title: Investigation of a novel, heritable bleeding diathesis of Thoroughbred horses and development of a screening assay.
abstract: BACKGROUND
  Bleeding in racing horses associated with exercise appears to be multifactorial, and clinical investigation into severe cases rarely occurs. Previously, we reported a severe bleeding diathesis in a Thoroughbred mare. Herein, we describe the cellular physiology of this defect, provide a diagnostic tool for identifying it, and demonstrate that the dysfunction is heritable.
  
  
  OBJECTIVE
  The subject has a heritable defect in platelet secretion that reduces thrombin generation in the absence of additional plasma factors and delays the onset of thrombin production even in the presence of these factors.
  
  
  METHODS
  The study included 3 clinically normal Thoroughbred horses: the subject and her offspring.
  
  
  METHODS
  Washed platelets were examined for their ability to (1) translocate phosphatidylserine to the outer leaflet of the platelet membrane as determined by annexin-V binding, (2) generate thrombin as assessed by the activity of the prothrombinase enzyme complex, and (3) bind fibrinogen and form aggregates as determined by flow cytometry.
  
  
  RESULTS
  Subject and offspring platelets created procoagulant surfaces by translocating phosphatidylserine. The subject's platelets demonstrated reduced prothrombinase activity, resulting in decreased production of thrombin relative to control platelets. Subject and offspring platelets bound less fibrinogen than control platelets when stimulated with thrombin.
  
  
  CONCLUSIONS
  The subject mare has a transmissible defect that involves reduced generation of thrombin by activated platelets, resulting in decreased aggregation and ineffective clotting. A flow cytometric assay of fibrinogen binding to washed platelets discriminates individuals with this platelet dysfunction and may be useful for discerning subclinical congenital or acquired platelet dysfunctions.
authors.affiliation: Department of Anatomy, School of Veterinary Medicine, University of California, Davis, CA 95616, USA. jwnorris@ucdavis.edu
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Norris
authors.forename: Suzanne M
authors.initials: SM
authors.lastname: Pratt
authors.forename: Joong-Hyuck
authors.initials: JH
authors.lastname: Auh
authors.forename: Sandra J
authors.initials: SJ
authors.lastname: Wilson
authors.forename: Dana
authors.initials: D
authors.lastname: Clutter
authors.forename: K Gary
authors.initials: KG
authors.lastname: Magdesian
authors.forename: Gregory L
authors.initials: GL
authors.lastname: Ferraro
authors.forename: Fern
authors.initials: F
authors.lastname: Tablin
mesh_terms: D000818:Animals; D017304:Annexin A5 / Q000378:metabolism; D001792:Blood Platelets / Q000737:chemistry* / Q000201:enzymology*; D016022:Case-Control Studies; D004789:Enzyme Activation; D005260:Female; D005340:Fibrinogen / Q000378:metabolism; D005434:Flow Cytometry / Q000662:veterinary; D006474:Hemorrhagic Disorders / Q000175:diagnosis / Q000453:epidemiology / Q000235:genetics / Q000662:veterinary*; D006734:Horse Diseases / Q000097:blood / Q000175:diagnosis* / Q000235:genetics*; D006736:Horses; D010718:Phosphatidylserines / Q000378:metabolism; D013917:Thrombin / Q000096:biosynthesis*
chemical_list: D017304:Annexin A5; D010718:Phosphatidylserines; D005340:Fibrinogen; D013917:Thrombin