Avian Bile Acids

Birds lack bilirubin reductase and instead excrete biliverdin into the bile. Small quantities of bilirubin are detectable in avian plasma due to apparent non-specific reduction. The biliverdin is unconjugated and does not accumulate in tissue. There are no commercial test kits available to measure avian biliverdin. Carotenoid pigments can contribute to yellow color in avian plasma, but controlled studies have not ruled out pathologic contributions to plasma
color.
Bile acids are synthesized in the liver from cholesterol. In seed-eating birds, chenodeoxycholic acid is the primary bile acid produced. Bile acids are stored in the gallbladder and released after a meal. In non-gallbladder containing birds an increased release has also been documented. Bile acids assist in digestion and absorption of fats. A majority of the bile acids are absorbed by the small intestine, transported to the liver by portal veins, and reclaimed by the liver.
Elevations in bile acids are associated with reduced hepatic function. Clinical experience with bile acid assays indicate that this procedure is much more sensitive than looking at liver-related enzymes. Enzymes measuring hepatocellular damage will often return to normal after acute insult. Persistent loss of liver hepatic function is better indicated by measuring bile acids. When single-point assays are performed, the patient should be fasted for up to twelve hours (raptors up to 24 hours). The clinician may want to avoid fasting the seriously ill bird. Post-prandial bile acid assays will show moderate elevations in normal birds.
Clinical elevations in bile acids do not indicate the type of hepatic pathology or dysfunction present. Persistent elevation in bile acids provide a strong indication for a liver biopsy. A biopsy will reveal the exact pathology, enabling the clinician to tailor specific therapy for the patient. Early recognition, diagnosis and treatment is the goal. This tool has allowed the clinician to recognize liver problems years before the onset of terminal disease, in some cases, when the patient is otherwise clinically normal physically and biochemically.
While reference ranges have not been established for many species, most birds appear to have fasting levels below 100 µmol/liter, with Amazons apparently running somewhat higher. We recommend liver biopsies in patients with values which repeatedly exceed 200 µmol/liter. Patients with values exceeding 500-700 µmol/liter are more likely to already have advanced hepatic pathology.
Bile acid testing should be considered in any bird who is acutely or chronically ill bird, when there is a long-term malnutrition history, or for avians over five years of age.
Most laboratories perform the photometric enzymatic assay rather than the radioimmunoassay (RIA) method. Artifactual elevations can be caused by lipemia or hemolysis with the photometric method.