MYELOID AND MEGAKARYOCYTIC HYPOPLASIA IN RELATED STANDARDBREDS

Myeloid and megakaryocytic bone marrow hypoplasia in association with moderate to profound neutropenia was observed in 8 young Standardbred horses sired by the same stallion; 7 horses were intermittently thromb ocytopenic. Evaluation of serial neutrophil counts in 2 horses suggest ed that a cyclic variation in neutrophil numbers was present, that lym phocyte numbers increased when neutrophil counts decreased, and that p latelet counts decreased when neutrophil counts decreased. Preliminary bone marrow cultures indicated that myeloid progenitor cells were pre sent and that these cells were able to respond to exogenous growth fac tors by differentiating. A bone marrow microenvironment or growth fact or defect is suspected. Seven of 8 horses died or were euthanized. One horse with moderate neutropenia and a normal platelet count has been racing for 3 years. Necropsies in 4 horses did not reveal a cause for the myeloid hypoplasia. A familial basis for the disease is suspected.