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

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Chapter | 10 Hemostasis
concentrations of serotonin, ADP, or collagen; platelet
ATP, ADP, and serotonin content was decreased; and secretion
of Ca 2 and Mg 2 by activated platelets was markedly
decreased ( Meyers et al ., 1981 ). More recent studies have
demonstrated that, in addition to the dense granule deficiency,
the collagen-induced increase in cytosolic Ca 2 ,
through the PLC γ pathway (see Sections II.B.3.a and c), is
depressed in the platelets of Japanese black cattle affected
with CHS ( Shiraishi et al ., 2002a, 2002b ). These results suggest
an additional mechanism for the decreased platelet function
in CHS. A dense granule storage defect, similar to that
in CHS, has been described in three families of American
cocker spaniel dogs with moderate to severe bleeding
(Callan et al ., 1995 ). Although these dogs had a decrease in
ADP content and an increased ATP:ADP ratio, in contrast
to CHS, the number and morphology of dense granules
on ultrastructural studies and concentrations of serotonin
appeared normal ( Callan et al ., 1995 ). The defect identified
in these dogs appears to be an isolated abnormality in the
δ -granule adenine nucleotide storage ( Callan et al ., 1995 ).
Platelet dysfunction in basset hounds has been attributed
to defective intracellular signaling. Ultrastructural appearance,
surface molecules, and granule constituents appear
normal in these dogs; however, secretion of dense granules,
expression of fibrinogen binding sites on GPIIb-IIIa,
and platelet aggregation are impaired in response to several
platelet agonists ( Catalfamo et al ., 1986 ). An elevation
of basal cAMP levels has been detected in these platelets
and is thought to result from a defect in cAMP metabolism
(Boudreaux et al ., 1986 ; Catalfamo and Dodds, 2000 ). As
previously mentioned, elevated cAMP levels reduce platelet
reactivity through a number of mechanisms (see Section
II.B.3.d, and Table 10-3) and thus would be detrimental during
hemostatic challenge. The thrombopathia of Simmental
cattle has also been attributed to defective intracellular signal
pathways. Whole blood from these cattle exhibit normal
clot retraction, but aggregation in response to ADP and collagen
is markedly impaired ( Searcy et al ., 1990 ). Similar to
basset hounds, these cattle express normal levels of GPIIb-
IIIa on the platelet surface; however, in contrast, dense
granule secretion is unimpaired ( Searcy et al ., 1990 ). The
cytoskeletal assembly in response to ADP was shown to be
incomplete, and this is thought to result in delayed activation
of GPIIb-IIIa and therefore delayed fibrinogen binding
(Frojmovic et al ., 1996 ; Searcy et al ., 1994 ).
4. Coagulation Factor Disorders
a. Fibrinogen Deficiency
As previously discussed, fibrinogen not only takes part in
primary hemostasis but is imperative for final clot formation
in secondary hemostasis (see Section II.C.4). Fibrinogen is
primarily synthesized in hepatocytes through production
and assembly of three polypeptide chains (A α , Bβ , and γ ),
which subsequently dimerize to form hexameric fibrinogen
(Asselta et al ., 2006 ). Each of the fibrinogen chains is
encoded by separate genes, and all are required for successful
assembly and secretion ( Roy et al ., 1991 ). Numerous
genetic defects have been identified in humans, with variable
influence on production, secretion, and function of the
molecule. Hereditary fibrinogen deficiencies are generally
categorized into quantitative (afibrinogenemia and hypofibrinogenemia)
and qualitative disorders (dysfibrinogenemia)
(Asselta et al ., 2006 ). In humans, the quantitative deficiencies
are generally inherited as autosomal recessive disorders,
whereas the qualitative deficiencies are generally autosomal
dominant ( Asselta et al ., 2006 ). Bleeding tendencies are variable;
however, umbilical cord bleeding, hemorrhages from
mucosal surfaces, hemarthroses, and hematomas are some
of the more frequently encountered symptoms ( Lak et al .,
1999 ). Hemarthrosis and hematomas tend to be less frequent
and severe than in the hemophilias ( Lak et al ., 1999 ). In afibrinogenemic
and hypofibrinogenemic patients, the majority
of bleeding episodes occurs as a result of trauma or surgery
(71% and 80%, respectively), with the remainder comprising
spontaneous episodes ( Acharya et al ., 2004 ). Thrombin time
is prolonged in fibrinogen defects; however, partial thromboplastin
time and prothrombin time are also prolonged
because of their dependency on a fibrin clot end point (see
Section III.B.3.d). Further investigation should include quantification
of fibrinogen, an absence indicating afibrinogenemia,
and decreased levels in hypofibrinogenemia. Although
fibrinogen concentration is generally within reference intervals
for dysfibrinogenemias, some present with low fibrinogen
concentrations ( Hayes, 2002 ). It is important to measure
activity in conjunction with concentration in order to reveal
functional deficiencies. Typically a 1:1 ratio of functional to
quantitative levels are present; however, in dysfibrinogenemia
the ratio is more typically 1:2, with disproportionately
low function compared to concentration ( Hayes, 2002 ).
Although significant progress has been made in the
characterization of human fibrinogen deficiencies, this disorder
is still exceedingly rare and poorly characterized in
animals. Fibrinogen defects have historically been identified
in goats and various breeds of dog ( Dodds, 2000 ).
More recent reports include a Border Leicester lamb with
disproportionately low fibrinogen function when compared
to concentration ( Fecteau et al ., 1997 ). However,
in this report, diagnostic limitations prevented definitive
distinction between dysfibrinogenemia and afibrinogenemia.
Clinical signs were consistent with those reported
in humans, including umbilical bleeding and a hematoma.
A case of congenital afibrinogenemia was confirmed,
through extensive testing, in a Bichon Frise. This patient
experienced excessive bleeding at sites of venipuncture, a
hematoma, and uncontrolled hemorrhage following ovariohysterectomy
( Wilkerson et al ., 2005 ).
Acquired deficiencies of fibrinogen are generally associated
with consumptive coagulopathies, but they can also
result from decreased production in liver failure.