decreased or increased in size. In acute hepatitis the liver is normal or increased in size with normal borders, whereas with chronic hepatitis the liver is normal or decreased in size and the borders may appear rounded. Decreased serosal detail may be found due to abdominal fluid in both acute and chronic hepatitis. Transabdominal ultrasound may reveal fluid and normal, nodular or heterogenous hepatic echotexture. If cirrhosis is present the surface of the liver may be irregular. Acquired portosystemic shunts, typical seen as aberrant vessels in proximity to the kidneys, are seen in some cases of chronic hepatitis when extensive fibrosis has led to portal hypertension.

Liver Sampling

Aspirates of the liver parenchyma for cytology are not recommended for the diagnosis of inflammatory liver diseases. Histology is also required to provide information about disease severity and to provide prognostic information such as degree of necrosis, the presence of nodular regeneration and amount and location of fibrosis. Liver biopsies can also be used to monitor treatment and progression of acute or chronic hepatitis. Clotting factor deficiencies may occur so a platelet count, PT and APTT or, alternatively, a PIVKA test should be performed within 24 hours of the biopsy procedure. If the PT or APTT are greater than twice normal or the PIVKA's are increased then 0.5 to 1.5 mg/kg vitamin K1 (phytonadione) may be given SQ or IM q 12 hours for 36 hours prior to biopsy. It is important to remember that vitamin K is only beneficial when cholestasis is the cause of the prolonged clotting times. If diffuse parenchymal disease is responsible, a fresh frozen plasma transfusion is recommended prior to biopsy to restore clotting factors. Biopsies require general anesthesia and can be taken via the tru-cut method, a surgical wedge technique or utilizing laparoscopy. These techniques are described under feline inflammatory diseases. Quantification of copper and iron is routinely performed in cases of chronic hepatitis in dogs.

With hepatitis, there is an initial insult that damages hepatocytes leading to apoptosis and necrosis. In most instances inflammatory cells are then recruited and, depending on the amount of damage to the structural framework of the liver, complete regeneration may occur. If extensive damage and cell death has occurred, focal hepatocellular and ductular proliferation may result in nodular regeneration. As a result of chronic inflammation, fibrosis may occur. Cirrhosis also occurs as a result of this distortion of normal architecture and is defined by the presence of non-functional regenerative nodules within the parenchyma. In both forms of hepatitis inflammatory cells include neutrophils, lymphocytes and plasma cells. In acute hepatitis, neutrophils are more common whereas in chronic hepatitis lymphocytes and plasma cells are the predominant inflammatory cells. In copper-associated hepatitis, accumulated copper causes hepatocellular necrosis leading to inflammation, chronic hepatitis and, if untreated, cirrhosis. Normal hepatic copper levels are < 500 ppm. Damage occurs at levels > 1,000 ppm. Bedlington terriers homozygous recessive for the defect in the COMMD1 gene may accumulate copper up to 6 years of age then it decreases but levels may increase up to 12,000 ppm. The diagnosis is made by repeated liver biopsies at 6 and 15 months. Unaffected dogs will be normal at 6 months. Heterozygotes will have increased copper levels at 6 months that have decreased at 15 months. Affected homozygotes will be increased at 6 months and even higher at 15 months. There is also a DNA test (VetGen®) available that identifies the COMMD1 deletion. In other breeds, copper levels are variable and a primary defect has not been identified.

Treatment

A detailed discussion of the treatment of hepatic encephalopathy will not be provided here but some of the therapeutics for inflammatory liver disease also addresses hepatic encephalopathy

Dietary recommendations depend upon the clinical presentation of the dog but diets designed for liver disease contain lower quantities of higher quality protein, increased digestible carbohydrates and lower sodium. Low protein will decrease the amount of ammonia produced in the intestinal tract and increasing the quality allows for continued cell repair. Soluble carbohydrates are metabolized by colonic bacteria allowing for increases in branched chain amino acids that trap ammonia within the colon so it cannot be absorbed. Soluble fiber also increases fecal mass and stimulates evacuation. Sodium restriction may be helpful with ascites due to portal hypertension. These diets may also have increased zinc and lower copper levels to decrease copper accumulation. There may be increased amounts of the antioxidants vitamins C and E.

Hepatitis is rarely due to a bacterial infection so antibiotics are not recommended. An exception to this would be infection with Leptospira spp.. Antibiotics are occasionally used in cases of fulminant acute hepatitis when there is concern about the liver's ability to combat infection but this is unlikely because of the reserve capacity of the liver.

Some bile acids are more hydrophilic than others. Less hydrophilic (more hydrophobic or lipophilic) bile acids are more toxic and induce cellular apoptosis and alter mitochondrial membrane permeability increasing free radical production and oxidative damage. Ursodeoxycholic acid also increases glutathione and metallothionein in hepatocytes that help reduce oxidative damage. Ursodeoxycholic acid is a natural hydrophilic bile acid that increases the bile acid pool which dilutes out more harmful bile acids and induces choloresis. This improves bile flow so there is less contact of potentially toxic bile acids with cell membranes thus reducing oxidative damage to cells. Ursodeoxycholic acid is commonly administered to dogs with acute and chronic hepatitis. In acute hepatitis it may be given for 2 weeks past resolution of disease. In chronic hepatitis it may be given life long.

Glucocorticoids are not administered in cases of acute hepatitis because most dogs respond well to supportive care and in rare cases in which infection is involved they would be contraindicated. The use of glucocorticoids is common with chronic hepatitis. Reasons for this were previously discussed under etiology. There is also evidence that glucocorticoids increase quality of life and survival times in dogs with chronic hepatitis. Doses of 1 – 2 mg/kg/day are given for 4 to 6 weeks then tapered. Affected dogs may require intermittent or lifelong treatment.

The primary sources of free radicals in the liver are the mitochondria in the hepatocytes, cytochrome P450 enzyme systems and Kupffer cells that have been activated by endotoxins. Free radicals take up electrons from neighboring molecules causing oxidative damage to lipids, proteins and DNA. Glutathione (GSH) peroxidase and superoxide dismutase (SOD) are normal cellular defenses. S-adenylmethionone (SAMe) is natural metabolite found in hepatocytes. It is a precursor for cysteine which is one of the amino acids that makes up GSH. SAMe is also a methyl donor for methylation reactions that are important in normal liver function. SAMe is most commonly given to dogs and cats with liver disease that are at a risk of oxidative damage. Silymarin (silibinin) is the active ingredient from the milk thistle fruit and increases SOD. Silymarin and SAMe have been shown to be effective in reducing oxidative damage in dogs and humans with mushroom (Amanita phalloides) and acetaminophen toxicity. Vitamin E is a fat soluble vitamin that decreases membrane peroxidation and free radical production. There are no studies supporting or refuting its use in dogs with liver disease. Because it is fat soluble, large doses may interfere with the absorption of other fat soluble vitamins such as vitamin K. For this reason it is not recommended in liver diseases in which vitamin K deficiency (severe cholestasis) is a concern.

Inflammation and damage to hepatocytes results in fibrosis. Colchicine is believed to stimulate collagenase which should break down collagen. There is no proven benefit in man or animals regarding efficacy.

Penicillamine and trientene are chelators used to mobilize copper from hepatocytes into circulation where it is excreted in the urine. Intestinal side effects can occur and they are teratogenic. There has been more experience with penicillamine as a chelator in dogs but trientene has been used and is effective. Follow-up biopsies are recommended to obtain hepatic copper levels < 500 ppm. Zinc increases the production of metallothionein in intestinal epithelial cells. Metallothionein binds dietary copper and it is lost as the intestinal epithelial cells are sloughed. Side effects are intestinal upset and hemolysis (at higher serum concentrations). These drugs must be given for several weeks to months before clinical improvement.

Extensive fibrosis and cirrhosis leads to portal hypertension and ascites. The ascites leads to a decrease in intravascular volume and activation of the renin-angiotensin-aldosterone system (RAAS) causing sodium retention (with water) and potassium loss. This exacerbates portal hypertension. Aldosterone antagonists, such as spirinolactone, are preferred because they antagonize the effects of aldosterone. Loop and thiazide diuretics cause intravascular volume depletion and further potassium loss which activate the RAAS. Unfortunately spirinolactone has weak diuretic effects and it is often necessary to add another more potent diuretic to control ascites.

Prognosis

The prognosis is dependent upon the underlying cause of hepatitis. In general acute hepatitis carries a better prognosis than chronic hepatitis. If, however, there is extensive damage to the liver complications such as DIC, multi-organ failure and death may occur. With persistent inflammation acute hepatitis may develop into chronic hepatitis. Follow-up biopsies 4 to 6 weeks after resolution of disease (clinical signs, laboratory data) can identify this. The prognosis for chronic hepatitis is more guarded. Dogs with chronic hepatitis can live for days to years with complete resolution in some dogs. Chronic hepatitis requires treatment until histologic recovery is noted which is life long in most cases. Fibrosis and cirrhosis are irreversible so dogs with extensive fibrosis or cirrhosis will not have resolution of their disease. Cirrhosis is a poor prognostic indicator.

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