von Willebrand factor is a large multimeric glycoprotein that circulates non-covalently with factor VIII coagulant protein. It used to be called factor-VIII related antigen, however this term is obsolete and should no longer be used. vWf is an entirely different protein from factor VIII; it is produced in different cells and has different roles in hemostasis.
vWf is produced by endothelial cells and megakaryocytes (factor VIII is produced by hepatocytes) and is stored in the alpha granules in platelets and in special organelles, called Weibel-Palade bodies, in endothelial cells. Both alpha-granules and Weibel-palade bodies serve as intracellular storage organelles of vWf. Dogs have a very small percentage of vWf in platelets (3%) compared to cats or human beings (20%). vWf is produced as a single protein chain (called a monomer), which then dimerizes within the cytoplasm of the megakaryocyte or endothelial cell. Therefore, the smallest component of vWf is a dimer. The dimer spontaneously forms long chains or polymers called multimers, which are held together by disulfide bonds. These multimers impart a very high molecular weight on vWf. The multimeric structure of vWf is important as the higher molecular weight multimers are more effective in hemostasis, so a relative deficiency of these multimers (which occurs in type II vWD) produces more severe bleeding.
Role of vWf
von Willebrand factor has an essential role in primary hemostasis, being important for platelet adhesion and spreading on the subendothelium, and platelet aggregation. Basically, vWf acts like a bridge or glue between platelets and the subendothelium, platelets and fibrin.
- Platelet adhesion: In vessels of
high shear rate (i.e. microvasculature), vWf is essential for the first
event in primary hemostasis, that is platelet binding to the
subendothelium. vWf mediates platelet adhesion by acting as a bridge
between the platelet receptor glycoprotein Ib-IX and subendothelial
collagen or elastin microfibrils.
- Platelet spreading: After
adhesion, vWf facilitates platelet spreading on the subendothelial
matrix, by attaching to matrix proteins and the platelet integrin
receptor GPIIb/IIIa on platelet surfaces.
- Platelet aggregation: vWf
aids platelet aggregation by acting as a bridge between GPIIb/IIIa on
adjacent platelets, although fibrinogen is the main platelet aggregating
agent in this regard. However, vWf may substitute for fibrinogen in
- Platelet plug stabilization:
vWf may aid in stabilization of the primary platelet plug by aiding the
incorporation of fibrin into the platelet plug. This is done by vWf by
binding to GPIb-IX on platelets and onto fibrin.
- Carrier for factor VIII: vWf acts as a carrier molecule for factor VIII, protecting it from degradation in the circulation and delivering it to sites of vessel injury. Thrombin breaks down the non-covalent association between the molecules, allowing factor VIII to participate in the coagulation cascade that takes place on platelet surfaces. In the absence of vWf, factor VIII coagulant (FVIII:C) activities in clotting assays are typically decreased (sometimes to very low levels, especially in human patients and less so in dogs). In human patients, there is a variant of vWD, which is due to a genetic defect in the factor VIII-binding domain of vWf. The defective vWf cannot bind factor VIII, therefore it is rapidly cleared from the circulation, resulting in very low factor VIII:C activities, but normal vWf:Ag values, which mimics hemophilia A.
von Willebrand disease has been described in over 50 breeds of dogs. The trait is most prevalent in the Doberman Pinscher, Pembroke Welsh Corgi, Airedale Terrier, Scottish Terrier, and Shetland Sheepdog, but severely affected individuals or families have been identified in many breeds (and even mixed breeds). There are differences between breeds in the proportion of carriers that actually express the vWD trait by showing abnormal or excessive hemorrhage, e.g. Airedale terriers rarely bleed, despite having a high prevalence of the trait, whereas Dobermans bleed quite commonly. The disease does occur in pigs, rabbits, cats and horses but has been recognized in only a few individuals of the latter 2 species.
Severity of bleeding is highly variable in dogs affected with vWD. In general, spontaneous bleeding tends to occur from mucous membranes lining the nose, mouth, urinary, reproductive, and intestinal tracts. Excessive bleeding in puppies may be noticed after tail docking, dewclaw removal, tattooing, or when the pup is teething. In less severely affected dogs, abnormal bleeding is seen only after surgery or trauma. Concurrent stress conditions such as viral and bacterial infections, hormonal fluctuations associated with heat cycles or pregnancy, and endocrine disorders causing deficiencies of steroid or thyroid hormones can all exacerbate signs of hemorrhage in dogs affected with vWD. Note that petechial hemorrhages are rarely seen in vWD, and if observed in a predisposed dog breed, should prompt a differential diagnosis of thrombocytopenia before vWD.
Diagnosis of vWD
Specific assays of canine vWf are needed to diagnose vWD in dogs as most dogs with vWD have normal platet counts and coagulation profiles (PT, APTT, and fibrinogen). However, some very severely deficient dogs have low enough FVIII:C activity to have a slightly prolonged APTT (especially if plasma is diluted before being assayed). Puppies that show signs of bleeding but have normal platelet counts and coagulation test results should be tested for vWD. The buccal mucosal bleeding time can be used as an in vivo test for vWD in the veterinary clinic (such as in a Doberman with unknown vWf results prior to surgery). However, this test is not sensitive or specific for vWD, therefore it does not replace more specific vWf assays. There are a variety of laboratory assays for vWD, however the most commonly used assay is that for vWf:Ag measurement in a citrated plasma sample. The following ranges for plasma vWf:Ag have been established at the Comparative Coagulation Laboratory at Cornell University:
|70 to 180%||Normal range|
|50 to 69%||Borderline range|
|0 to 49%||vWD|
- Dogs testing in the normal range are considered clear of the vWD
trait, and at low risk for expressing or transmitting the vWD trait.
- Dogs testing in the borderline range cannot be accurately
classified as a carrier or clear of the trait on the basis of the vWf:Ag
result. This is an overlap region of plasma vWf:Ag, where some
individuals are clear and some are carriers of vWD. On a second test,
some borderline range individuals fall in the normal or abnormal range,
thereby enabling a prediction of their genetic status. A test mating
can be performed by breeding a borderline range individual to a
high-testing clear mate. If the borderline parent is clear of vWD, then
all pups in the litter are predicted to be clear. The presence of one
or more abnormal range pups indicates that the borderline parent is a
carrier of vWD. Alternatively, these borderline dogs should be tested
using a new genetic test for vWD. This test is useful for confirming the
carrier status in dogs with questionable vWf:Ag results, but should not
be used to diagnose the disease.
- Dogs testing in the abnormal range are considered carriers of the vWD trait, and are at risk for transmitting an abnormal vWf gene to offspring, and in some individuals, for expressing an abnormal bleeding tendency. In general, the lower the value of plasma vWf:Ag, the more at risk an individual dog is for expressing a bleeding tendency. Most dogs with bleeding due to vWD have vWf:Ag values < 35%.
von Willebrand disease is inherited as an autosomal trait and is categorized into 3 types based on the amount and multimeric composition of the molecule.
Type I: This is due to a deficiency
in the amount of vWf. All multimers are present but in reduced amounts.
This is the type reported in most breeds of dogs, including the
Doberman. Bleeding is variable in this disorder and is dependent on the
value of vWf:Ag and breed (as mentioned previously, Airedales have Type I
vWD but rarely exhibit bleeding, in contrast to Dobermans, which also
have type I vWD, but do bleed). The inheritance of this condition is
currently under dispute. It was thought to be autosomal dominant, with
incomplete penetrance, however some authors believe it is inherited in
an autosomal recessive fashion.
- Type II vWD: This
has been reported in dogs (German shorthair and wirehair pointers) and
in a Quarterhorse. It is characterized by low vWf:Ag and a relative
decrease in high molecular weight multimers. The bleeding is more severe
than in type I vWD and is not related to vWf:Ag (as vWf:Ag values do
not correlate to multimers).
- Type III vWD: This has been reported in dogs (Chesapeake bay retrievers, Scottish Terriers, Dutch Kooiker dogs and Shetland Sheepdogs), pigs and a monkey. It is the most severe form of vWD and is characterized by an almost complete absence of vWf and very long BMBT. In the Scottish Terrier, it is inherited as an autosomal recessive trait.
- Unknown types have been reported in a cat (severe deficiency-Type III?) and a calf (? Type II).
Severe hemorrhage in vWD patients can be controlled with transfusion of fresh, fresh frozen plasma or cryoprecipitate. Whole blood should only be used in dogs that are hypoxic from anemia, and even then, component therapy (packed red cells and cryoprecipitate) is preferrred as whole blood will not provide sufficient vWf to stop the hemorrhage. In addition, plasma products are optimal since these dogs usually require repeated transfusions through life and repeated exposure to red cell antigens increases the risk of transfusion reactions.
Another treatment is desmopressin acetate or DDAVP. DDAVP stimulates the release of vWf from stores (Weibel-Palade bodies) in endothelial cells and increases vWf:Ag values and decreases the BMBT for up to 4 hours in Dobermans with type I vWD. Repeated administration has diminishing effectiveness due to depletion of stores. In addition, this drug does not work in dogs with type III vWD as they lack endothelial stores of vWf. The response to DDAVP in a single dog is repeatable and predictable but not all dogs with type I vWD will respond to the drug, therefore it should not be relied upon to achieve hemostasis in surgery (but may useful as an adjunct to transfusion therapy). The dose of DDAVP is 1 µg/kg diluted in sterile saline given 30 minutes before surgery.