Von Willebrand's Disease

 

Julie B. Anderson, DVM; Kenneth S. Latimer, DVM, PhD; Perry J. Bain, DVM, PhD; Heather L. Tarpley, DVM

Class of 2004 (Anderson), College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996 and the Department of Pathology (Latimer, Bain, Tarpley), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

Von Willebrand's disease (vWD) is a congenital, extrinsic platelet defect resulting in platelet dysfunction.1-3 It is characterized by a deficiency of von Willebrand factor (vWF), a glycoprotein that is involved in platelet adhesion to the vessel wall during formation of the primary hemostatic plug. Although vWD is most commonly reported in dogs, it also has been observed in swine, horses, cattle, and cats.

Disease Mechanism

vWD is characterized by a lack of functional vWF, resulting in abnormal primary hemostasis (platelet plug formation) and prolongation of bleeding time in vivo. vWF is a multimeric, plasma glycoprotein that plays a central role in hemostasis by supporting platelet adhesion to subendothelial collagen at sites of vascular injury. vWF is produced primarily by endothelial cells, but also is produced in small quantities by platelets and megakaryocytes. In dogs, only 2% of vWF is of platelet origin. In contrast, approximately 10 to 20% of vWF in humans and about 25% of vWF in cats is of platelet origin. vWF is found in endothelial cells, plasma, alpha granules of platelets and megakaryocytes, and subendothelial matrix of blood vessel walls. In endothelial cells, vWF is stored in cytoplasmic granules (Weibal-Palade bodies, Fig. 1). vWF is secreted from endothelial cells via a constitutive pathway directly into the circulation or the subendothelium. It also can be stored in endothelial cell organelles and released in response to stimuli such as thrombin and epinephrine. The release of vWF is stimulated by an assortment of other substances such as histamine, fibrin, and estrogen. The release of alpha granules of platelets is stimulated by a variety of substances including collagen, platelet-activating factor, thrombin, and adenosine diphosphate. 1,11

Figure 1. Endothelial cells with stored von Willebrand factor (red) in the cytoplasmic granules (Weibal-Palade bodies). University of California – San Diego Medical School.

Small, medium, and large multimers of vWF exist. The largest ones (high molecular weight) are most active in hemostasis because they presumably have increased numbers of binding sites per molecule or because their physical characteristics alter under certain conditions of blood flow.1,2,10 vWF circulates bound to factor VIII and appears to prolong the half life of factor VIII. In dogs with vWD, factor VIII concentrations usually remain at 20% or more of normal values. The role of vWF in the release and production of factor VIII has not been clarified.1

When the endothelium is disrupted, vWF binds to collagen of the subendothelium. In areas of low blood flow rate, platelets (Fig. 2) may adhere to vessel walls independently of vWF. In areas with a high blood flow rate, vWF is necessary for the platelets to adhere to the subendothelium. Although some vWF is located in the subendothelium, additional vWF is needed for an optimal platelet adhesion. In such instances, additional vWF is released from platelet or endothelial cell granules. As vWF binds to exposed collagen, its conformation changes allowing an increased binding affinity for glycoprotein Ib located on the platelet membrane. Platelets subsequently bind to vWF and adhere to the vessel wall. Once the platelets are activated, they expose their fibrinogen-binding sites (glycoprotein IIb-IIIa). Fibrinogen adheres and further aggregation of platelets occurs. vWF may also bind to platelets to stabilize their attachment.1

 

Figure 2. Scanning electron micrograph of blood. Erythrocytes appear as biconcave disks (red), leukocytes have surface ruffles (yellow), and platelets appear as small, discoid structures (pink to magenta). National Cancer Institute.

 

Von Willebrand Disease In Animals

Dogs - vWD is the most common canine hereditary bleeding disorder and has been reported in over 50 different breeds of dogs. However, vWD is most prevalent in the Corgi, Doberman Pinscher, German Shepherd Dog, German Shorthaired Pointer, Golden Retriever, Shetland Sheepdog, and Standard Poodle (Fig. 3) .3

Figure 3. The Corgi, Doberman Pinscher, German Shepherd Dog, German Shorthaired and Wirehaired Pointers, Golden Retriever, Shetland Sheepdog, and Standard Poodle breeds of dogs are most commonly affected affected with von Willebrand's Disease. American Kennel Club

Swine - Type II vWD has been reported in swine as a severe bleeding disorder in a strain of inbred male and female Poland China pigs.3 Hemorrhage from minor wounds such as castration, ear notching, or after farrowing commonly results in death. When compared with normal animals, there is a decreased concentration of factor VIII, decreased platelet retention time, and increased ear-bleeding times.4,5 Porcine vWD has a recessive inheritance pattern and non-bleeding carriers can be identified by laboratory testing. Affected swine have been used as an animal model of human von Willebrand disease.

Horses - vWD in horses has been described as a quantitative and qualitative deficiency of vWF. The prevalence and clinical importance of vWD has yet to be determined, although the disease has been reported in Quarter Horses and Thoroughbreds. Affected individuals experience mild hemorrhage following surgery or trauma. Recurrent epistaxis may be observed. vWD may not be recognized early in life. The platelet count, one-stage prothrombin time (OSPT), and thrombin clotting time (TCT) are within the reference interval. The activated partial thromboplastin time (APTT) may be decreased if concurrent deficiency in factor VIII is present. Skin and mucosal bleeding times usually are prolonged. The definitive diagnosis of vWD depends on quantitative and qualitative evaluations of vWF. The plasma von Willebrand factor antigen (vWF:Ag) concentration variably is decreased. Qualitative assays may show a disproportionately greater decrease in the functional activity of this protein. Cases of equine vWD to date have had a multimer profile similar to human and canine type II vWD where the high molecular weight (and more hemostatically effective) multimers are reduced. Treatment is targeted at minimizing injury and avoiding the use of drugs known to suppress equine platelet function (nonsteroidal antiinflammatory drugs and sulfonamides). Transfusions of fresh blood or fresh or fresh-frozen plasma may be of value in controlling bleeding episodes. Because vWD is an inherited disorder, affected animals should not be used for breeding purposes.6, 7, 8

Cats - vWD is rare in cats; only one case has been reported at this time. The patient was an 8-year-old Himalayan cat with excessive bleeding after a tooth extraction. The cat had hematuria, petechia, and melena as well as laboratory evidence of disseminated intravascular coagulation. Eight months later, the cat showed spontaneous gingival hemorrhage. Plasma concentration of vWF:Ag was less than 7%. This cat was suspected to have congenital vWD.1

Cattle - Type II vWD has been reported in a Simmental calf.3

Clinical Signs

In animals with vWD, bleeding episodes commonly involve mucosal surfaces. Excessive hemorrhage also may be observed after trauma or surgery. Paradoxically, the measurement of vWF:Ag does not always accurately predict the risk of hemorrhage in patients. Clinical signs of vWD include the following: 1,10

• Epistaxis
• Gingival hemorrhage
• Hematuria
• Excess vaginal hemorrhage
• Gastrointestinal bleeding – melena
• Multiple small bruises
• Purpura
• Spontaneous bleeding from mucosal surfaces
• Excessive bleeding from tooth extractions
• Prolonged bleeding from wounds
• Increased cutaneous bleeding, especially after venipuncture
• Petechia usually not seen

Uremia, hyperproteinemia, anemia, and liver disease usually have associated platelet dysfunction. If an animal has concurrent vWD, they may experience signs of hemorrhage that complicates management of these disorders. Concurrent congenital or acquired coagulopathies and thrombocytopenias can be associated with bleeding in a dog that has been subclinically affected with vWD.10

Presumptive Diagnosis of von Willebrand's Disease

In vWD, the platelet count and morphology are normal. The one-stage prothrombin time (OSPT) and thrombin clotting time (TCT) usually are within reference ranges. Table 1 presents various features that are useful in differentiating bleeding due to vascular and platelet disorders from that occurring with coagulation factor deficiency.11

 

Buccal Mucosal Bleeding Time - The buccal mucosal bleeding time (BMBT) measures the length of time (minutes) required for the platelets to plug a small laceration in blood vessels. This test evaluates primary hemostasis or platelet status in vivo. The BMBT should not be prolonged with deficiency of coagulation factors. In vWD, the BMBT is usually prolonged because platelet function is abnormal. Mild to moderate vWD will have a BMBT of ~ 5-10 minutes with a reference interval of 2-4 minutes. With severe vWD, the BMBT may be prolonged for 12 or more minutes. Although the BMBT is prolonged in vWD, it is not specific for vWD. The BMBT also may be prolonged in patients with thrombocytopenia and other functional platelet defects. The BMBT is within the reference interval with warfarin toxicity, hemophilia A, hemophilia B, or factor VII deficiency. The advantage of the BMBT is that it is a simple, rapid, and economical way to evaluate in vivo hemostasis.1, 2, 10

Activated Partial Thromboplastin Time (APTT) - Coagulation factor VIII deficiency usually accompanies the deficiency of vWF; however, but factor VIII activity is rarely <30% of the reference interval. Therefore, the APTT generally is not prolonged unless the factor deficiency is <50% of normal values.1,2,10

Definitive Diagnosis of von Willebrand's Disease

Antigenic Measurement - Quantitative ELISA assays are the most common, rapid, sensitive, and reproducible laboratory test to measure vWF concentrations in plasma.11 This test uses anti-vWF antibodies to quantitate vWF antigen.1, 2 Test results usually are reported as units per deciliter or as a percentage with assigned values of 100 U/dl or 100%. vWF:Ag values of less than 50% (<50 U/dl) fall below the reference range and are considered vWF deficient.10 General guidelines for interpretation of the vWF antigen test are as follows:

• 70-180% within reference interval (vWD not present)
• 50-69% borderline (suspicious for vWD)
• 0-49% abnormal (vWD present)

The correct sample for the vWF antigen test is citrated plasma. Blood is collected in 10% sodium citrate anticoagulant (ratio of 1 part whole blood to 9 parts anticoagulant). After the blood specimen is centrifuged, the citrated plasma should be removed and frozen in plastic tubes. Frozen citrated plasma specimens should be shipped to the laboratory for analysis within two weeks of plasma collection. Cat and horse vWF antigen may be determined using the canine-specific reagents.2

Multimeric Assays - Multimeric assays are performed routinely only in research laboratories and are used to classify vWD by subtype. Multimer distribution is measured with protein immunoelectrophoresis. Multimeric assays are used to differentiate type I and type II vWD, whether there is an absence or presence of high molecular weight multimers. 10

Classification

The classification of vWD is used to recommend therapeutic protocols. The three main categories of vWD are as follows:

Type I - Type I vWD is the most common form of vWD in animals and accounts for >90% of reported cases. Type I vWD is associated with a decreased concentration of vWF:Ag. All vWF multimers are present, yet there is a partial quantitative deficiency (<50%). The vWF that is present is both structurally and functionally competent. Clinical signs of vWD are not seen until the concentration of vWF is <20% of normal. The presentation of clinical disease can vary from mild to severe bleeding.1, 2

Type II - In type II vWD, the concentration of vWF is decreased and there are qualitative abnormalities of vWF structure and function. The decreased plasma vWF concentration is associated with a disproportionate loss of high molecular weight multimers. This type of vWD is very rare and clinical disease is manifested by severe bleeding. One or more episodes of hemorrhage typically occur by the time the animal is one year old. Type II vWD is reported primarily in German Shorthaired Pointers and German Wirehaired Pointers.1, 2

Type III - Type III vWD is a severe quantitative deficiency of vWF. This condition is characterized by extremely low concentrations to undetectable concentrations of all multimers (<0.1%). This form of vWD is uncommon. Clinical disease is associated with severe hemorrhage.1, 2 Similar to type II vWD, several bleeding episodes usually occur by the time the animal reached adulthood.

Type of vWD	Concentration of Plasma vWF:Ag	Multimeric Structure of Plasma vWF	Affected Breeds % of dogs with plasma vWF:AG <50%
Type I	Decreased	All multimers decreased	Doberman Pinscher 73% Corgi 43% German Shepherd Dog 35% Golden Retrievers 30% Poodles 30% Shetland Sheepdogs 23%
Type II	Decreased	Disproportionate decrease in high molecular weight multimers	German Shorthair Pointer (NR*)
Type III	Undetectable	Undetectable	Scottish Terrier 30% Shetland Sheepdog 23% Chesapeake Bay Retriever (NR)

* = not reported.

Inheritance of von Willebrand's Disease

vWD is transmitted as an autosomal trait; males and females have an equal chance of inheriting the disorder. The mode of inheritance is still being studied by genetic testing. At this time, researchers are unsure whether vWD is dominant or recessive trait.10

Acquired von Willebrand's Disease

An observation has been made associating hypothyroidism and vWD in dogs, as has been the case in human beings. Levothyroxine supplementation has resulted in a rise in vWF concentration in hypothyroid and euthyroid human beings. A similar observation has not been made in dogs, but further study is warrented.9

Treatment

Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.

Treatment of vWD should be directed toward preventing exacerbations of bleeding and controlling any current bleeding episodes. Prevention of vWD bleeding episodes is accomplished by minimizing injuries and avoiding administration of sulfa drugs, nonsteroidal anti-inflammatory drugs, dextran, heparin, and other drugs that may impair hemostasis.

Emergency treatment of bleeding episodes is accomplished by transfusion, the objective of which is to supply active vWF. Products that will supply active vWF include fresh plasma, fresh-frozen plasma, whole blood, and cryoprecipitate. Cryoprecipitate is preferred because it contains the highest concentration of active vWF in the smallest volume (~ one-tenth of the volume of the plasma from which the cryoprecipitate was derived). Besides vWF (high molecular weight forms), cryoprecipitate also is enriched with factor VIII and fibrinogen. The dosage of cryoprecipitate is 1U /10 kg of body weight (1U of activity is derived from 200 ml of plasma). The second best product to treat vWD-associated bleeding disorders is fresh frozen plasma given at a dosage of 6 to 12ml per kilogram of body weight. Fresh whole blood will provide vWF if it is transfused within 6 hours of collection. Neither stored whole blood nor packed red cells have therapeutic concentrations of vWF.1, 10

1-Deamino-8-D-arginine vasopressin (DDAVP), a vasopressin analogue, is an alternative treatment for vWD-associated bleeding episodes. It is theorized that DDAVP causes a release of vWF from storage sites. The dosage of DDAVP for dogs is 1 µg /kg of body weight given either SC or IV. DDAVP is effective for pre-operative prophylaxis in Type I vWD and can be administered 30 minutes before surgery at a dosage of 1 µg /kg SQ; however, Type I vWD responds poorly to long term DDAVP therapy. Furthermore, DDAVP will increase the vWF concentrations in the blood of healthy dogs, if given 30 minutes prior to blood collection.1, 10