Haemophilia A and B | BSAVA Library

Haemophilia A and B

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Haemophilia A and B are hereditary, X-linked bleeding disorders caused, respectively, by deficiencies of coagulation factors FVIII and FIX. Haemophilia is among the most common hereditary human disorders; both haemophilia A and B have been identified in domestic animals and, as in humans, haemophilia A is the most common form. This chapter considers function of factors VIII and IX; prevalence and inheritance of haemophilia; clinical signs; diagnosis; management of haemophilia; genetic counselling.

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28.1 The intrinsic tenase coagulation complex. The serine protease FIXa assembles on a phospholipid membrane surface with calcium and its cofactor, FVIIIa, to form an active enzyme complex. The tenase complex interacts with its substrate, the zymogen FX, to induce rapid generation of the enzyme, FXa.
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28.3 X-linked inheritance of haemophilia A and B. Propagation of haemophilia by a carrier female. Carrier females transmit a mutant gene (depicted as a dark bar on the X chromosome) on average to half their sons and half their daughters. The sons that inherit this mutation are affected with haemophilia and the daughters are haemophilia carriers. Propagation of haemophilia by an affected male. Males with mild to moderate haemophilia may survive to reproductive age. If bred to an unaffected female, all the daughters of an affected male are carriers of haemophilia and all the sons are clear.
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28.4 Clinical signs of haemophilia. Haemarthrosis of the stifle joint and thigh muscle haematoma in a mixed-breed dog affected with severe haemophilia B (factor IX activity < 1%). Dorsal temporal muscle haematoma in an Irish Setter puppy affected with severe haemophilia A (factor VIII activity < 1%).
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28.5 Intrinsic coagulation pathway. The intrinsic pathway leading to fibrin formation is initiated by pre-incubation of plasma with a reagent containing phospholipid and a particulate surface. Upon the addition of calcium, activation of the contact pathway proceeds through a series of steps culminating in the common pathway (outlined by a dashed line), which produces a burst of thrombin (factor IIa) that cleaves fibrinogen to form an insoluble fibrin clot. MW, molecular weight.
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28.6 Thromboelastography (TEG) tracings from a dog with severe haemophilia A. Superimposed tracings depict assays performed on whole blood samples activated by the addition of a tissue factor reagent (black tracing) or with no activating agent (green tracing). The activated sample shows only a slight delay in the time for initial fibrin formation (denoted by the first separation of the solid black line into two mirror images), and has no abnormalities in TEG parameters denoting fibrin clot strength. In contrast, in the absence of tissue factor to bypass the intrinsic pathway, a stable fibrin clot fails to form in the reaction cup (denoted by the single flat green line).
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