Acute blood loss can lead to shock and even death if >30%–40% of blood is lost and the hypovolemia that develops is not treated aggressively with IV fluids or compatible blood, or both. Causes of acute loss can be known (eg, trauma, surgery) or occult (eg, GI blood loss). Coagulopathies, bleeding tumors, gastric ulceration, and external or internal parasites should be excluded as causes. GI parasites, such as Haemonchus in ruminants and hookworms in dogs, can lead to severe blood loss, especially in young animals. Low-grade, chronic blood loss eventually results in iron-deficiency anemia, although some degree of reticulocytosis may persist even after iron stores become depleted. The hallmark of an iron-deficiency anemia is microcytic, hypochromic anemia. This chronic blood loss can be due to some type of parasitism in young animals (eg, fleas, lice, intestinal parasitism), but in older animals, bleeding from GI ulcers or tumors is more common.