Anemia is a blood disorder, it is not a disease, but it is a sign for a disease. Anemia is defined as the decrease in red blood cells count or hemoglobin concentration or both per unit volume of blood.
This means that the decrease of either red blood cells count alone or hemoglobin concentration alone or both of them is defined as anemia. The most important clinical signs of anemia include paleness of mucous membranes, increase respiratory and heart rates, and decrease production and reproduction performance of the animals.
There are different classifications of anemia, the most acceptable is the etiological classification, the latter classify anemia into morphological, hemorrhagic, hemolytic, nutritional and bone marrow depression anemia.