THE MERCK VETERINARY MANUAL
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Overview of Anemia

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Anemia is defined as an absolute decrease in the red cell mass as measured by RBC count, hemoglobin concentration, and PCV. It can develop from loss, destruction, or lack of production of RBC. Anemia is classified as regenerative or nonregenerative. In a regenerative anemia, the bone marrow responds appropriately to the decreased red cell mass by increasing RBC production and releasing reticulocytes. In a nonregenerative anemia, the bone marrow responds inadequately to the increased need for RBC. Anemias due to hemorrhage or hemolysis are usually regenerative. Anemias that are caused by decreased erythropoietin or an abnormality in the bone marrow are nonregenerative.

Clinical signs in anemic animals depend on the degree of anemia, the duration (acute or chronic), and the underlying cause. Acute anemia can result in shock and even death if more than a third of the blood volume is lost rapidly and not replaced. In acute blood loss, the animal usually presents with tachycardia, pale mucous membranes, bounding or weak pulses, and hypotension. The cause of the blood loss may be obvious, eg, trauma. If no evidence of external bleeding is found, a source of internal or occult blood loss must be sought, eg, a ruptured splenic tumor, other neoplasia, coagulopathy, GI ulceration, or parasites. If hemolysis is present, the patient may be icteric. Patients with chronic anemia have had time to adjust, and their clinical presentation is usually more indolent with vague signs of lethargy, weakness, and anorexia. These patients will have similar physical examination findings, pale mucous membranes, tachycardia, and possibly splenomegaly or a new heart murmur, or both.

A complete history is an important part of the evaluation of an anemic animal. Questions might include duration of clinical signs, history of exposure to toxins (eg, rodenticides, heavy metals, toxic plants), drug treatments, vaccinations, travel history, and any prior illnesses.

A CBC, including a platelet and a reticulocyte count, will provide information on the severity of anemia and degree of bone marrow response, and also allow for evaluation of other cell lines. A blood smear should be evaluated for abnormalities in RBC morphology or size and for RBC parasites. The RBC indices (measures of size and hemoglobin concentration) are calculated by automated cell counters calibrated for the species in question. RBC size is expressed by the mean corpuscular volume (MCV) in femtoliters and usually reflects the degree of regeneration. Macrocytosis (an increase in the MCV) usually correlates with a regenerative anemia. Macrocytosis can be a heritable condition in Poodles without anemia and may occur in anemic cats infected with feline leukemia virus. Microcytic RBC are the hallmark of iron-deficiency anemia. The hemoglobin concentration of each RBC, measured in g/dL, is defined as the mean corpuscular hemoglobin concentration. Abnormalities in RBC morphology, such as basophilic stippling, can indicate lead intoxication. Heinz body formation indicates oxidant injury to RBC, secondary to toxin exposure (see Table 1: Toxic Causes of AnemiaTables). Cats are more susceptible to Heinz body formation than other species, and even cats without anemia can have a small number of Heinz bodies.

Table 1

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The reticulocyte count is usually reported as a percent of the RBC mass. This value should be corrected for the degree of anemia in order to evaluate the degree of regeneration. An absolute reticulocyte count (measured by RBC/μL × reticulocyte percentage) of >50,000/μL or >60,000/μL in cats or dogs, respectively, is considered regenerative. In order to correct the percent reticulocytes, the formula (see below) can be applied. A corrected reticulocyte percent >1% indicates regeneration in the dog and cat. After acute blood loss or hemolytic crisis, reticulocytosis usually takes 3–4 days to become evident.

A serum chemistry panel and urinalysis evaluate organ function. If GI blood loss is suspected, an examination of the feces for occult blood and parasites can be useful. Radiographs can help identify occult disease, such as a penny (zinc toxicity) in the stomach of a puppy with hemolytic anemia. Bruising or bleeding may be signs of a coagulopathy and indicate the need for a coagulation profile. If hemolytic disease is suspected, blood can be evaluated for autoagglutination and a direct Coombs' test might be indicated. A test for autoagglutination can be done by placing a drop of saline on a slide with a fresh drop of the patient's blood; the slide should be gently rotated to mix the drops together, then evaluated grossly and microscopically for macro- and microagglutination. Serology for infectious agents like feline leukemia virus, Ehrlichia, equine infectious anemia virus, and Babesia may also be helpful in defining the cause of anemia (see Table 2: Infectious Causes of AnemiaTables).

Table 2

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Bone marrow evaluation by aspiration and/or biopsy is indicated in any animal with an unexplained, nonregenerative anemia. If the CBC reveals a decrease in more than one cell line, possibly indicating a hypoplastic marrow, a biopsy would be indicated along with an aspirate. Biopsies and aspirates are complementary: biopsies are better for evaluating the architecture and degree of cellularity of the marrow, and aspirates allow for better evaluation of cellular morphology. Aspirates also allow for an evaluation of orderly maturation of the red and white blood cell lines, the ratio of red to white blood cell precursors (M:E ratio), and the number of platelet precursors. Iron store can also be evaluated by Prussian blue staining. An M:E ratio of <1 indicates that red cell production is greater than white cell production; with an M:E ratio >1, the opposite is likely. The M:E ratio is always interpreted in light of a recent CBC, because changes in the ratio could also be due to suppression of one cell line compared with the other.

Last full review/revision July 2011 by Sarah E. Kraiza, DVM, DACVIM (Oncology)

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