ANEMIA IN PERSPECTIVE
Anemia, defined as a lower than normal blood hemoglobin (Hb) concentration or packed cell volume (PCV; hematocrit [HCT]), occurs if RBC production is acutely or chronically insufficient to replace RBC losses, which may be caused by normal RBC senescence, accelerated RBC destruction (hemolysis), or extracorporeal blood loss (bleeding). Some examples of anemia result from relatively uncomplicated alterations in single factors, for example, transient anemia following an acute hemorrhagic event in an otherwise healthy individual. In most cases, however, the pathophysiology of anemia involves the interplay of several disturbances in RBC homeostasis, including limitations of production as well as abnormal red cell survival. Our first obligation is to examine these factors in a general sense and to evaluate the significance and limitations of the techniques for distinguishing and quantitating them.
MORPHOLOGY OF ERYTHROPOIESIS
After approximately 24 hours, the reticulocyte loses its intracellular materials (which stain with new methylene blue). The cell is now an adult RBC destined to live approximately 80 days in a normal blood volume of about 70 ml/kg (a number different than in other common domestic animals).
Nucleated RBCs may appear in peripheral blood of adult cats without evidence of intensified erythropoiesis and may be a sign of systemic stress of disease or splenic inactivity. This finding is unique to the cat. Nucleated RBCs do acutely accompany increased reticulocyte numbers in early, active erythropoiesis. The presence of increased nRBCs without reticulocytes or in nonanemic states also suggests bone marrow disease or cardiopulmonary dysfunction. These cells are often observed in myeloproliferative disease.
Feline Reticulocytes—Feline RBCs retain stainable reticulum for several weeks. Type I forms have a punctate reticulum staining and are not counted in traditional peripheral blood reticulocyte counts. Types II and III with dense aggregates of reticulum and conforming to the morphologic appearance of reticulocytes of other species are counted in peripheral blood as a gauge of erythroid response. A reticulocyte concentration of aggregate reticulocytes greater than 50,000/µl is evidence of regeneration.
Basophilic stippling, a dark bluish stippling of some immature RBCs, is commonly found in some RBCs released into peripheral blood in response to anemia. This finding has no special significance insofar as the etiology of anemia in the cat is concerned.
FELINE HEMOGLOBIN
Two hemoglobins, designated major and minor, in the cat differ from other mammalian hemoglobins in that they have the largest number (eight) of reactive sulfhydryl groups. The physiologic occurrence of small eccentric refractile bodies (erythrocyte refractile bodies; Heinz bodies; Schmauch bodies) indicates the unusual propensity for hemoglobin denaturation in cats. This is most probably related to the unique structure of feline hemoglobins.(6) This may also be related to the fact that cats have a unique nonsinusoidal spleen, which does not readily remove atypical RBCs.
ADULT FELINE RED BLOOD CELL ANALYTES
With respect to PCV, Hb, and RBC number as well as RBC indices, cats generally attain adult values by 10 or 11 months of age. There are significant differences between kitten and adult values, as discussed in more definitive texts on the subject.
PHYSICAL EXAMINATION
A thorough physical examination is essential in the diagnosis of feline anemia. For example, icterus can be caused by hemolysis; cats with lymphatic diseases often have concurrent or associated anemia and cats with petechiae and/or ecchymoses most probably have thrombocytopenia and/or thrombocytopathia or vasculitis.
ANEMIA AS A DIAGNOSTIC SIGN
Anemia is the result of only a few processes and when these processes are considered, a series of diagnostic steps may be set in motion, often revealing the underlying cause. Iron deficiency anemia in kittens may result from portosystemic shunts (microcytosis without evidence of RBC regeneration), all milk diets, or from external or internal parasitism; in adults, parasitism would be lower on a differential list and blood loss through ulceration or neoplasia would be more common causes.
Decreases in total protein (TP) and the RBC indices along with decreases in serum iron (SI) and increases in total iron-binding capacity (TIBC) would be anticipated. Anemia of inflammatory disease (AID; anemia of chronic disease) is mild if detectable and is associated with decreases in both SI and TIBC. Anemia associated with hemorrhage is not difficult to diagnose and is usually associated with a history of trauma.
Hemorrhagic anemia and hemolytic anemia are unique in terms of increasing reticulocyte percentage once the bone marrow reaches increased productive status three to four days after the insult. Often in intravascular hemolysis or hemolysis induced by Heinz bodies, the mean corpuscular hemoglobin concentration (MCHC) is above the reference interval. Whenever this analyte is increased, the preceding causes should be considered.
Hypohormone anemia, which is anemia associated with renal disease, hypothyroidism, and hypoadrenocorticism, requires specific and incisive testing. Anemia associated with bone marrow disease is usually accompanied by cytoses or cytopenias in at least one other cell line.
EVALUATING LABORATORY TESTS
A complete hemogram complete with all three RBC indices (mean cell volume [MCV], hemoglobin concentration by percentage [MCHC] and hemoglobin by weight [MCH]), HCT, Hb, RBC number, reticulocyte and nRBC numbers, TP, examination of RBCs, white blood cells (WBC) and platelet morphology, and a differential in absolute numbers, is the first step in solving the mystery of anemia. Regenerative anemias are those with elevated reticulocyte counts (see above) and are indicative of hemolysis or hemorrhage. Only when they are accompanied by large numbers of reticulocytes can nRBCs be associated with RBC regeneration. All other forms of anemia are considered nonregenerative or hypoproliferative. When these anemias are severe, transfusion is indicated.
BONE MARROW BIOPSY
Bone marrow aspiration or core biopsy is reserved for any unexplained cytopenia or when neoplasia is staged. When bone marrow examination is used to help explain cytopenia, it is imperative that the bone marrow preparations be accompanied by a concurrent hemogram (i.e., blood drawn at the same time as the marrow sampling). Because normal diurnal and day-to-day variation is quite significant and the hematologist's diagnostics are always referable to peripheral blood numbers, it is incorrect and poor science to draw information from a bone marrow aspirate without reference to a concurrent hemogram. A core biopsy is indicated when attempts at aspiration do not yield bone marrow particles or sufficient cells.
FELINE BLOOD GROUPS AND FELINE TRANSFUSION MEDICINE
Three feline blood groups have been described: A, B, and AB. The feline blood group system is different than that in dogs because the occurrence of natural isoantibodies is common. Approximately 70% of all type B cats have anti-A antibody in a high enough titer to cause decreased RBC survival and acute hemolysis. As little as 5 ml of incompatible blood is enough to cause a fatal reaction. In 35% of all type A cats anti-B is present but usually in low titer; reaction in these animals is less frequent. Although the incidence of type B cats in the United States is low, many purebred cats (excepting the Siamese) such as the Cornish and Devon Rex, British shorthair, Abyssinian, and Himalayan cats have a high frequency of B blood types. Crossmatching is strongly recommended for all cats about to receive blood or blood products. The presence of natural isoantibody always result in decreased RBC survival post-transfusion. Mean RBC survival is approximately 30 days in cats of the same blood type and less than 10 to 14 days in cats with differing blood types.
Donor cats should be screened for red cell parasites, heartworms and feline leukemia virus (FeLV), feline infectious peritonitis (FIP), and feline immunodeficiency virus (FIV). At two-week intervals 10 ml/kg can be collected. The use of citrate-phosphate-dextrose-adenine (CPD-A1) is the recommended anticoagulant. Heparin is contraindicated as it activates platelets and antithrombin III and can result in many unwarranted and disparate reactions including hemorrhage.
When delivering feline whole blood taken in a citrate anticoagulant care must be taken not to cause hypocalcemia, which may be severe and even lethal. Citrate is a strong calcium chelator. Caution must also be used not to induce volume overload. Generally, if less than 20% of blood volume is delivered to a normovolemic but anemic feline patient during an eight-hour period, volume overload does not occur. As mentioned, cats have a normal blood volume of approximately 70 ml/kg.
CONSIDERATIONS
Acepromazine, Chloramphenicol, Trimethoprim Sulfa, Griseofulvin. These drugs can cause hemolytic anemia in the cat or cytopenias due to bone marrow suppression. Pancytopenia commonly occurs in cats when griseofulvin is used in the treatment of dermatomycosis.
Oxidative RBC Injury. Denaturation of feline hemoglobin and Heinz body anemia results from excessive doses of Vitamin K. In dosages in excess of 5 mg/kg/day vitamin K results in severe Heinz body hemolysis. Other drugs that can cause oxidative injury are benzocaine, acetaminophen, methylene blue, and acepromazine. Onions can also cause Heinz bodies.
Severe Heinz Body Hemolysis can result when tiny amounts of lidocaine are sprayed into the oral cavity before intubation or when urinary antiseptics containing methylene blue are used. MPS removal of these cells leads to dramatic and severe anemia.
Propylene Glycol in Semi-Moist Cat Foods. These materials cause Heinz bodies in cats. In cats receiving these foods red cell masses are generally lower than in other cats.
Retroviruses. FeLV and FIV induce a variety of bone marrow diseases including anemia. These viruses also cause erythroid dysplasia. The peripheral blood manifestations of these viral incursions are macrocytic RBCs that are not reticulocytes and nucleated cells that are unclassifiable. These anemias are not regenerative, but often the MCV is elevated significantly above the reference interval.
Immune-Mediated Hemolysis. Positive direct antiglobulin test (DAT; direct Coombs' test), albeit useful in detecting immune-mediated hemolysis, is positive when there is a detectable concentration of immunoglobulin on the red cell surface for any reason.(4,5) For example, hemobartonellosis can cause a positive DAT which will remain so until this organism is cleared from the body. In treating immune-mediated hemolytic anemia (IHA) in the cat, prednisone at 2 to 4 mg/lbis administered every 12 hours (in contrast to the dog in which low dosage glucocorticoid is successful). In confirmed cases of IHA in the cat, chlorambucil, which is not expensive and has few side reactions, has also been reported to be successful as a second drug. Chlorambucil comes in 2 mg tablets; typically, cats receive two to three tablets of chlorambucil once every other week. It is an excellent immunosuppressive agent. Azathioprine is not recommended as an immunosuppressive agent in the cat.
RBC Morphologic Changes. There are numerous morphologic changes in RBCs that are diagnostically helpful. Unfortunately, spherocytes are difficult to recognize in feline blood smears. Schistocytes, (fragmented RBCs) are recognized in disseminated intravascular coagulation (DIC). Leptocytes and acanthocytes, which are suggestive of hepatic diseases in other domestic species, are rare in the cat. Basophilic stippling, although rare, may be associated with lead toxicity.
Howell-Jolly bodies (remnants of nuclear materials) are observed when erythropoiesis is active and suggest bone marrow activity in the erythroid cell line. Often, Howell-Jolly bodies are associated with increased reticulocyte numbers in a new methylene blue stain or with polychromasia in a Wright's stained smear.
Heinz bodies (sometimes described as “signet ring cells”) are evidence of hemoglobin denaturation and may involve certain drugs as has been described. They may also be seen in anorectic cats that are not receiving vitamin B complex in their diets.
Although not common, autoagglutination is indicative of immune-mediated hemolytic processes, most probably involving immunuoglobulin M or some forms of immunoglobulin G.
Hemobartonellosis. The DAT is often positive in cats with hemobartonellosis. This disease is always secondary to another physiologically stressing event and an infectious etiology must be considered. Most commonly, these are viral- or bacterial-induced primary processes. If hemobartonellosis is suspected but not observed, the examiner should place the ethylene diamine tetracetate (EDTA) anticoagulated blood in the refrigerator for eight to 10 hours and look for autoagglutination on the test tube walls or hemolysis in plasma.
Doxycycline is the drug of choice in treating hemobartonellosis in cats and has the advantage of once a day dosing. Tetracycline generally suppresses the feline appetite and sometimes induces fever. Hypersalivation and vomiting are other feline tetracycline reactions.
Total Protein, Total Bilirubin, Urine Biliruin. Hemorrhaging in cats is associated with decreases in total plasma or serum proteins accompanying the decreases observed in red cell mass whereas hemolysis is characterized by normal to increased total proteins. Increases are often artifactual and are caused by the amount of hemolysis or red cell membrane debris in the plasma or serum. Cats with hemolysis often have increases in both total bilirubin and urine bilirubin. Cats that are hemorrhaging would not have increases in these analytes.
Zinc. Ingestion of zinc-containing pennies and the resultant intravascular hemolysis is seen more often in dogs than cats. Nevertheless, many of the cat carriers have zinc-containing materials in the carrier locks or clamps. Stressed or angry cats that chew on these could be subject to severe intravascular hemolysis.
Insulin. Cats being aggressively treated with insulin for diabetes mellitus have reduced serum phosphate content, often to a concentration incompatible with RBC integrity. When intracellular phosphorus is reduced, adenosine triphosphate ion and water exchanges are also reduced, causing spherocytosis and intravascular hemolysis.
Erythropoietin as a Therapeutic Agent. Erythropoietin (Epogen[TM] - Amgen) is used almost exclusively in treating anemia associated with feline chronic renal disease. Dosage is 100 to 150 units/kg subcutaneously every two to three days. As it is a human recombinant product, about one fourth to one third of cats receiving this hormone will produce antibodies against it in six to eight weeks. Care must be taken against overzealous use of this product, which can lead to as polycythemia with severe to fatal results. As the Hct reaches the reference level, the interval dosage of erythropoietin is reduced to every fourth or fifth day. The underlying renal disease is unaffected, but the quality of life is markedly improved.
REFERENCES ARE AVAILABLE ON REQUEST