Hereditary Erythrocyte Disorders

Several hereditary erythrocyte defects have now been discovered in both dogs and cats, and recent research has resulted in a great deal of new information. Urs Giger presents an overview of the current situation and offers some pointers for the diagnosis and management of these conditions.

Urs Giger
DVM, MS, PD, Dipl. ACVIM-SA, Dipl. ECVIM-CA, Dipl. ECVCP
 

Introduction

Anemia is one of the most common clinical signs in companion animals and is a frequent finding on laboratory blood tests. Although acquired conditions (such as infections, immune disorders, toxicity, blood loss, and chronic organ failure) represent the main causes of anemia, hereditary erythrocyte defects leading to anemia are becoming increasingly important in clinical practice. Several hereditary erythrocyte defects have been clinically reported in companion animals, and much more new information, including the molecular basis of some disorders, has emerged to readily permit specific clinical diagnoses (1,2). Frequently, these causes are only considered after speculative treatments for immune and infectious diseases have failed, or if the anemia is recurring or persisting, despite the fact that in some breeds such conditions are relatively common and well recognized. This brief review focuses on the peculiarities of erythrocytes and key clinical, diagnostic and management aspects of hereditary erythrocyte defects in dogs and cats.

Canine and feline erythrocytes

Although the major structural features and functions of erythrocytes are similar among all mammals, some noticeable differences exist between canine and feline erythrocytes. Feline erythrocytes are much smaller than canine erythrocytes and thus spherocytes cannot be recognized in cats. However, the erythrocytic hemoglobin concentrations (mean cellular hemoglobin concentration, MCHC) are the same for both species. Interestingly, a few normal variations are recognized; for example, erythrocyte microcytosis is observed in many Akitas, whilst erythrocyte macrocytosis is seen in Miniature Poodles. The normal lifespan of erythrocytes in dogs and humans are similar (100-120 days), but only 70-75 days in cats.

Devoid of a nucleus and mitochondria, erythrocytes have a limited and specialized metabolism that enables them to survive in the circulation and to adequately transport oxygen. Energy is generated almost exclusively through anaerobic glycolysis (the Embden-Meyerhof pathway). The hexose monophosphate shunt reduces pyridine nucleotides and glutathione, which are necessary for degradation of oxidants, thereby preventing membrane damage and hemoglobin denaturation (oxidation). In addition, the methemoglobin or cytochrome-b5 reductase system reduces heme iron from the ferric (Fe3+) to the ferrous (Fe2+) form; only reduced hemoglobin can bind and carry oxygen. The Rapoport-Luebering pathway is responsible for the synthesis of 2,3-diphosphoglycerate (DPG), which influences the oxygen affinity of canine – but not feline – hemoglobin. Indeed, the erythrocytic DPG concentration is similar in dogs and humans, but very low in cats. [...]