Jaundice [Icterus]

Jaundice or icterus is characterized by the yellow discoloration of unpigmented skin, mucosal and conjunctival membranes, as well as membranes over the sclera and is caused by elevated blood bilirubin concentrations. Jaundice often arises in diseases of the liver and biliary system. However, jaundice does not always occur with hepatic or biliary disease and may be conspicuously absent in acute hepatitis.

Jaundice is classified into three categories depending on its etiology: prehepatic or hemolytic, hepatic or hepatocellular and posthepatic or cholestatic.

1. Prehepatic or Hemolytic Jaundice

Hemolytic jaundice is caused by massive intravascular or extravascular hemolysis resulting in the release of red blood cell hemoglobin. The breakdown of the increased amounts of hemoglobin results in elevated concentrations of unconjugated (or indirect) bilirubin, which needs to be converted into conjugated (or direct) bilirubin by the liver before being excreted through the biliary system.

Hemolytic jaundice is common in animals and may be associated with bacterial toxins, invasion of erythrocytes by protozoa or viruses, inorganic and organic poisons, and immunologic reactions.

Diseases in which bacterial toxins cause intravascular hemolysis are bacillary hemoglobinuria of cattle and leptospirosis. The common protozoan and viral diseases in which hemolysis occur include babesiosis, anaplasmosis, eperythrozoonosis, and equine infectious anemia. Chronic copper poisoning, selenium poisoning in sheep, phenothiazine poisoning in horses and bites by snakes are other common causes.

3. Hepatic or Hepatocellular Jaundice

Hepatocellular jaundice is the result of impaired capacity of the liver to conjugate indirect to direct bilirubin, which is required for excretion of bilirubin with bile. The cause may be any of those diffuse diseases of the liver that cause degeneration of hepatic cells.

Additionally, swelling and edema in the liver caused by inflammation can result in a mechanical obstruction of the biliary flow within the liver. Mechanical stasis of the biliary flow can also be caused by fibrous tissue constriction and obliteration of the small biliary canaliculi after hepatitis and in many forms of fibrosis. Cholelithiasis, the formation of biliary calculi, is frequently reported as a cause of cholestasis in humans and has been reported in horses and cattle.

Serum concentrations of total bilirubin are increased primarily because of retention of direct bilirubin.

3. Posthepatic Jaundice

Obstruction of the bile ducts or common bile duct by nematodes, flukes, or biliary calculi, as well as compression by tumor masses, is a possible cause of posthepatic jaundice. Inflammation of the bile ducts by extension from enteritis or by infestation with trematodes can also impair the bile flow and result in elevated concentrations of direct bilirubin.

Obstruction is usually complete and results in the disappearance of bile pigments from the feces. Serum concentrations of conjugated bilirubin rise, causing a marked elevation of total bilirubin in the serum. Excretion of the conjugated bilirubin in urine occurs on a large scale.


Clinical Findings

The staining of jaundice is caused by staining of tissues, especially elastic tissue, and not by accumulation in tissue fluids, which makes it best detected clinically in the sclera. Jaundice is usually much more severe with impairment of bile flow and when bile pigments are absent from the feces.

However, obstructive jaundice can occur with only partial occlusion of hepatic flow provided at least half the bile flow is obstructed. In such cases jaundice may occur even though bile pigments are still present in the feces. With lesser obstruction the portion of the liver and biliary tract that is functioning normally excretes the extra load of bile pigments.