all aboutjaundice/icterus Dr. Stuart Walton, BVSc (Hons), BScAgr (Hons), MANZCVS (SAIM), DACVIM Jaundice/Icterus is a yellow discoloration of the skin, mucous membranes, sclera, and bodily fluids (plasma and urine) secondary to an increased concentration of bilirubin in serum and tissues. It poses a diagnostic challenge to the veterinary practitioner because it has several vastly different etiologies. In most instances, serum bilirubin > 1 mg/dl is considered abnormal with clinically detectable icterus occurring when the bilirubin is > 2 mg/dl (35 μmol/L) or greater than 5 – 10-fold above normal. It occurs either when the rate of bilirubin production exceeds the rate of bilirubin uptake by the hepatocytes (i.e., prehepatic e.g., hemolysis), when bilirubin cannot be handled by the liver (i.e., hepatic e.g., hepatic insufficiency), or bilirubin cannot be excreted by the biliary tract (i.e., post-hepatic e.g., biliary tract disease). Each designation may have a different or combined etiopathogenesis. Approximately 80% of bilirubin is formed after red blood cells are recycled. Approximately 20% of bilirubin is formed from the breakdown of myoglobin, cytochromes and other hemecontaining proteins within the liver. Within macrophages of the liver and spleen, heme from phagocytized senescent erythrocytes is cleaved by the enzyme, heme oxygenase, to form biliverdin, a green pigment. Biliverdin is reduced to bilirubin by biliverdin reductase. Unconjugated bilirubin (free or direct), a yellow orange pigment, is then released into the circulation and is bound to albumin. Plasma bilirubin (unconjugated) is removed from the circulation by the liver (uptake) and converted to conjugated bilirubin by hepatocytes. From here, it is secreted from hepatocytes into the biliary system, and then excreted into the intestines with bile. Most conjugated bilirubin can be deconjugated in the duodenum by gut bacteria into urobilins (e.g., urobilinogen). A small amount of this is oxidized to form stercobilin, while the rest is reabsorbed via enterohepatic circulation and then re-excreted by the liver. A small portion of urobilins are excreted into the
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urine by the kidneys. Bilirubin is not detectable in normal feline urine. In cats, the presence of bilirubin in the urine is indicative of conjugated hyperbilirubinemia. The differentials for hyperbilirubinemia and jaundice should be organized by location; 1. Prehepatic – Increased bilirubin production from destruction of erythrocytes (hemolysis) 2. Hepatic – Decreased hepatocyte uptake, conjugation and secretion of bilirubin 3. Post-hepatic - Impaired biliary excretion The first step toward an effective and efficient diagnostic workup of icteric cats is to determine whether jaundice falls into the category of prehepatic, hepatic or post-hepatic disease. Essential to a diagnosis is a thorough history and detailed physical examination which incorporates the cat's signalment. History taking should question whether there are other cats in the household and whether they are clinically well. Indoor/Outdoor status should also be ascertained. A thorough medical and dietary history should be elucidated as well as parasitic prophylaxis. The diagnostic workup should also be able to address and answer the following questions: 1. Is the source of the jaundice prehepatic, hepatic or posthepatic? Differentiating prehepatic disease from hepatic and post-hepatic disease is relatively simple and involves obtaining a small sample of blood to measure the packed cell volume (PCV) and total solids. Typical manifestations of anemia include: pale mucus membranes, lethargy, weakness/
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the yellow cat
