Platelets from thrombocytopenic ponies acutely infected with equine infectious anemia virus are activated in vivo and hypofunctional

Thrombocytopenia is a consistent finding and one of the earliest hematologi cal abnormalities in horses acutely infected with equine infectious anemia virus (EIAV), a lentivirus closely related to human immunodeficiency virus. Multifactorial mechanisms, including immune-mediated platelet destruction and impaired platelet production, are implicated in the pathogenesis of EIA V-associated thrombocytopenia. This study was undertaken to investigate whe ther regenerative thrombopoiesis and platelet destruction occurred in ponie s acutely infected with EIAV. Circulating large, immature platelets were in creased in ponies acutely infected with EIAV late in the infection when pla telet count was at a nadir. Morphometric analysis of bone marrow from acute ly infected ponies revealed significant increased in megakaryocyte area and megakaryocyte nuclear area. A trend toward increased numbers of megakaryoc ytes was also observed. Platelets from acutely infected ponies had increase d surface-bound fibrinogen and ultrastructural changes consistent with in v ivo platelet activation. Platelets also had hypofunctional aggregation resp onses to three agonists in vitro. We conclude that thrombocytopenia in poni es acutely infected with EIAV is regenerative and suggest that bone marrow platelet production is not severely compromised in these ponies. Our findin gs reveal that in vivo platelet activation occurs in ponies acutely infecte d with EIAV, and as a result platelets are hypofunctional in vitro. Activat ion of platelets in vivo may cause platelet degranulation or formation of p latelet aggregates, which would result in removal of these damages platelet s from circulation. This may represent a form of nonimmune-mediated platele t destruction in ponies acutely infected with EIAV. (C) 1999 Academic Press .