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Icterus (view source)

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==Description==
 
==Description==
 
[[File:Jaundiced cat.jpg|thumb|right|Image of a jaundiced cat, note the discoloured pinnae<br>Copyright Sabar 2007]]
 
[[File:Jaundiced cat.jpg|thumb|right|Image of a jaundiced cat, note the discoloured pinnae<br>Copyright Sabar 2007]]
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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 slcera and mucous membranes in life and in the tunica intima of the aorta at post mortem examination.
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Icterus refers to the staining of tissues by [[Bilirubin|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 slcera and mucous membranes in life and in the tunica intima of the aorta at post mortem examination.
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Bilirubin is a pigment of red blood cells which is released into plasma cells when these cells are degraded in the '''monocytic phagocyte system (MPS)'''.  This '''unconjugated''' bilirubin is transported in plasma bound to albumin and taken up into hepatocytes where it is conjugated with glucuronic acid.  The resultant '''conjugated''' bilirubin is secreted into bile which then moves into the small intestine.  The conjugated bilirubin is degraded to '''urobilinogen''' by gastro-intestinal bacteria and a small proportion of this product is reabsorbed and excreted in the urine.  The remaining urobilinogen is further degraded to '''stercobilin''', a brown pigment which contributes to the colour of faeces.  Therefore, in animals with complete biliary obstruction, urobilinogen is absent from the urine and the faeces have a white/grey 'acholic' colour due to the absence of stercobilin.  Small quantities of conjugated bilirubin are found in the urine of normal dogs because it has a low renal threshold.   
      
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.
 
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.
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