An Overview of Microcytic Anemia

Matthew W. Woods, DVM; Heather L. Tarpley, DVM; Melanie E. Johnson, DVM; Kenneth S. Latimer, DVM, PhD

Class of 2005 (Woods) and Department of Pathology (Tarpley, Johnson, Latimer), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

Initial clinical diagnosis of anemia is generally based upon classification of the anemia as regenerative or nonregenerative. Regenerative anemias are characterized by increased production of erythrocytes. Conversely, nonregenerative anemia lacks evidence of erythrocyte production. In addition, anemias are classified by certain indices that give the clinician some knowledge of cell size (microcytic, normocytic, microcytic) as well as hemoglobin content (hypochromic, normochromic, or hyperchromic). This discussion will focus on microcytic anemias.

The mean corpuscular volume (MCV) or average erythrocyte size is usually determined directly by automated hematology analyzers, but it can be calculated manually by the formula:

(Hct x 10)/ RBC count (millions) = MCV.¹

Microcytosis is indicated by a MCV value that is below the reference interval. Furthermore, microcytosis often is accompanied by a high RBC distribution width (RDW) value.⁴ This value is a measure of anisocytosis or variation in erythrocyte size.

Several factors influence the MCV value. Factors that associated with microcytosis include immature animals, iron lack or deficiency, portosystemic shunts, Asian dog breeds (Akita, Shiba Inu, Chow Chow), and certain nutritional deficiencies.² The primary focus of this discussion is microcytic anemia.

Iron deficiency is the most common cause of microcytic anemia in animals. Most instances of iron deficiency anemia usually are classified as microcytic and hypochromic. Evidence of erythroid regeneration may be minimal or absent. Microcytic anemias are characterized by the presence of smaller sized erythrocytes

Iron Metabolism

Body iron is regulated by the rate of iron absorption rather than iron excretion. In addition, iron absorption is regulated by the amount of storage iron and rate of erythropoiesis. Ceruloplasmin, a copper containing protein, is necessary for the transfer of iron from intestinal epithelium and macrophages to plasma transferrin. Iron is transported in the plasma bound to transferrin and is measured clinically as serum iron. The total serum transferrin concentration is measured as total iron binding capacity (TBIC). Usually only 1/3 of the transferrin binding sites are occupied by iron. This is expressed as percent saturation. TBIC is usually elevated in iron deficiency except in dogs and there is a low percent saturation.

Iron is stored in macrophages as ferritin and hemosiderin. Ferritin is a water soluble, iron-protein complex. Small amounts of ferritin circulate in the plasma and can be measured as an indirect indicator of the storage iron pool. However, the laboratory test to measure ferritin concentration is species-specific and only available for specimens from dogs, cats, horses, and human beings. Serum ferritin is usually decreased in iron deficiency.¹ It is important to note that ferritin is also an acute phase protein; its concentration can be elevated in inflammation and some forms of neoplasia.² Cytologically, hemosiderin can be demonstrated in the bone marrow of most healthy animals except cats by Perl’s (Prussian blue) staining. Hemosiderin is insoluble in water and will persist in processed cytologic and histologic bone marrow specimens. In iron deficiency anemia, there is a paucity or absence of stainable iron (hemosiderin) in the bone marrow.⁴

In iron deficiency, a decrease in the MCV will precede a decrease in the mean corpuscular hemoglobin concentration (MCHC).¹ Hypochromic cells have a narrow rim of lightly stained hemoglobin and greater than normal central pallor due to decreased hemoglobin (Hg) concentration and cells being thin (leptocytes).² Affected erythrocytes also may be smaller or microcytic because extra cell divisions occur before a critical hemoglobin concentration is reached to arrest mitosis.

Causes of Iron Deficiency Anemia

Blood Loss - Blood sucking parasites (fleas, ticks, hookworms), bleeding intestinal neoplasms, ² transitional cell carcinoma with urogenital bleeding,⁵ gastrointestinal ulcers, thrombocytopenia, inherited hemostatic disorders, hemorrhagic colitis, chronic intravascular hemolysis with hemoglobinuria, and excessive blood draws for blood donation and diagnostic purposes can promote iron deficiency anemia.

In acute blood loss, some degree of erythrocyte regeneration (reticulocytes, except in horses) may keep the MCV within the reference interval. With chronic blood loss, iron is progressively depleted resulting in insufficient iron stores for reticulocyte production. The MCV subsequently decreases.² A negative iron balance can occur with loss of as little as 3-4 ml of blood per day. A decreased serum protein concentration or panhypoproteinemia (low normal to low albumin and globulin concentrations) if the blood loss is sustained. Concurrent thrombocytosis due to an increase in erythropoietin production.⁹

Growing animals - Because milk contains little iron, dietary iron deficiency may be apparent in nursing animals. In young, rapidly growing animals on an all-milk diet, transient iron deficiency may lead to mild anemia. This situation occurs within first week of life in piglets that are reared on concrete flooring without access to soil. Cautious parental or oral supplementation with iron may be necessary. Deficiencies of vitamin E, which is required for heme synthesis, also result in hypoferremia (low concentration of circulating iron). Excessive iron supplementation, in the presence of hypoferremia, may increase free iron concentration in the circulation causing peroxidation of cellular membranes and necrosis within the heart, liver, and skeletal muscle.³

Copper deficiency - Ceruloplasmin is a copper-containing protein that is synthesized by liver. This protein is necessary for transfer of iron from gut epithelium and macrophages to transferring (an iron transporting protein). Generally, copper deficiency is rarely encountered.¹

Iatrogenic iron deficiency - Although copper deficiency is rare, some Bedlington Terriers are genetically predisposed to the development of copper storage disease. Affected individuals may become copper deficient as a result of long term dietary copper restriction and administration of copper chelating drugs.⁶ Microcytic, hypochromic anemia has been documented with long-term copper restriction and chelation therapy. This condition may be due to chelation of copper and other bivalent cations including iron.

Pyridoxine deficiency - Pyridoxine or vitamin B₆ is a cofactor for heme synthesis. Deficiency of this vitamin is rare but may leads to failure of iron utilization.

Figure 1. Normal canine erythrocytes are uniformly sized and have a slight area of central pallor. Wright-Leishman stain.	Figure 2. Dog with iron deficiency. Notice anisocytosis, microcytes, and marked hypochromia (marked central pallor). Wright-Leishman stain.

Figure 3. Bone marrow from a healthy dog. Hemosiderin within macrophages stains blue. Perl’s stain.	Figure 4. Bone marrow from a dog with iron deficiency anemia. Hemosiderin (blue) particles are not present, indicating depletion of bone marrow iron stores. Perl’s stain.

Treatment of Iron Deficiency Anemia

Any underlying disease condition (dietary deficiency, parasitism, neoplasia, etc.) should be diagnosed and appropriately treated. This may allow resumption of normal iron status and reversal of hematologic abnormalities.

Iron supplementation may be considered in selected individuals. Because animals with severe iron deficiency may have impaired intestinal absorption, oral iron supplementation may be of little benefit until partial iron repletion has occurred. Therefore, parenteral iron supplementation should be initiated and followed by oral iron supplementation for 1 to 2 months or until clinical signs of anemia have resolved. Kittens will usually undergo a spontaneous recovery from anemia coinciding with intake of solid foods at 5 to 6 weeks of age.

For parenteral iron supplementation, iron dextran may be injected intramuscularly. This form of iron is released slowly from the injection site. Intravenous administration of iron dextran should be avoided because it can cause hypersensitivity due to the long carbon chain in dextran. Calculation of the dosage of iron dextran is as follows:

(15 - Patient’s Hg(g/dl)) x BW(kg) x 3 = Iron injected (mg)

Oral supplementation of iron may be accomplished by adding ferrous sulfate powder or ferrous gluconate to the food or drinking water. The dosage of these iron compounds is 3 mg /kg of body weight /day given orally.⁹

Other Causes of Microcytosis with or without Anemia

Portosystemic Shunts - Microcytosis with mild anemia is commonly found in dogs with portosystemic shunts.¹ One study suggests that ~66% of affected dogs have low MCV and Hct values that are slightly below the reference interval. The mechanism of microcytosis is not completely understood, but decreased serum iron concentrations, normal to increased ferritin concentration, and accumulation of stainable iron in the liver suggest that microcytosis is associated with abnormal iron metabolism rather than absolute iron deficiency.⁸ Whether this condition involves impaired iron transport or sequestration of stored iron is undetermined. Portosystemic shunts can be congenital or acquired. Affected animals may also have increased activity of hepatic enzymes, elevated pre- and post-prandial bile acid concentrations, elevated ammonia levels, decreased BUN concentration, hypoalbuminemia, and hypoglycemia. Clinically, these individuals may be lethargic, depressed, appear stunted, or exhibit polydypsia and polyuria.⁷

Diserythropoieisis in English Springer Spaniels - Some English Springer Spaniels have been observed to have a microcytic, non-regenerative anemia with circulating metarubricytes. Furthermore, dysplastic changes have been observed in erythroid precursors in the bone marrow. This condition also may be associated with polymyopathy and cardiac disease.

Asian Dog Breeds – Microcytosis has been observed in Akitas, Shiba Inus, and Chow Chows. In these dogs, the microcytosis is unaccompanied by anemia. Thus, the MCV has a lower reference interval for these breeds.¹

Extreme Spherocytosis - A rare cause of microcytosis is extreme spherocytosis. This observation has only been made in certain strains of laboratory mice and humans with hereditary spherocytosis. Marked spherocytes in immune-mediated hemolytic anemia does not present as microcytosis because of marked erythrocyte regeneration. (reticulocytes have an increased MCV).

Differentiation of Iron Deficiency Anemia, Anemia of Chronic Inflammatory Disease, and Microcytic anemia of Portosystemic Shunts

Differentiation of iron deficiency anemia, anemia of chronic inflammatory disease, and microcytic anemia of portosystemic shunts may be challenging. Table 1 presents laboratory features that may aid in distinguishing these three diagnostic possibilities.

Table 1. Parameters to aid in the differentiation of anemias caused by iron deficiency, chronic inflammatory disease, and portosystemic shunts.

References

1. Latimer KS, Mahaffey EA, Prasse KW: Duncan and Prasse’s Veterinary Laboratory Medicine, Clinical Pathology, 4th ed. Iowa State Press / Blackwell Publishing Co., Ames, 2003, pp 3-45.

2. Feldman BF: Schalm's Veterinary Hematology, 5th ed. Lippincott Williams & Wilkins, Baltimore, 2000, pp. 190-204.

3. Merck Veterinary Manual, 8th ed. Merck & Co, Inc., White House Station, 1998, pp. 11-12.

4. Tilley LP: The 5 Minute Veterinary Consult, Canine and Feline, 2nd ed. Lippincott Williams & Wilkins, Philadelphia, 2000, pp. 1436-1437.

5. Nelson RW: Small Animal Internal Medicine, 3rd ed. Mosby, St. Louis, 2003.

6. Seguin MA: Iatrogenic copper deficiency associated with long-term copper chelation for treatment of copper storage disease in a Bedlington Terrier. J Am Vet Med Assoc 218:1593-1597, 2001.

7. Ferrell EA, Graham JP: Simultaneous congenital and acquired extrahepatic portosystemic shunts in two dogs. Vet Radiology Ultrasound 44:38-42, 2003.

8. Johnson SE: http://www.maxshouse.com/Portosystemic shunts.htm

9. Greene CE: Hematology class notes, University of Georgia College of Veterinary Medicine, 2003, pp.1-15.

Acknowledgement

Picture of the Shiba Inu needlepoint kit is from HotDiggityDog.com and is used with permission.

^  Top of page