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

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Chapter | 28 Avian Clinical Biochemistry
This is slightly increased postprandially because of the
intrahepatic effects of intestinal hormones like secretin,
avian vasoactive intestinal peptide (VIP), or cholecystokinin
(CCK). The sites of the increased bile secretion and the
regulatory mechanism are unknown ( Lumeij, 1991 ). Via
the enterohepatic recirculation, over 90% is reabsorbed in
the jejunum and ileum ( Hill, 1983 ). Plasma BA concentrations
including their salts and corresponding glycine and
taurine conjugates are a reflection of the clearing capacity
for BA of the liver. All liver functions such as extraction,
conjugation, and excretion are involved in this process,
and the BA provides information on the combined effects
of these functions. The plasma concentration of BA is a
sensitive and a specific indicator of liver function, and it
is widely used in humans and domestic animals, including
birds ( De Bruijne and Rothuizen, 1988 ; Hoffmann
et al ., 1987 ; Lumeij, 1988 ; Rutgers et al ., 1988 ). Reference
values for BA have been established for the racing pigeon,
the most commonly kept psittacine species ( Lumeij and
Overduin, 1990 ; Lumeij and Wolfswinkel, 1988 ), ostriches,
and peregrine falcons ( Table 28-3 ). Lumeij (1987f) found
that BA was the single most useful plasma constituent for
detecting liver function disorders in the racing pigeon,
where it is both specific and sensitive, the main advantage
over AST being that the latter is not liver specific. In
experimental liver disease, a 5- to 10-fold increase of BA
over the upper limit of the reference range is common.
G . Postprandial Effects
There is a significant postprandial increase in BA in granivorous
birds, with and without a gallbladder ( Lumeij,
1991 ), as well as in carnivorous birds ( Fig. 28-17 ; Lumeij
and Remple, 1992 ). Although up to a 4.5-fold postprandial
increase of BA was observed in individual birds, the concentrations
were never increased more than 1.65 fold over
the upper limit of the reference range, whereas in hepatobiliary
disease 5- to 10-fold increases are common ( Lumeij
et al ., 1988a ). Even though postprandial increases might
complicate interpretation of BA, differentiation between
postprandial increases and increases that result from hepatobiliary
disease is possible. Experimental findings suggest
that values 70 μ mol/L in fasted racing pigeons and
values 100 μ mol/L postprandially should be considered
increased and suggestive for hepatobiliary disease.
H . Plasma Ammonia:
Hepatoencephalopathy
A tentative diagnosis of hepatoencephalopathy is often
made when neurological signs are observed in birds with
liver disease. The syndrome, however, has not been well
documented in birds. In mammals, hepatoencephalopathy
and hepatic coma are most often observed in portosystemic
shunting in which portal blood and its ammonia are shunted
away from the liver. Hepatoencephalopathy is not a disease
in itself, except for the anatomical anomaly, but a medical
condition characterized by neurological symptoms caused
by intoxication of the brain by products of protein digestion,
namely ammonia (NH 3 ). Most likely, degradement products
from protein act as false neurotransmitters. Protein-rich
foods often trigger neurological symptoms in these patients.
Fasting plasma NH 3 levels and plasma NH 3 levels 30min
after oral loading with NH 4 Cl in the form of the NH 3 tolerance
test (ATT) can be used to establish the ability of the
liver to convert NH 3 into UR. Fasting plasma NH 3 in healthy
psittacines ranged from 36 to 274 μ mol/L, which is well
above the fasting concentrations reported in dogs ( Lumeij
and Peccati, 1993 ). Furthermore, some avian species will
normally have up to an 8-fold increase of plasma NH 3 concentration
on the ATT using the canine protocol. Therefore,
an abnormal ATT is not diagnostic for portosystemic shunting
in birds ( Lumeij and Peccati, 1993 ).
I . Exogenous Markers
The hepatic clearance of exogenous drugs or compounds
plays a central role in the diagnosis and monitoring of
hepatic disease in humans, but so far it has received little
attention in avian diagnostics. Jaensch et al. (2000a) performed
galactose and indocyanine green clearance assays
in normal chickens and following celiotomy and compared
values with those after 13% hepatectomy. Partial hepatectomy
resulted in elevation of galactose single point concentrations,
but, paradoxically, galactose clearance values
did not alter significantly. Partial hepatectomy did not alter
clearance values of indocyanine green.
In galahs (Eolopus roseicapillus ) a significant reduction
in galactose clearance and galactose clearance as a
function of body surface area (GEC-SA) was observed
after 18% hepatectomy, compared to no reduction after
celiotomy alone or 6% hepatectomy. Although galactose
single-point concentrations did not elevate significantly,
they were strongly correlated with galactose clearance and
GEC-SA values ( Jaensch et al ., 2000b ).
Further work is needed in birds to explore the potential
of galactose clearance for use as a noninvasive method to
monitor hepatic function.
J . Iron Storage Disease
A distinction should be made between hemosiderosis, which
includes a range of disorders that lead to a buildup of iron
pigment in hepatic tissue, and hemochromatosis, which
is a genetic disorder in humans and possibly other species.
There is no conclusive evidence that hemosiderosis has any
clinical significance in birds, with the possible exception of
the hemochromatosis syndrome in the Indian hill mynah