Anemia is a condition wherein the blood contains an abnormally low number of red blood cells. Horses can have a low red blood cell count due to actual blood loss, the lack of red blood cell production, or red blood cell destruction by some process.

Horses suffering from any chronic disease may produce fewer red blood cells. Cancer of the bone marrow crowds out development of red and white blood cells, reducing these cell types. Certain toxins and drugs can destroy red blood cells (hemolytic anemia).

Traumatic blood loss is relatively common in horses and can cause anemia. In many chronic cases, anemia is mild and goes unnoticed until a complete blood count is performed. Anemia must be relatively severe to cause obvious signs of disease.