A Morphometric Study of Bone Marrow Megakaryocytes in Foals Infected with Equine Infectious Anemia Virus

Morphometric evaluation of bone marrow core biopsies was used to determine megakaryocyte (MK) numbers and MK size in nine foals with equine infectious anemia virus (EIAV)-induced thrombocytopenia. Both immunocompetent normal foals and foals with severe combined immunodeficiency (SCID) were used. Platelet counts were made three times weekly following viral infection. Bone marrow core biopsies were taken from the ilium of each foal prior to experimental infection, immediately after the onset of thrombocytopenia, and at necropsy. All foals developed thrombocytopenia by 23 days postinfection. The bone marrow MK density did not change in response to the thrombocytopenia. MK area did not change significantly; however, the MK nuclear area at necropsy was significantly higher than that preinfection. The presence of thrombocytopenia in the SCID foals showed that immune-specific responses were not required for the production of EIAV-induced thrombocytopenia. Furthermore, the lack of a compensatory megakaryocytopoiesis in both SCID and normal foals was consistent with the theory that altered platelet production plays a role in the development of this thrombocytopenia.