Von Willebrand Disease

Von Willebrand Disease (vWD) is the most common inherited bleeding disorder in dogs, characterized by a quantitative or qualitative deficiency of von Willebrand factor (vWF), a large multimeric glycoprotein essential for primary hemostasis. vWF mediates platelet adhesion to subendothelial collagen at sites of vascular injury and stabilizes coagulation factor VIII (FVIII) in circulation. The disease is classified into three major types: Type 1 (quantitative partial deficiency), Type 2 (qualitative defect with decreased high-molecular-weight multimers), and Type 3 (complete absence of vWF). Type 1 is the most prevalent in dogs, while Type 3 is the most severe. vWD results in mucosal and cutaneous bleeding tendencies, with severity varying from mild to severe depending on the type and residual vWF activity. The condition is inherited as an autosomal trait, with specific breed predilections and genetic mutations identified. Diagnosis relies on measurement of vWF antigen (vWF:Ag), vWF activity assays, and buccal mucosal bleeding time (BMBT). Treatment focuses on supportive care, desmopressin acetate (DDAVP) for mild to moderate cases, and transfusion of blood products for severe bleeding or surgical prophylaxis.

Von Willebrand Disease is primarily an inherited genetic disorder. In dogs, the most common form is Type 1, caused by autosomal dominant mutations with variable penetrance, leading to reduced plasma vWF levels. Specific mutations have been identified in breeds such as the Doberman Pinscher (a splice site mutation in the vWF gene), Scottish Terrier, Shetland Sheepdog, and German Shorthaired Pointer. Type 2 vWD results from missense mutations that impair multimer assembly or secretion, leading to dysfunctional vWF with absent high-molecular-weight multimers; this is rare in dogs but has been reported in German Shorthaired Pointers. Type 3 vWD is autosomal recessive, with complete deficiency of vWF due to nonsense or frameshift mutations; it is seen in breeds like the Scottish Terrier, Shetland Sheepdog, and Chesapeake Bay Retriever. Acquired vWD can occur secondary to hypothyroidism, due to reduced synthesis or increased clearance of vWF, and has been reported in dogs with autoimmune thyroiditis. Additionally, certain neoplasms, such as hemangiosarcoma, may cause acquired vWD via adsorption of vWF onto tumor cells. The underlying molecular defect leads to impaired platelet adhesion and aggregation, resulting in prolonged bleeding times.

Von Willebrand Disease is the most common inherited coagulopathy in dogs, with an estimated prevalence of 1-2% in the general canine population, but much higher in predisposed breeds. Over 50 breeds have been identified with vWD, with the Doberman Pinscher having the highest prevalence, where up to 70% of individuals may be affected with Type 1 vWD, though many are asymptomatic carriers. Other commonly affected breeds include Scottish Terriers, Shetland Sheepdogs, German Shepherds, Golden Retrievers, and Poodles. Type 3 vWD is less common but severe, with a higher incidence in Scottish Terriers, Shetland Sheepdogs, and Chesapeake Bay Retrievers. The disease is inherited as an autosomal trait; Type 1 is autosomal dominant with incomplete penetrance, while Type 3 is autosomal recessive. There is no sex predilection, and the disease is typically diagnosed in young to middle-aged dogs, often when they present with bleeding episodes or during pre-surgical screening. Geographic distribution is global, but breed-specific prevalence varies with regional breeding practices. In cats, vWD is rare, with a few case reports in domestic shorthair cats, but the disease is not as well characterized.

Von Willebrand factor is a large multimeric glycoprotein synthesized by endothelial cells and megakaryocytes. It is stored in Weibel-Palade bodies in endothelial cells and alpha-granules in platelets. Upon vascular injury, vWF binds to exposed subendothelial collagen via its A3 domain and undergoes conformational changes under shear stress, allowing its A1 domain to bind to platelet glycoprotein Ib (GPIb) receptors. This interaction mediates platelet adhesion and subsequent platelet activation and aggregation, forming a primary hemostatic plug. vWF also binds and stabilizes coagulation factor VIII (FVIII) in plasma, protecting it from proteolytic degradation. In vWD, quantitative or qualitative defects in vWF lead to impaired platelet adhesion, prolonged bleeding time, and secondary deficiency of FVIII (especially in Type 3). The severity of bleeding correlates with the degree of vWF deficiency and the absence of high-molecular-weight multimers, which are most effective in supporting platelet adhesion under high shear stress. The molecular defects include mutations that impair synthesis, dimerization, multimerization, storage, or secretion of vWF. In Type 1, there is reduced synthesis or increased clearance of vWF, leading to low plasma levels but normal multimer distribution. Type 2 involves abnormal multimer formation, with loss of high-molecular-weight multimers, resulting in a qualitative defect. Type 3 is characterized by complete absence of vWF, leading to severe bleeding and significant FVIII deficiency. The clinical consequence is a primary hemostatic defect, manifesting as mucosal bleeding (epistaxis, gingival bleeding, hematuria, gastrointestinal bleeding) and prolonged bleeding after trauma or surgery.

The primary predisposing factor for vWD is genetic inheritance, with specific breed predispositions and known mutations. Dogs with Type 1 vWD may have variable clinical expression, and some may never exhibit bleeding unless challenged by surgery, trauma, or concurrent disease. Hypothyroidism is a significant acquired predisposing factor, as thyroid hormone influences vWF synthesis and release; dogs with hypothyroidism may have reduced vWF levels, unmasking or exacerbating vWD. Other endocrine disorders, such as hyperadrenocorticism, may also affect hemostasis. Concurrent use of medications that impair platelet function, such as nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin, or other antiplatelet agents, can increase bleeding risk. Severe systemic diseases, including neoplasia (e.g., hemangiosarcoma), liver disease, and uremia, can cause acquired vWD or exacerbate bleeding tendencies. Stress, exercise, and pregnancy can transiently increase vWF levels, potentially masking the diagnosis. Age may also play a role, as vWF levels increase with age in some dogs, but this is not consistent. Environmental factors such as trauma or surgical procedures are common triggers for bleeding episodes in affected dogs.

Clinical signs of vWD vary depending on the type and severity of the disease. In Type 1, many dogs are asymptomatic, and bleeding may only occur after surgery, trauma, or during estrus. When present, signs include epistaxis (nosebleeds), gingival bleeding, prolonged bleeding from minor wounds, hematuria (blood in urine), and gastrointestinal bleeding (melena or hematochezia). In Type 2, bleeding is more frequent and severe, with similar mucosal bleeding but also potential for joint bleeding (hemarthrosis) in severe cases. Type 3 is the most severe, with spontaneous bleeding episodes, including severe epistaxis, oral bleeding, hematuria, and gastrointestinal hemorrhage; these dogs may also develop hematomas and bleeding into body cavities. Physical examination findings may include pale mucous membranes if anemia is significant, petechiae and ecchymoses (though less common than in thrombocytopenia), and signs of blood loss such as tachycardia and weak pulses. In chronic cases, iron deficiency anemia may develop due to recurrent blood loss. Bleeding may be exacerbated by concurrent conditions such as hypothyroidism or use of antiplatelet drugs. In severe cases, bleeding can be life-threatening, especially if it occurs in critical areas such as the central nervous system or airway.

Differential diagnoses for vWD include other primary hemostatic disorders such as thrombocytopenia (immune-mediated, infectious, or drug-induced), thrombopathia (e.g., Glanzmann's thrombasthenia, Chediak-Higashi syndrome), and other inherited coagulopathies like hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency). Secondary hemostatic disorders, such as vitamin K deficiency or rodenticide toxicity, can also cause bleeding but typically present with hematomas and hemarthrosis rather than mucosal bleeding. Acquired conditions that cause platelet dysfunction, such as uremia, liver disease, and multiple myeloma, should be considered. Additionally, vasculitis and disseminated intravascular coagulation (DIC) can cause bleeding. Key distinguishing features: vWD typically presents with mucosal bleeding and prolonged buccal mucosal bleeding time (BMBT) with normal platelet count and normal coagulation times (PT, aPTT). In contrast, thrombocytopenia has low platelet count; hemophilia A and B have prolonged aPTT with normal platelet count and BMBT; vitamin K deficiency/rodenticide toxicity have prolonged PT and aPTT. Specific assays for vWF:Ag and vWF activity confirm vWD. Genetic testing for known mutations is also available for certain breeds.

The diagnostic approach to vWD begins with a thorough history and physical examination, focusing on bleeding episodes, breed, and family history. If vWD is suspected, initial screening tests include a complete blood count (CBC) to assess platelet count and rule out thrombocytopenia, and a buccal mucosal bleeding time (BMBT) to evaluate primary hemostasis. A prolonged BMBT (>4 minutes) with normal platelet count suggests platelet dysfunction or vWD. Coagulation panel (PT, aPTT) should be performed to rule out secondary hemostatic defects; these are typically normal in vWD. If vWD is suspected, specific testing includes measurement of vWF antigen (vWF:Ag) via ELISA, which quantifies the amount of vWF protein. Low vWF:Ag (<50% of normal) is consistent with Type 1 vWD, while very low or undetectable levels suggest Type 3. For Type 2 vWD, vWF activity assays (e.g., ristocetin cofactor activity, collagen binding assay) are needed to detect functional defects; a discrepancy between vWF:Ag and activity indicates a qualitative defect. Multimer analysis by gel electrophoresis can confirm Type 2 by showing loss of high-molecular-weight multimers. Genetic testing for specific mutations is available for breeds with known mutations (e.g., Doberman Pinscher, Scottish Terrier) and can confirm the diagnosis and identify carriers. In cases where acquired vWD is suspected, additional tests for hypothyroidism (thyroid panel) or other underlying diseases may be warranted.

In vWD, the complete blood count (CBC) is typically normal, with a normal platelet count and morphology. However, if significant bleeding has occurred, anemia may be present, which can be regenerative or non-regenerative depending on chronicity. Serum biochemistry is usually unremarkable, but iron deficiency anemia may develop in chronic cases, characterized by microcytic hypochromic anemia, low serum iron, low ferritin, and increased total iron-binding capacity. Coagulation tests, including prothrombin time (PT) and activated partial thromboplastin time (aPTT), are normal in vWD, as the secondary hemostatic pathway is intact. However, in Type 3 vWD, aPTT may be mildly prolonged due to severe FVIII deficiency. The buccal mucosal bleeding time (BMBT) is prolonged (>4 minutes) and is a useful screening test. The definitive laboratory finding is a decreased plasma vWF:Ag concentration, measured by ELISA. Normal canine vWF:Ag levels are typically >70% of a reference standard; levels <50% are diagnostic for Type 1 vWD, and levels <1% are seen in Type 3. vWF activity assays, such as ristocetin cofactor activity (vWF:RCo) or collagen binding assay (vWF:CB), are reduced in all types, but in Type 2, the activity is disproportionately lower than the antigen level. Multimer analysis shows a loss of high-molecular-weight multimers in Type 2. Genetic testing for specific mutations (e.g., in the vWF gene) can confirm the diagnosis and identify carriers. Additional tests may include thyroid function tests to rule out hypothyroidism as a cause of acquired vWD.

Imaging is not typically used for the diagnosis of vWD, but it may be employed to evaluate bleeding complications. Thoracic radiography may be indicated if there is suspicion of pulmonary hemorrhage or hemothorax, which can occur in severe cases. Abdominal ultrasonography can be used to assess for gastrointestinal bleeding, hematuria, or retroperitoneal hemorrhage. In cases of acute bleeding, ultrasonography can detect free fluid in body cavities (e.g., hemoabdomen) and guide abdominocentesis. Echocardiography is not directly relevant to vWD but may be performed if cardiac disease is suspected as a comorbidity. Advanced imaging such as CT or MRI is rarely needed but may be used to evaluate intracranial hemorrhage in neurological cases. Imaging findings are nonspecific and reflect the consequences of bleeding, such as organomegaly, fluid accumulation, or masses (hematomas). Therefore, imaging is primarily supportive and does not contribute to the diagnosis of vWD itself.

Cytology and histopathology are not typically used for the diagnosis of vWD, as the disease is a plasma protein deficiency. However, if a bleeding disorder is suspected, bone marrow examination may be performed to rule out primary bone marrow disorders such as thrombocytopenia or leukemia. Bone marrow cytology in vWD is usually normal, with adequate megakaryocytes. Histopathology of tissues may reveal evidence of hemorrhage, such as extravasated red blood cells, hemosiderin-laden macrophages, and fibrosis in chronic cases. In cases of acquired vWD secondary to neoplasia, histopathology of the tumor may be diagnostic. For example, hemangiosarcoma may show malignant endothelial cells. Immunohistochemistry for vWF can be performed on tissue sections to assess endothelial cell vWF content, but this is not a standard diagnostic test for vWD. In research settings, electron microscopy can demonstrate reduced or absent Weibel-Palade bodies in endothelial cells, but this is not clinically applicable. Overall, cytology and histopathology play a limited role in the diagnosis of vWD, primarily to exclude other causes of bleeding.

The treatment of vWD depends on the severity of the disease and the clinical situation. For minor bleeding episodes, local compression and wound care may be sufficient. For more significant bleeding or to prevent bleeding during surgery, the following strategies are employed: 1. Desmopressin acetate (DDAVP) is the first-line therapy for Type 1 vWD and some cases of Type 2. DDAVP is a synthetic analog of vasopressin that stimulates the release of vWF from endothelial cells, transiently increasing plasma vWF levels. The recommended dose is 1-4 mcg/kg subcutaneously or intravenously, diluted in saline and given over 15-30 minutes, or 1-2 drops of intranasal solution (100 mcg/mL) per nostril. The effect peaks in 30-60 minutes and lasts 1-2 hours. It can be repeated, but tachyphylaxis may occur after repeated doses. DDAVP is not effective in Type 3 vWD because there is no endogenous vWF to release. 2. For severe bleeding or surgical prophylaxis, transfusion of blood products is necessary. Fresh frozen plasma (FFP) or cryoprecipitate (if available) provides vWF and FVIII. The dose of FFP is 10-20 mL/kg IV, repeated every 8-12 hours as needed. Cryoprecipitate is more concentrated and is given at 1 unit per 10 kg body weight. Whole blood may be used if anemia is also present. 3. Antifibrinolytic agents, such as epsilon-aminocaproic acid (EACA) or tranexamic acid, can be used as adjunctive therapy to stabilize clots. EACA is given at 500 mg/kg PO or IV q8h (max 4 g), and tranexamic acid at 15-25 mg/kg PO or IV q8h. These are particularly useful for mucosal bleeding. 4. In cases of acquired vWD due to hypothyroidism, thyroid hormone replacement therapy (levothyroxine 0.02 mg/kg PO q12h) may improve vWF levels over several weeks. 5. Supportive care includes fluid therapy, blood transfusion for anemia, and iron supplementation if iron deficiency develops. 6. Avoidance of drugs that impair platelet function, such as NSAIDs and aspirin, is crucial. 7. For elective surgery, pre-operative treatment with DDAVP or plasma transfusion is recommended, and post-operative monitoring for bleeding is essential.

The prognosis for vWD varies with the type and severity. Dogs with Type 1 vWD generally have a good prognosis, especially if they are asymptomatic or have mild bleeding episodes that can be managed with DDAVP. Many affected dogs live normal lives without significant bleeding unless challenged by surgery or trauma. Type 2 vWD is less common and may cause more frequent bleeding, but with appropriate management, the prognosis is still fair to good. Type 3 vWD carries a guarded prognosis due to the severity of bleeding episodes, which can be life-threatening. However, with aggressive treatment including plasma transfusions and careful management, affected dogs can survive, but they require lifelong vigilance and may have a reduced quality of life. Negative prognostic indicators include severe spontaneous bleeding, bleeding into critical sites (e.g., central nervous system), and the development of complications such as anemia or iron deficiency. The response to DDAVP is generally good in Type 1, but repeated use may lead to tachyphylaxis. Overall, with proper diagnosis and management, many dogs with vWD can have a good quality of life, but owners must be educated about the risks and the need for preventive measures.

Follow-up for dogs with vWD depends on the severity and treatment. After a bleeding episode or surgical procedure, dogs should be monitored closely for 24-48 hours for signs of continued bleeding. Serial hematocrit and platelet counts may be checked to assess blood loss. If DDAVP is used, vWF levels can be measured before and after administration to assess response. For dogs on long-term management, regular veterinary check-ups every 6-12 months are recommended to monitor for any complications, such as iron deficiency anemia. If hypothyroidism is diagnosed, thyroid hormone levels should be monitored and levothyroxine dose adjusted accordingly. Owners should be educated to avoid medications that affect platelet function and to seek immediate veterinary care if bleeding occurs. For breeding dogs, genetic testing is recommended to identify carriers and prevent the spread of the disease. In severe cases, a referral to a veterinary hematologist may be beneficial. Long-term follow-up should include periodic assessment of vWF levels, especially if the dog is to undergo surgery or if there is a change in clinical status.

Pearls: 1. Always consider vWD in any dog with mucosal bleeding and a normal platelet count. 2. The buccal mucosal bleeding time (BMBT) is a simple, inexpensive screening test for primary hemostatic defects. 3. DDAVP is a valuable tool for managing Type 1 vWD, but it is ineffective in Type 3. 4. For surgical prophylaxis, administer DDAVP or plasma 30-60 minutes before the procedure. 5. In breeds with known vWD mutations, genetic testing can identify carriers and affected dogs. 6. Hypothyroidism can cause acquired vWD; always check thyroid function in dogs with unexplained bleeding. Pitfalls: 1. Do not rely solely on aPTT to rule out vWD; it is normal in most cases. 2. Avoid using NSAIDs or aspirin in dogs with vWD, as they can exacerbate bleeding. 3. Do not use DDAVP in Type 3 vWD, as it will not be effective. 4. Be cautious with repeated DDAVP doses, as tachyphylaxis can occur. 5. Do not forget to check for concurrent diseases that may worsen bleeding, such as liver disease or uremia. 6. In emergency situations, do not delay transfusion if severe bleeding is present; plasma or cryoprecipitate is the treatment of choice.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are recommended for vWD: 1. Desmopressin acetate (DDAVP): For Type 1 and some Type 2 vWD, administer 1-4 mcg/kg SC or IV (diluted in 10-20 mL saline over 15-30 minutes) or intranasal (1-2 drops of 100 mcg/mL solution per nostril) 30-60 minutes before surgery or at the time of bleeding. The effect lasts 1-2 hours; may repeat in 12-24 hours if needed, but tachyphylaxis may occur. 2. Fresh frozen plasma (FFP): For severe bleeding or surgical prophylaxis, administer 10-20 mL/kg IV over 2-4 hours, repeated every 8-12 hours as needed. 3. Cryoprecipitate: If available, administer 1 unit per 10 kg body weight IV, repeated as needed. 4. Epsilon-aminocaproic acid (EACA): For adjunctive therapy in mucosal bleeding, administer 500 mg/kg PO or IV q8h (maximum 4 g per dose). 5. Tranexamic acid: 15-25 mg/kg PO or IV q8h. 6. Levothyroxine: For acquired vWD due to hypothyroidism, administer 0.02 mg/kg PO q12h, adjusting dose based on thyroid hormone levels. 7. Iron supplementation: For iron deficiency anemia, administer ferrous sulfate 100-300 mg/day PO (dogs) divided, or as directed. 8. Avoid drugs that impair platelet function, such as aspirin, NSAIDs, and other antiplatelet agents. 9. For emergency stabilization, administer IV fluids (e.g., lactated Ringer's solution) and consider blood transfusion if anemia is severe. 10. In cases of severe bleeding, consider recombinant activated factor VII (rFVIIa) as a rescue therapy, though its use is limited and expensive.

Von Willebrand disease in dogs has been extensively studied. Key findings from the literature include: 1. The prevalence of vWD in Doberman Pinschers is high, with up to 70% affected, but many are asymptomatic (Stokol et al., 1995). 2. Genetic mutations have been identified for Type 1 vWD in Doberman Pinschers (a splice site mutation) and Scottish Terriers (a missense mutation), enabling DNA testing (Rieger et al., 1998; Kramer et al., 2004). 3. DDAVP has been shown to increase vWF levels in dogs with Type 1 vWD, but the response is variable and not sustained (Kraus et al., 1999). 4. A study by Brooks et al. (1991) demonstrated that cryoprecipitate is more effective than fresh frozen plasma in increasing vWF levels and controlling bleeding. 5. The ACVIM consensus statement on the diagnosis and management of vWD in dogs provides guidelines for testing and treatment (Brooks et al., 2005). 6. Hypothyroidism has been associated with acquired vWD, and thyroid hormone replacement can improve vWF levels (Panciera et al., 1992). 7. A retrospective study by Callan et al. (2002) reported that dogs with Type 3 vWD have severe bleeding episodes and require aggressive transfusion therapy. 8. The use of antifibrinolytic agents such as tranexamic acid has been reported to be beneficial in reducing bleeding in vWD (Boudreaux et al., 2010). 9. Genetic testing is now widely available and recommended for breeding programs to reduce the incidence of vWD (ACVIM consensus). 10. Overall, the evidence supports the use of DDAVP for Type 1 vWD, plasma products for severe cases, and careful perioperative management to prevent bleeding complications.