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

862
Chapter | 28 Avian Clinical Biochemistry
deposition coincides with increased osteoblastic activity.
When the bird starts to secrete the eggshell the medullary
bone is resorbed by osteoclastic activity. Ca is deposited in
the eggshell as Ca-carbonate and the P is excreted from the
body. Medullary bone might be mistaken for a pathological
condition when radiographs are being evaluated.
D . Hypocalcemia Syndrome in African
Grey Parrots
In birds of prey and African grey parrots a hypocalcemia
syndrome is known, characterized by hypocalcemic seizures.
A striking feature of this syndrome in African grey parrots,
which is not known in other birds, is that demineralization of
the skeleton is not obvious at the moment the seizures occur.
The hypocalcemia syndrome is an important differential
diagnosis in an African grey parrot that repeatedly falls of
its perch. Reference values for tCa concentrations in African
grey parrots range from 2.0 to 3.25 mmol/L ( Rosskopf et al .,
1982 ). Lumeij (1990) , studying a population of 72 African
grey parrots found reference values of 2.1 to 2.6 mmol/L
(inner limits of the percentiles P 2.5 to P 97.5 , with a probability
of 90%) and a range from 2.0 to 3.4 mmol/L. Hochleithner
(1989b) , studying 68 African grey parrots and using a dry
chemistry system (Kodak Ektachem), reported reference
values for Ca of 1.75 to 2.38 mmol/L (inner limits of the
percentiles P 2.5 to P 97.5 ). Hochleithner (1989a) reported five
cases of hypocalcemia in African grey parrots with plasma
calcium concentrations ranging from 0.75 to 1.5 mmol/L.
Rosskopf et al . (1985) stated that the one consistent finding
of the hypocalcemia syndrome is a “ blood calcium level ”
below 1.5 mmol/L. Values as low as 0.6 mmol/L have been
reported ( Rosskopf et al ., 1985 ). When borderline calcium
concentrations are found, the correction formula reported
in Section VIII.A should be used. Stanford (2005) reported
that 5/19 cases of hypocalcaemia in African grey parrots as
diagnosed by low iCa concentrations had normal tCa concentrations
and therefore concluded that measurement of
iCa concentration is vital to diagnose hypocalcaemia. This
conclusion, however, was not supported by data on corrected
tCa. With the current availability of ion selective electrodes,
however, measurement of iCa seems to be the practical way
of evaluating Ca status in birds. When timely treatment with
parenteral Ca and vitamin D 3 preparations starts and sufficient
dietary uptake of Ca is taken care of, clinical signs will
regress in a short time. It is therefore likely that the disease
is caused by Ca and vitamin D 3 deficiency. The higher incidence
of the hypocalcaemia syndrome in African grey parrots
might be related to the relatively higher dependence on
ultraviolet light in this species ( Stanford, 2005, p. 136 ).
E . Alkaline Phosphatase in Bone Disease
Alkaline phosphatases (APs) form a group of membranebound
glycoproteins that hydrolyze monophosphate esters
at alkaline pH. Three different isoenzymes have been identified.
Although there is a significant activity of AP in
various tissues, the physiological role is unclear, except for
AP in bone tissue. AP activity in bone reflects the activity
of osteoblasts, and this enzyme is involved in the formation
and mineralization of the bone matrix. In humans,
increased AP activity is observed during growth and in
osteoproliferative disorders ( Savova and Kirev, 1992 ).
Different techniques have been used to identify fractions
responsible for increased plasma activities. The heat
inactivation test has been developed to distinguish AP activity
of bone origin from that of liver origin ( Johnson et al .,
1972 ; Posen et al ., 1965 ). In humans, residual activities after
heat inactivation at 56°C higher than 35% indicate hepatic
disease, whereas residual values lower than 25% indicate
bone disease with increased osteoblastic activity ( Fennely
et al ., 1969 ; Fitzgerald et al ., 1969 ; Stolbach, 1969 ). Using
a guinea fowl model with bone tumors induced by osteopetrosis
virus, Savova and Kirev (1992) were able to confirm
these findings also for an avian species. They showed, by
comparing the findings with the more sensitive wheat germ
lectin method ( Brixen et al ., 1989 ; Rosalki and Foo, 1984 ),
that for guinea fowl the AP activity of bone origin can be
inactivated at 58°C rather than 56°C. Savova and Kirev
(1992) found that AP activity of bone origin in 15-week-old
guinea fowl was twice as high as that of 1-year-old birds.
They also confirmed the positive correlation between the
intensity of virus-induced excessive bone growth and serum
AP activity reported previously by Sanger et al . (1986) and
Barnes and Smith (1977) . The presumed high proportion of
AP of bone origin was supported by the low values of residual
activity after heat inactivation at 58°C (14.7 3.7%) and
after precipitation with wheat germ lectin (13 1.2%) during
the period of active bone tumor formation ( Savova and
Kirev, 1992 ).
IX . DIABETES MELLITUS AND PLASMA
GLUCOSE
The basic metabolic regulation of glucose metabolism in
birds is identical to that in mammals, but there is a quantitative
difference. Reference values for plasma glucose in birds
range somewhere between 11 and 25 mmol/L (Lumeij and
Overduin, 1990 ; Rosskopf et al ., 1982 ). Physiological values
up to 33 mmol/L have been observed postprandially in
pigeons ( Lumeij, 1987b ). As a result of stress, plasma glucose
concentrations up to 33 mmol/L may also be observed
( Jenkins, 1994 ). The insulin content of the pancreas of granivorous
birds is about one-sixth that of mammalian pancreata,
whereas the glucagon content is about two to five times
greater. Circulating plasma concentrations of glucagon (1 to
4 ng/ml) are 10 to 50 times higher in birds than in mammals.
Insulin is synthesized in the B cells of the pancreas, whereas
glucagon is synthesized in the A cells.