Bleeding diathesis associated with variant von Willebrand factor in a Simmental calf.

A 10-month-old Simmental heifer was examined because of a 10-day history of epistaxis and aural hematomas. Examination of the calf also revealed hemarthrosis. Initial laboratory data indicated that platelet count, platelet size, prothrombin time, and partial thromboplastin time were not different from a clinically normal (control) cow. Mucosal bleeding time was prolonged, and platelet adhesion to glass beads was less than expected. The clinical signs, prolonged bleeding time, and platelet adhesion defect were corrected by infusion of bovine plasma. Subsequent laboratory testing revealed that the affected calf had a truncated multimeric structure of von Willebrand factor (vWF), low vWF activity, and impaired platelet aggregation in response to adenosine diphosphate, but concentration of vWF was not different from that in clinically normal control animals. These data were consistent with a diagnosis of variant von Willebrand disease. The relationship of this disease to the thrombopathy of Simmental cattle is unclear.