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