Iron deficiency

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Blood smear of a dog with severe iron deficiency[1]

Iron deficiency is a relatively uncommon disorder of dogs, characterized by iron levels below the normal range of 46 – 241 μg/dL.[2].

Traditionally it was associate with insufficient meat intake or lack of access to iron-containing soils, but modern diets have virtualy eradicated this disease in domestic dogs in developed countries.

Disorders of iron in the body include iron deficiency anemia, anemia of inflammatory disease, exercise-induced iron deficiency[3] and iron overload.

Iron in the form of heme is vital to many metabolic functions including oxygen transportation in hemoglobin. Iron is also a component of multiple enzymes, including cytochromes, necessary for energy generation and drug metabolism[4].

Through the donation or acceptance of an electron, iron exists in either a reduced ferrous (Fe2+) or an oxidative ferric (Fe3+) state. The majority of functional iron is contained in hemoglobin, with smaller quantities found in myoglobin and cytochromes[5]. The liver, which is the site of production of iron transport proteins, contains the largest non-functional iron stores either as ferritin or hemosiderin[6]. Ferritin is both diffuse and soluble, and is the primary iron storage protein[7]. Hemosiderin is similar in structure, but has more iron relative to protein and is insoluble. Iron is also stored in reticuloendothelial cells of the bone marrow and spleen.

Ferrous iron is absorbed mainly in the duodenum, then bound to transferrin in the plasma and transported for use in target organs or storage.

Iron deficiency results when either dietary intake does not meet the dog’s requirement or when there is chronic external blood loss. The dietary iron requirement for adult dogs s 80 mg/kg and is higher in puppies due to their rapid growth[8].

Inadequate intake is rare except in nursing pups due to the low concentration of iron in milk. Inadequate dietary iron intake does not occur in dogs fed commercial pet foods, but can rarely occur with home-cooked and vegetarian diets without appropriate iron supplementation[9][10].

High iron content is found in meat products (such as liver, heart, and muscle), but also in brewer’s yeast, wheat germ, egg yolks, oysters, some dried beans, and some fruits. Green vegetables, cereals, fish, and fowl have an intermediate amount of iron.

Foods low in iron include milk, milk products, and most non-green vegetables.

Clinical signs in affected dogs are often vague but include lethargy, decreased exercise tolerance, weight loss, retarded growth, pica and generalized malaise[11]. In more severe cases, bounding pulses, arrhythmia and a systolic heart murmur may be evident.

Diagnosis is based on blood tests which reveal a non-regenerative anemia characterized by hypochromic microcytosis and thrombocytosis[12].

Initially, reticulocytosis is present due to increased production and release of reticulocytes secondary to anemia[13]. However, as iron stores are depleted and the iron deficiency becomes more severe, the absolute reticulocyte count becomes inadequate for the degree of anemia.

A differential diagnosis would include other causes of anemia such as enteric- or hemo-parasites.

In mild cases, oral iron therapy is usually preferred over parenteral iron administration due to its low cost and higher safety.

Iron injections are usually curative, with iron dextran can be given at 10 mg elemental iron/kg weekly[14], but in dogs with PCV <15, blood transfusions may be required.

References

  1. Naigamwalla DZ et al (2012) Iron deficiency anemia. Can Vet J 53(3):250-256
  2. McCown JL & Specht AJ (2011) Iron homeostasis and disorders in dogs and cats: a review. J Am Anim Hosp Assoc 47(3):151-160
  3. Kenyon CL et al (2011) Influence of endurance exercise on serum concentrations of iron and acute phase proteins in racing sled dogs. J Am Vet Med Assoc 239(9):1201-1210
  4. Crichton R (2009) Iron Metabolism: From Molecular Mechanisms to Clinical Consequences. 3rd ed. 17–56. West Sussex, UK: John Wiley and Sons. pp:141–325
  5. Harvey JW (2008) Iron metabolism and its disorders. In: Kaneko JJ, Harvey JW, Bruss ML, editors. Clinical Biochemistry of Domestic Animals. 6th ed. Burlington, Massachusetts: Elsevier. pp:259–285
  6. Harvey JW (2000) Microcytic anemia. In: Feldman BF, Zinkl JG, Jain MC, editors. Schalm’s Veterinary Hematology. 5th ed. Philadephia, Pennsylvania: Lippincott, Williams and Wilkins. pp:200–204
  7. Knovich MA et al (2009) Ferritin for the clinician. Blood Rev 23:95–104
  8. Dzanis DA (1994) The Association of American Feed Control Officials dog and cat food nutrient profiles: Substantiation of nutritional adequacy of complete and balanced pet foods in the United States. J Nutr 124(12):2535S–2539S
  9. Gross Kl et al (2000) Nutrients. In: Hand MS, Thatcher CD, Remillard RL, et al., editors. Small Animal Clinical Nutrition. 4th ed. Marceline, Missouri: Walsworth Publishing Company. pp:75–77
  10. Michel KE (2006) Unconventional diets for dogs and cats. Vet Clin North Am Sm Anim Practice 36:1269–1281
  11. Giger U (2005) Regenerative anemias caused by blood loss or hemolysis. In: Ettinger SJ, Feldman EC, editors. Textbook of Veterinary Internal Medicine. 6th ed. St. Louis, Missouri: Elsevier Saunders. pp:1886–1908
  12. Neel JA et al (2012) Thrombocytosis: a retrospective study of 165 dogs. Vet Clin Pathol 41(2):216-222
  13. Fry MM & Kirk CA (2006) Reticulocyte indices in a canine model of nutritional iron deficiency. Vet Clin Pathol 35(2):172-181
  14. Plumb DC (2008) Plumb’s Veterinary Drug Handbook. 6th ed. Ames, Iowa: Blackwell Publ. pp:329–331