Icterus
Also known as: Jaundice
Introduction
Icterus refers to the staining of tissues by bilirubin pigment or bilirubin complexes. Bile pigments such as bilirubin have particular affinity for elastic tissues and the typical yellow/orange colour of icterus is therefore evident in the sclera and mucous membranes in life and in the tunica intima of the aorta at post-mortem examination.
The serum bilirubin concentration of an animal indicates the degree of icterus and the condition only becomes clinically evident at levels above 2 mg/100 ml (normal range below 0.5 mg/100 ml). Bilirubin should be measured by a technique which measures the large proportion which is bound to plasma albumin.
Causes of Icterus
Pre-hepatic Jaundice
Also sometimes called Haemolytic Icterus. This condition results from increased red blood cell destruction, overwhelming the capacity of the liver to conjugate and excrete the bilirubin which is released into the plasma. The majority of the bilirubin is therefore unconjugated and, unlike the conjugated form, this cannot be excreted by the kidney. Possible causes of haemolysis and prehepatic jaundice include:
- Haemolytic bacteria, including Clostridium haemolyticum in cattle and Leptospires in various species.
- Haemolytic parasites, including Babesiosis in cattle and dogs and Mycoplasma haemofelis in cats.
- Immune reactions to red blood cells, including:
- Neonatal isoerthryolysis, resulting from the production of antibodies by the dam which are ingested by the neonate in colostrum and subsequently cause destruction of red blood cells. Bilirubin is able to cross the immature blood brain barrier and cause direct damage to the neurones of the brain, a phenomenon called kernicterus.
- Autoimmune haemolytic anaemia or immune-mediated haemolytic anaemia.
- Destruction of red blood cells of lambs fed with bovine colostrum.
- Hypophosphataemia, which may occur in cattle with post-parturient haemoglobinuria, in animals with diabetic ketoacidosis (DKA) which are rapidly stabilised with insulin and in refeeding syndrome.
- Inherited defects of red blood cell enzymes, including phosphofructokinase and pyruvate kinase.
- Microangiopathic damage to red blood cells as they pass through narrow or damaged blood vessels, as in haemangiosarcomata, disseminated intravascular coagulation (DIC) or vasculitis.
- Oxidative damage to red blood cells, caused by paracetamol in cats, onion poisoning in dogs and copper toxicity in many species. Ingestion of red maple leaves may also cause haemolysis in horses, as may brassicas (such as rape and kale) in cattle and sheep.
Haemolysis which is sufficiently severe to cause icterus is likely to be life-threatening due to the reduction in oxygen-carrying capacity of the blood. Animals affected acutely may require transfusions of whole blood, packed red blood cells or synthetic bovine haemoglobin and it may be advisable to provide oxygen by nasal catheter, flow-by or mask. In addition, the presence of large amounts of haemoglobin may cause acute intrinsic renal failure (in addition to the pre-renal failure caused by reduced oxygen delivery to the kidneys) and neonates may suffer from kernicterus, direct damage to the central nervous system caused by bilirubin.
Hepatic Jaundice
Also referred to as Toxic Icterus. Liver cell damage may lead to jaundice by two main mechanisms:
- In acute hepatic necrosis and hepatic lipidosis, damaged cells swell to such a degree that flow of bile in the canaliculi is obstructed. There is therefore intrahepatic obstruction and conjugated bilirubin accumulates in the blood.
- In chronic liver failure, so much hepatic function is lost that the bilirubin produced by the constant turnover of red blood cells cannot be taken up and conjugated, leading to an accumulation of unconjugated bilirubin in the blood.
- In reality, these situations occur together. There is both excessive unconjugated and conjugated bilirubin in the blood.
- Examples:
- Leptospira icterohaemorrhagica in the dog.
- Rift Valley fever virus.
- Some plant poisonings (Lantana)
- Some fungal infections (Aflatoxicosis)
Post-hepatic Jaundice
Also called Obstructive Icterus. This occurs due to an obstruction in the biliary tract which normally carriers bile from the liver and gall bladder to the duodenum. Conjugated bilirubin is found in the urine but, in complete obstruction, urobilinogen will be absent from the urine and stercobilin from the faeces. Possible causes of post-hepatic jaundice include:
- Intraluminal obstructions:
- Choleliths ('gall stones') are much less common in animals than they are in humans. They are usually composed of bilirubin salts in dogs and calcium carbonate in cats, although they are very rare in the latter species.
- Gall bladder mucoceles produce a kiwi sign on radiographs and may be a sequel to cystic mucinous hyperplasia of the gall bladder mucosa.
- Biliary neoplasia, most commonly cholangiocellular cystadenoma (in cats) or carcinoma (in dogs).
- Aberrant migration of intestinal parasites, including Ascaris suum in pigs and Parascaris equorum in horses. This occur particularly when worms have been treated with an anthelmintic that causes spasticity of the muscles.
- Extraluminal obstructions:
- Pancreatitis, pancreatic abscess or neoplasm
- Biliary tract rupture
- Pyloric or duodenal mass
Animals suffering from extra-hepatic biliary obstruction (EHBO) are often profoundly unwell. The reduced flow of bile salts into the gastro-intestinal (GI) tract allows GI bacteria to proliferate and eventually translocate across the intestinal wall. In addition, biliary stasis reduces the function of Kupffer cells within the liver, reducing their ability to remove and neutralise translocated bacteria from the portal blood. These animals should be stabilised adequately before any surgical repair is attempted.
Obstructive icterus results in excessive conjugated bilirubin in the blood. Out of the three types of icterus, it causes the most elevated levels of bilirubin in the blood and the greatest discoloration of the tissues of the body.
Links
Also see Bilirubin.
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References
Gorman N (1998) Canine Medicine and Therapeutics Blackwell Sciences
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