Liver disease in avian patients (Proceedings)

This article will discuss methods of recognizing and diagnosing clinical liver disease in avian species including clinical signs, laboratory testing, diagnostic imaging, biopsy, histopathology and culture.

Clinical Signs

Clinical signs associated with liver disease are highly variable and often vague enough to be attributable to many other disease processes. Because the liver is such a large organ and has an amazing ability to regenerate, single insults to the liver may not cause significant disease, and many birds will remain asymptomatic.1,2 When clinical are evident they may be nonspecific and include partial or complete anorexia, lethargy, depression, diarrhea, polyuria, polydipsia, dyspnea, poor feather quality and weakness.1,2 More specific signs of liver disease include abdominal distension, overgrowth and bruising of the beak, melena, green or yellow coloration of the urates associated with biliverdinuria, petechial or ecchymotic hemorrhage, acute hemorrhage and death.1,2 In our practice liver disease is most commonly noted in cockatiels (Nymphicus hollandicus), Amazon parrots (Amazona sp) and macaws (Ara sp).

Clinical Pathology

The liver serves several functions so diseases affecting this organ may result in both biochemical and morphologic changes. Therefore, hematology, biochemical analysis, protein electrophoresis, bile acids and cytologic evaluation of fluids or tissue are all useful tools for evaluating liver disease in avian patients. Complete blood count (CBC) changes suggestive of liver disease include leukocytosis or leukopenia (depending upon the etiology), non-regenerative or hemorrhagic anemia, monocytosis and hypo- or hyperproteinemia (depending upon the chronicity of disease). Testing for other diseases such as psittacosis, aspergillosis and mycobacteriosis may be warranted if hematologic changes so indicate.

Cholesterol

Elevated cholesterol concentrations are associated with a variety of disease conditions in companion species including endocrine disorders (hypothyroidism, hyperadrenocorticism, and diabetes mellitus), nephrotic syndrome, protein losing nephropathy, obstructive hepatobiliary disease, post prandial increase, acute pancreatitis and hepatic dysfunction. Assays which quantify cholesterol give a measured value of total cholesterol which consists of free cholesterol and cholesterol esters. The majority of cholesterol is synthesized in the liver, but a portion is also esterfied and degraded to bile acids by the liver. Reference ranges for cholesterol are poorly documented in birds; however, normal values for birds are highly variable. Elevation of cholesterol may be associated with lipemia (as seen in some species of macaws, rose-breasted cockatoos and Amazon parrots with fatty liver disease), other forms of liver disease, hypothyroidism, bile duct obstruction, starvation, high fat diets, xanthomatosis or associated with atherosclerosis.3,4 Decreased plasma cholesterol concentrations have also been associated with some cases of hepatic insufficiency and decreased cholesterol synthesis resulting from severe liver disease or malnutrition, aflatoxicosis, decreased dietary fat, E. coli endotoxemia and spirochetosis.4,5

Glucose

Hypoglycemia may result from hepatic insufficiency/dysfunction, septicemia, neoplasia, infectious/inflammatory diseases, malabsorptive disorders, starvation, and urinary disease.

Uric Acid

Uric acid is the major end product of protein breakdown in birds. It is produced in the liver and eliminated by tubular secretion independent of the patient's hydration status. Hypouricemia is infrequently seen in birds compared to hyperuricemia since the majority of UA is synthesized in the liver, hypouricemia may suggest severe hepatic disease.4

Hepatic Enzymes

The detection of hepatocellular disease in birds through enzymology can be difficult due to the lack of a truly sensitive indicator of hepatic disease. Interpretation of changes in plasma enzyme activity should be performed judiciously and with the understanding that correlation of enzyme levels with hepatocellular disease can be difficult at best and elevations of these enzymes give no indication of overall liver function or the cause of liver damage.

Alkaline Phosphatase (ALP) may arise from the liver, although activity of this enzyme in the liver is considered low.4 Elevations in plasma ALP activity in birds are more commonly associated with osteoblastic activity, bone growth and repair, osteomyelitis, nutritional secondary hyperparathyroidism and ovulatory activity.6 Increases in ALP associated with liver disease has been reported; however, ALP is associated with cell membranes and increased activity does not indicate hepatocellular damage.6

Aspartate Aminotransferase (AST, SGOT) is a cytosolic enzyme, like Alanine Aminotranoferase, and also originates from a variety of tissues including liver, heart, skeletal muscle, brain and kidney. Although interspecies variation of AST distribution in avian tissues occurs, significant elevations are usually an indicator of recent hepatocellular or muscle damage in most species.3,4,5 Although AST is not liver specific, is the most useful plasma enzyme for the detection of hepatobiliary disease in psittacine and other avian species.5 AST should be used in conjunction with other diagnostic tests such as creatinine kinase and bile acids as a means of increasing its diagnostic value.4 Decreased AST activity may be indicative of decreased liver mass.6,7

Lactate Dehydrogenase (LDH) is found in most avian tissues and is not specific for any particular organ, plasma or serum value may increase with liver or muscle damage.3,4,8 Decreased LDH may result form severe loss of hepatocellular mass.8

Protein Electrophoresis

Protein electrophoresis (EPH) is an extremely useful diagnostic tool for assessing serum or plasma protein distribution as well as diagnosis and therapeutic monitoring of many diseases conditions in many species of animals. Serum EPH identifies 5 distinct peaks including prealbumin, albumin, and alpha, beta and gamma globulins.9,10 A decrease in albumin and the globulin fractions can indicate hepatic insufficiency. An increase in albumin is commonly indicative of dehydration, while an increase in the alpha, beta and gamma globulin fractions may be indicative of systemic inflammation, possibly as a result of a hepatopathy.9,10

Bile acids

Elevations in bile acids appear to be a sensitive indicator of hepatic disease and reduced hepatic function in birds except in the case of iron storage disease where bile acids may not rise. In psittacine species a 2-to-4 fold increase in plasma bile acids may indicate a significant decrease in hepatobiliary function11,12 Bile acids levels less that 100µmol/L are considered normal, levels between 90-120 µmol/L are considered suspect, and values greater than 120 µmol/L are considered diagnostic for liver disease.1 Postprandial elevations in bile acids have also been seen in several avian species, and healthy birds with gall bladders show no significant difference in post prandial bile acids concentration when compared to species which lack a gall bladder.4,12,13 Although bile acids concentrations may be a sensitive indicator of hepatic disease and impaired hepatic function they do not indicate the type of disease present. Biopsy accompanied by histopathology or microbiology would be necessary to determine the etiology of a diseased liver. Bile acid levels which are lower than expected are associated with cirrhosis, microhepatica, poor feather quality, and overgrown misshaped beaks. Bile acid levels may also be used to monitor response to therapy.

Abdominocentesis

Indications for abdominocentesis include ascites, peritonitis, hemoperitoneum or other abdominal fluid accumulation.14,15 Abdominocentesis is performed by aseptically preparing a small area on ventral midline caudal to the point of the sternum. A small gauge needle or butterfly catheter is inserted on ventral midline just caudal to the sternum and directed towards the right side of the coelomic cavity15 and fluid is aspirated into a sterile syringe. Abdominal effusions are classified as pure transudates, modified transudates, or exudates (nonseptic, septic, malignant or hemorrhagic) based upon cellularity, color, total protein and specific gravity.15 Pure transudates resulting from changes in oncotic pressure associated with hypoproteinemias, cardiac disease, or hepatic cirrhosis are characterized by a low cellularity (total count less than 1000/(l), specific gravity of 1.020 or less, total protein of 3.0 g/dl or less, and a clear to pale yellow color.15 Modified transudates have an increased cellularity (total cell counts greater that 1000/(l but less that 5000/(l) of mononuclear cells, granulocytes and reactive mesothelial cells, and protein content.15 Exudates typically have a high cellularity (greater than 5000/(l), high specific gravity (greater that 1.020) and total protein levels greater that 3.0 g/dl.15 The predominate cell type accompanying exudates may indicate the source. For example septic exudates may have degenerate heterophils with intracellular or extracellular bacteria while neoplastic exudates may contain exfoliated tumor cells.

Cytology

Cytological samples from internal organs are made from smears, "squash" preparations, tissue imprints, aspiration or excisional biopsies. Samples may be obtained antemortem through ultrasound, endoscopic or surgical laparotomy techniques.16

Diagnostic Imaging

Radiographs are often very helpful when assessing liver size, although some variation in appearance of the liver may within and between species groups.2 Hepatic enlargement due to an infectious, inflammatory, or neoplastic etiology may appear as widening of the cardio-hepatic silhouette (loss of hour-glass appearance between the heart and liver) or displacement of ventriculus caudally or laterally on ventral-dorsal view. On a lateral project of the coelomic cavity an enlarged liver may also displace viscera from its normal location; especially elevating the proventriculus and ventriculus dorso-caudally.17 Conversely, a small liver due to microhepatica, chronic disease and cirrhosis will appear as a narrowing of the cardio-hepatic silhouette.

Ultrasonography

The benefits of using ultrasound to assess the coelomic cavity and organ systems of avian species is well documented and is best used to characterize lesions within the liver parenchyma.1,18 Certainly, there can be some difficulty gaining access to a good acoustic window due limitations in patient size and presence of air sacs; however, if the probe is kept centrally along the caudal edge of the sternum the air sacs are avoided.1 Zebisch, Krautwald-Junghanns and Willuhns (2004) describe the use of a biopsy splint to aid in the collection of ultrasound guided liver aspirates or biopsies.18 Neoplastic or granulomatous lesions, including abscesses, may appear as discrete hyperechoic lesions throughout the liver.19 A diffuse hyperechogenicity of the liver may be observed with inflammatory or infectious hepatopathies, hepatic lipidosis.19 In patients with an enlarged liver ultrasound can be used to obtain liver aspirates or percutaneous biopsies in larger species.

Endoscopic and Percutaneous Biopsy

Indications for liver biopsy include consistent elevations in AST, bile acids and abnormal findings on radiographs or ultrasound. An approach to the caudal thoracic air sacs allows visualization of the liver encapsulated within the paired peritoneal cavities—ventral and dorsal hepatic peritoneal cavities. To gain access to both liver lobes simultaneously a ventral midline approach is best. The ventral midline approach is also very useful for obtaining percutaneous biopsies. When obtaining a biopsy of the liver either percutaneously or by endoscopy it is preferable to elevate the cranial aspect of the body (approximately 30-45-degrees) in either lateral or dorsal recumbency to prevent fluid from entering into the cranial airsaces and lungs. Culture and sensitivity of liver samples as well as Histologic examination is tantamount to obtaining a definitive diagnosis of liver disease.

Hepatic Scintigraphy and Micro Positron Emission Tomography (PET)/Computed Tomography (CT) Scan

Hadley et al (2006) evaluated the use of quantitative hepatic hepatobiliary scintigraphy to assess liver function in pigeons (Columba livia). The results of this study clearly show good correlation between increased histologic damage to the liver after toxin (ethylene glycol) exposure and worsening of hepatic cellular function as determined by scintigraphy using 99m Tc-mebrofenin.20 Based upon these results, the area under the heart time-activity curve can be used to determine hepatic extraction as a measure of hepatic parenchymal cell function in pigeons.20 Certainly further studies are need to determine the sensitivity of hepatobiliary scintigraphy and its applications in avian medicine; however, the results are very promising.20

Positron Emission tomography examines the functional nature of a lesion versus its anatomy which is examined by computed tomography (CT) or magnetic resonance imaging (MRI). PET scans require that the patient receive a radiopharmaceutical agent (3 F-fluorodeoxyglucose [FDG]) which is distributed to tissues with higher blood flow or metabolic rates such as the brain, kidneys, inflammatory lesions and neoplastic lesions.21 The information gained from a PET scan can then be combined with 3-D reconstruction of a CT scan to further evaluate the functional and anatomical nature of a disease.21

Treatment of Liver disease

Treatment of liver disease is multifactorial involving medical and dietary management. If a definitive diagnosis is known then specific therapy should be aimed at resolution of clinical illness with appropriate therapy. Antibiotics, anti-inflammatory agents, fluid therapy, nutritional support and metabolic support are key to helping the patient recover from insult to the liver.

Nutritional therapy requires placing the bird on a low-fat diet such as a commercial pelleted diet supplemented with small amounts of fresh fruits and vegetables.22,23 Additionally, the aim of dietary management should be to increase soluble fiber as well as reduce substrates (such as protein) and thereby reduce the amount of work the liver must perform.24 Roudybush (1999) recommends reducing the level of protein to approximately 8% with a reduction of aromatic amino acids, which are poorly metabolized in the liver, to a minimum.24 Although too little protein may exacerbate lipidosis due to decreased synthesis of lipoproteins necessary to transport lipids from the liver.24 Additionally, vitamin A should be reduced to 1,500 IU/kg of diet and branched chain amino acids (leucine, isoleucine and valine) should be increased in a ration of 2:1 to aromatic amino acids.24 Excessive carbohydrates should be avoided.

Without a definitive diagnosis medical therapy consists of supportive care and adjunct therapy. Patients that are depressed or exhibiting neurologic symptoms may benefit from the use of lactulose (0.3-0.7/kg q 8-12h) (Cephulac, Marion Merrell Dow, Inc., 9300 Ward Pky, Kansas City, MO 64114).22,24,25 Birds in critical condition should receive appropriate symptomatic therapy including correction of fluid and electrolyte imbalances. Fluids containing lactate should be avoided in patients with liver disease or. Adjunct therapy has also gained favor in the treatment of liver disease in avian species. Nutriceuticals such as the active ingredients in milk thistle, particularly Silymarin, are useful in the prevention or treatment of various liver diseases. A suggested dosages range is 100-150 mg/kg PO q 8-12 hours.26 Johnson-Delaney suggests mixing 750mg Silymarin (milk thistle) in 10ml of lactulose and administering 75 mg/kg PO q 12 hours.27 Birds with elevated cholesterol and lipidemia may benefit from policosanol.27 SAM-e has also been recommended for its hepatoprotective abilities. Long-term management required routine evaluation of liver enzymes as well as bile acids.

Colchicine is known for its anti-inflammatory properties and is used in the treatment of gout or hepatic fibrosis/cirrhosis at a dose of 0.04 mg/kg PO q 12-24, although it may cause gout in some cases.28 Ritchie and Harrison (1997) recommend a dose of 0.2 mg/kg PO q 12h.29 Ursodeoxycholic acid has also been used to treat liver disease in birds.

References provided on request