Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 13 Hepatic Function
perfect. Elevated plasma bilirubin values are usually present
for one or more days before clinical icterus is apparent,
and there may be a delay between the time plasma bilirubin
returns to normal and the clearance of the yellow discoloration
of tissues. Conjugated bilirubin is said to have a
greater affinity for connective tissue than the unconjugated
pigment, possibly because conjugated pigment is less
avidly bound to albumin ( With, 1968 ).
Visible yellow discoloration of tissues is readily recognized
in animals in the unpigmented sclera. The normal red
color of the visible mucous membranes makes detection of
a slight yellow cast more difficult. It is possible to apply
pressure to the mucous membranes and temporarily reduce
blood flow to the area, so that the underlying discoloration
of the tissue can be better assessed.
The color of plasma (icteric index) may be useful clinically
in the evaluation of icterus. Normal canine, feline,
and ovine plasma is often water clear and free of yellow
color. The finding of yellow plasma in these species is
highly suggestive of hyperbilirubinemia. Cattle absorb
and transport significant quantities of carotene in plasma.
Because in cattle the icteric index varies with the dietary
intake of carotene, measurement has limited use in this
species. Equine plasma normally has a high icteric index,
which in part is due to a plasma bilirubin concentration
that normally is higher than that of other domestic species.
There are other, as yet uncharacterized, noncarotene pigments,
however, that may contribute to the color of equine
plasma.
Notable species differences occur in the frequency with
which icterus is observed in association with liver disease.
In sheep and cattle with terminal hepatic insufficiency, there
usually is a significant biochemical elevation in plasma bilirubin,
but elevation may be insufficient to result in clinical
icterus ( Finn and Tennant, 1974 ; Hjerpe et al., 1971 ).
This is due possibly to the residual capacity of the liver to
excrete bilirubin or to extrahepatic mechanisms for bilirubin
excretion or degradation. Clinical icterus in ruminants
is often associated with hemolytic anemia in which production
of bilirubin exceeds excretory capacity (e.g., anaplasmosis
in cattle and copper poisoning in sheep). In severe
fatty liver, cattle that are critically ill may exhibit some
degree of clinical icterus.
The assessment of clinical icterus in the horse is somewhat
more complicated than in other species. The sclera
and visible mucous membranes of most normal horses do
not appear icteric, but in 10% to 15% of normal horses, a
slight but definite yellow discoloration of the sclera or oral
mucous membranes can be detected ( Tennant et al., 1975 ).
Scleral icterus of a moderate degree may also be observed
in horses with a variety of illnesses that do not involve
the liver directly (e.g., pneumonia, impaction of the large
intestine, enteritis). Reduction in food intake is a common
factor in such disorders, and fasting in the horse causes a
rapid increase in plasma bilirubin concentration. In both
hemolytic anemia and hepatic failure in the horse, the
degree of icterus is usually remarkably greater than that
seen under physiological conditions or that is associated
with reduced food intake. In the horse, severe clinical
icterus is almost invariably present in acute hepatic
necrosis ( Tennant et al., 1975 ; Thomsett, 1971 ). However,
in chronic hepatic disease, icterus may be a more variable
sign. In a series of 34 cases of hepatic cirrhosis in
the horse, significant icterus was a presenting sign in
70% ( Tennant et al., 1975 ). Icterus was even less frequent
(40%) in another series of horses with cirrhosis ( Gibbons
et al., 1950 ).
The dog and cat appear to be intermediate between
ruminants and the horse in the propensity to develop
clinical icterus. Hemolytic disease, severe hepatocellular
dysfunction, and extrahepatic bile duct obstruction
are characteristically associated with icterus in dogs and
cats. In experimental extrahepatic bile duct obstruction in
the dog, the plasma bilirubin increases at once following
obstruction and clinical icterus is observed within 1 to 3
days. After 2 to 3 weeks, however, the plasma bilirubin of
some dogs declines. As in sheep and cattle, this may be
related to adaptation of extrahepatic mechanisms of bilirubin
excretion, particularly the kidney. The kidney of the
dog is capable of adapting so that the rate of renal excretion
of bilirubin equals the rate of formation. In cats with
complete extrahepatic bile duct obstruction, however, no
such decrease is observed, and persistent hyperbilirubinemia
and deep icterus are characteristic.
B . Hepatic Encephalopathy
Hepatic encephalopathy is the syndrome of disturbances
in cerebral function that is caused by hepatic insufficiency
or hepatic failure. Severity of neurological signs may vary
from subtle and intermittent changes in behavior associated
with lethargy or stupor, to bizarre, belligerent behavior,
mania, convulsions, and hepatic coma. Typically, such
signs are attributable to severe acute or chronic liver disease
or in dogs to congenital malformation of the portal
vein (portosystemic shunt).
Hepatic encephalopathy is a prominent clinical feature
of hepatic failure in the horse. In one series of cases, 82%
of horses presenting with acute hepatitis and 32% with cirrhosis
had prominent neurological abnormalities ( Tennant
et al., 1973 ). Varying degrees of CNS derangement may
be observed. Some horses stand quietly with the feet apart
and the head lowered, nodding the head occasionally and
appearing somnolent. Pupillary response to light may be
normal or moderately sluggish, but, in some cases, vision is
lost. Compulsive walking in a circle or in a single direction
may be observed, and affected individuals may appear
oblivious to their surroundings, walking over or through
objects in their path ( “ walking disease ” ; Rose et al., 1957 ).