Why are reticulocytes not reliable for assessing RBC regeneration in horses?

Reticulocytes are immature red blood cells that indicate bone marrow response to anemia, as their presence in circulation suggests active red blood cell production. However, in horses, assessing RBC regeneration using reticulocytes is problematic because reticulocytes are typically not found in significant numbers in the peripheral blood. Unlike species such as dogs or cats, where reticulocytosis can be a clear indicator of regeneration, horses have a unique physiological response to anemia.

In healthy horses, the bone marrow compensates for anemia not by releasing reticulocytes but by releasing mature erythrocytes directly into circulation. Therefore, the lack of reticulocytes in peripheral blood makes them an unreliable indicator of red blood cell regeneration in this species. This fundamental difference in response is what challenges the efficacy of reticulocyte counts in equine medicine, therefore making other methods, such as hemoglobin concentration or hematocrit, necessary to assess the response of the bone marrow to an anemic state in horses.