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 deter mine megakaryocyte (MK) numbers and MK size in nine foals with equine infectious anemia virus (EIAV)-induced thrombocytopenia. Both immunoco mpetent normal foals and foals with severe combined immunodeficiency ( SCID) were used. Platelet counts were made three times weekly followin g viral infection. Bone marrow core biopsies were taken from the ilium of each foal prior to experimental infection, immediately after the o nset of thrombocytopenia, and at necropsy. All foals developed thrombo cytopenia by 23 days postinfection. The bone marrow MK density did not change in response to the thrombocytopenia. MK area did not change si gnificantly; however, the MK nuclear area at necropsy was significantl y higher than that preinfection. The presence of thrombocytopenia in t he 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 fo als was consistent with the theory that altered platelet production pl ays a role in the development of this thrombocytopenia.