A two year old Quarter Horse gelding presented on emergency to Cornell’s Equine and Nemo Farm Animal Hospital (ENFAH) for severe anemia, as part of a pancytopenia. The original complaint to the primary care veterinarian was a one month history of progressive lethargy which had begun impacting the horse’s performance. On presentation the patient was tachycardic (90 bpm), tachypneic, and had pale mucous membranes; the remainder of his physical exam was unremarkable. Point of care blood work revealed a packed cell volume (PCV) of 10%, thrombocytopenia (12 thou/uL), and leukopenia (4 thou/uL), as well as a mild increase in lactate. There was no evidence of macroscopic agglutination of red blood cells when a slide agglutination test was performed. Thoracic and abdominal ultrasound was performed to identify masses and assess for hemorrhage, but no abnormalities were noted. A blood transfusion was performed to stabilize the patient.

Based on the examination at presentation and further work up of the case, our problem list was severe anemia, thrombocytopenia, leukopenia, and tachycardia. Our differential diagnoses broadly included bone marrow suppression due to bone marrow injury (toxic, infectious, or drug) or neoplasia, hemorrhage, and hemolysis. Blood samples were submitted to test for equine infectious anemia and Anaplasma phagocytophilum, as well as for performing a complete blood count with blood smear evaluation, chemistry, and a Coombs test. A bone marrow biopsy was performed to elucidate a possible cause of intramedullary disease. A repeat complete blood count revealed several promonocytes in the smear and rare round mononuclear cells with fine chromatin and 1-3 indistinct nucleoli were seen (suspected to be circulating blasts). The bone marrow biopsy showed that 60% of the bone marrow consisted of neoplastic cells. It was determined that the neoplastic cells were most likely of monocyte origin based on the evidence of monocytic differentiation. Due to the poor prognosis associated with the diagnosis of acute myeloid leukemia, euthanasia was elected.  

This presentation will discuss the differential diagnoses for pancytopenia in an equine patient, with emphasis on acute myeloid leukemia.