Proceeding of the LAVC Latin American Veterinary Conference Oct , 2009 Lima, Peru

Transcription

1 Close this window to return to IVIS Proceeding of the LAVC Latin American Veterinary Conference Oct , 2009 Lima, Peru Reprinted in the IVIS website with the permission of the LAVC

2 Dr. Michael Fry COMMON LEUKOGRAM ABNORMALITIES IN DOGS AND CATS ANEMIA CLASSIFICATION, LABORATORY EVALUATION, MECHANISMS OF DISEASE CYTOLOGY BASICS AND COMMON SKIN/SUBCUTANEOUS LESIONS Página 200

3 ANEMIA CLASSIFICATION, LABORATORY EVALUATION, MECHANISMS OF DISEASE Anemia refers to subnormal circulating red blood cell mass, characterized by decreases in one or more (usually all) of these complete blood count (CBC) parameters: Erythrocyte concentration Hematocrit (or packed cell volume) Hemoglobin concentration Anemia causes clinical signs referable to decreased oxygen-carrying capacity (e.g., pallor of mucous membranes, lethargy, weakness, exercise intolerance) and may also cause other laboratory abnormalities secondary to tissue hypoxia (e.g., increased liver enzyme activities as a result of hypoxia-induced damage to hepatocytes). Anemia also causes decreased viscosity of the blood, and in marked cases frequently causes heart murmurs as a result of a decrease in laminar blood flow. Classification of anemia Anemia No reticulocytosis Reticulocytosis Impending reticulocytosis Non-regenerative Regenerative Pre-regenerative Classifying anemia as regenerative or nonregenerative is clinically useful because it provides information about the mechanism of disease, as discussed in more detail below (also see Appendix A). Página 205

4 The hallmark of regenerative anemias (except in horses) is reticulocytosis i.e., increased concentration of immature erythrocytes in circulation. Reticulocytosis indicates increased erythropoiesis characterized by erythroid hyperplasia in the bone marrow, and release of erythrocytes into the circulation before they are fully mature (further back in the production pipeline ). Thus, reticulocytosis inidicates an appropriate compensatory response to anemia. Normal reticulocyte counts are species-dependent. Here are some guidelines (ideally, the reference values should be laboratory-specific): Reticulocyte reference Reticulocytes with Species values (#/ul) regenerative response? Dogs & cats 80,000 Yes Cattle None Yes Horses None No Absence of reticulocytosis indicates that erythropoiesis is being inhibited in some way. There are many potential underlying causes of non-regenerative anemia, as will be discussed further. The term preregenerative is sometimes used to describe anemia with a regenerative response that is impending, but not yet apparent on the CBC. Confirming a regenerative response in such cases requires either evidence of erythroid hyperplasia in the bone marrow or emergence of a reticulocytosis on subsequent days. Recall that the stimulus for increased erythropoiesis is increased secretion of erythropoietin (Epo) in response to hypoxemia. Although the action of Epo on erythropoiesis is rapid, evidence of a regenerative response is not immediately apparent in a blood sample. One of the main effects of Epo is to expand the pool of early stage erythroid precursors, and it takes time for these cells to differentiate to the point where they are released into circulation. In a case of acute blood loss or hemolysis, it typically takes 3 to 4 days until reticulocytosis is evident on the CBC, and several more days until the regenerative response peaks. Página 206

5 Qualitative v. quantitative assessment of regeneration Reticulocytes differ from mature erythrocytes in size, content, and staining properties: Relative Relative Contain Routine New Methylene Blue RBC type Size [Hgb] rrna Staining Staining Mature Normal Normal No Pink Negative (Normochromasia) Reticulocyte Larger Lower Yes Purple Positive* (Polychromasia) *In cats, only count aggregate (not punctate) reticulocytes. Polychromatophilic RBC Aggregate reticulocytes Punctate reticulocyte(cat) The best qualitative indicator of a regenerative response (again, except in horses) is increased polychromasia i.e., increased numbers of polychromatophilic RBCs. This is accomplished by microscopic examination of a routinely stained, well-made blood smear. Becoming Página 207

6 proficient at evaluating blood smears requires being able to distinguish normal morphology, abnormal morphology, and artifact and depends mostly on experience. Accurately quantifying reticulocytes requires special staining techniques. This is done automatically by some hematology analyzers, and can also be performed manually using new methylene blue (NMB) or a similar stain (see Appendix B). In feline samples, punctate reticulotyes are not counted as reticulocytes, and they have the same appearance as mature erythrocytes on routine blood smear examination. MCV & MCHC alterations with regenerative anemias A strong regenerative response may result in macrocytosis (MCV above the reference limit) and hypochromasia (MCHC below the reference limit), because reticulocytes are larger and have a lower hemoglobin concentration than mature erythrocytes. A macrocytic, hypochromic anemia is almost certainly regenerative. However, the large majority of regenerative anemias do not fit this pattern. In most cases of regenerative anemia, even though the concentration of reticulocytes is increased, they comprise only a relatively small proportion of all erythrocytes and there are not enough of them in relation to the number of mature RBCs to push the MCV and MCHC values into the abnormal range. Here's a hypothetical example in which a normal dog has a hemolytic episode and its red cell mass drops by approximately one-half, and a week later there is evidence of a strong regenerative response: Patient Patient (1 week Reference RBC parameter Unit (baseline) post-hemolysis) values RBC x 10 6 /ul Hct % Retics x 10 3 /ul < 80 MCV fl MCHC g/dl Página 208

7 In this case, the regenerative response is associated with an increased MCV and a decreased MCHC but both values are still within reference limits. Of course, whether or not the patient becomes macrocytic & hypochromic depends on many factors, including its baseline MCV and MCHC values, the severity of anemia, the magnitude of the regenerative response, and the size and hemoglobin concentration of the reticulocytes. Most of the time it doesn't happen. nrbcs and regenerative anemias Another finding that may accompany regeneration, in addition to reticulocytosis, is the presence of nucleated erythroid cells (nrbcs). However, the presence of circulating nrbcs is not in itself definitive evidence of an appropriate compensatory regenerative response to anemia, and in fact may signify dyserythropoiesis (e.g., because of lead poisoning or bone marrow disease) or splenic dysfunction. When nrbcs are present as part of an appropriate regenerative response to anemia, they should be in low numbers relative to the numbers of reticulocytes. Mechanisms of disease: regenerative anemias Regenerative anemia almost always occurs due to hemorrhage or hemolysis. Some find it useful to remember these as the 2 Hs or, alternatively, the 2 Ls (loss or lysis). In the case of hemorrhage, erythrocytes and the other components of blood escape from the vasculature. Hemorrhage may be acute or chronic, internal or external. Causes of hemorrhage include trauma, abnormal hemostasis, some forms of endo- or ectoparasitism, ulceration, and neoplasia. In general, regenerative anemias do not occur because of a problem with erythropoiesis, but one that occurs after erythrocytes are made. However, it is important to note that chronic hemorrhage can deplete the body s iron stores, leading to iron-deficiency anemia, which may be either regenerative or nonregenerative. A regenerative response may occur when the deficiency is resolving or temporarily compensated (e.g., when hemorrhage ceases, or when a patient suddenly gains access to increased dietary or parenteral iron). Nonregenerative anemias and iron-deficiency anemia specifically, are discussed in more detail below. Página 209

8 Hemolysis may be intravascular in which case erythrocytes release their contents, mostly hemoglobin, directly into the blood or extravascular, in which case macrophages phagocytose erythrocytes, and little or no hemoglobin is released into the blood. Both forms (mostly extravascular hemolysis) occur as part of normal homeostasis, and involve pathways to conserve iron and other reusable components in hematopoiesis. However, some diseases are associated with increased destruction of erythrocytes by one, or both, of these mechanisms. Here are some clinical and laboratory abnormalities associated with hemolytic anemia: Abnormality Cause(s) Hyperbilirubinemia / icterus Hemolysis (intra- or extravascular) Other causes (e.g., liver disease, Cholestasis) Heinz bodies Eccentrocytosis Increased MCH & MCHC Hemolysis (intra- or extravascular) Hemolysis (intra- or extravascular) Intravascular hemolysis Artifactual hemolysis Lipemia Hemoglobinuria Splenomegaly Intravascular hemolysis Extravascular hemolysis Other causes (e.g., congestion, neoplasia) Spherocytosis Autoagglutination Extravascular hemolysis Extravascular hemolysis Mechanisms of disease: nonregenerative anemias Nonregenerative anemias are characterized by a lack of reticulocytosis on the CBC (recall that reticulocytosis does not occur in horses even in the context of regeneration). Most often the absence of reticulocytosis is due to decreased production in the marrow (i.e., erythroid hypoplasia). The most common form of nonregenerative anemia is known as anemia of inflammation or anemia of chronic disease. Página 210

9 The key mediator of anemia of inflammation is the hormone hepcidin, an acute phase protein synthesized mainly in the liver and which was first identified as an antimicrobial peptide. Hepcidin expression increases with inflammation, infection, or iron overload, and decreases with anemia or hypoxia. The bioactive form is a 25-amino acid peptide that exerts its effects by binding to the cell surface iron efflux protein, ferroportin, and inducing its internalization and degradation. The effect of this interaction is to inhibit both absorption of dietary iron from intestinal epithelium and export of iron from macrophages and hepatocytes. Inflammation Iron overload Hepatocyte..... expression of hepcidin Ferroportin Iron Enterocyte Macrophage Hepcidin binds to ferroportin and causes its internalization and degradation, thus inhibiting efflux of iron into the plasma. Anemia of inflammation almost certainly involves factors besides decreased iron availability. For example, experimentally induced sterile inflammation in cats resulted in a shortened erythrocyte life span, suggesting anemia of inflammation is also a function of accelerated erythrocyte destruction. True (or absolute) iron deficiency has long been recognized as a cause of anemia. Iron deficiency in domestic animals occurs most commonly because of chronic blood loss, and thus loss of iron-rich hemoglobin, and much less frequently due to nutritional deficiency. Although iron deficiency often results in nonregenerative anemia, it is not always so, especially if caused by chronic hemorrhage and when nutritional iron is not a limiting factor. There are many underlying conditions that involve hemorrhage-induced iron deficiency anemia, including primary or secondary gastrointestinal disease (e.g., hookworms, neoplasia, ulceration) and marked ectoparasitism. Página 211

10 The classic hematologic picture with iron deficiency is microcytic (subnormal MCV), hypochromic (subnormal MCHC) anemia. Microcytosis typically develops before hypochromasia. In severe cases, microcytosis and hypochromasia may be discernible on review of a blood smear as RBCs that are abnormally small, or paler staining because of their subnormal hemoglobin concentration, respectively. However, microscopic examination is not a reliable means of detection, especially in the case of mild abnormalities. Other causes of decreased erythropoiesis include the following: Decreased hormonal stimulation (e.g., Epo production secondary to chronic renal failure; hypothyroidism) Infection of erythropoietic cells (e.g., FeLV) Toxic insult to the bone marrow likely to result in decreased hematopoiesis in general Other disease involving the bone marrow (e.g., fibrosis, neoplasia) likely to result in decreased hematopoiesis in general Immune-mediated destruction of erythroid precursors see below Malnutrition Inherited conditions It is important to point out that nonregenerative anemia is not always caused by decreased erythropoiesis. For instance, immune-mediated hemolytic anemia (IMHA) is typically strongly regenerative, but there are also nonregenerative forms of IMHA. Bone marrow findings in dogs with severe nonregenerative IMHA range from a complete absence of erythropoiesis, known as pure red cell aplasia, to erythroid hyperplasia in a majority of patients in one study. The latter situation is an example of ineffective hematopoiesis (in this case, ineffective erythropoiesis), in which cells are being produced at normal or increased levels but are destroyed, presumably because of an immune-mediated mechanism, before they enter the circulation. Appendix A: Anemia Classification & Mechanisms of Disease Regenerative Non-regenerative CBC hallmarks Reticulocytosis & Absence of reticulocytosis polychromasia (except horses) +/- MCV, MCHC +/- MCV, MCHC (iron def.*) Página 212

11 Causes Hemorrhage Hemolysis Trauma Extravascular Anemia of inflammation Hemostasis defect Immune-mediated Iron sequestration Neoplasia Hemoparasitism RBC life span GI ulceration Toxicity Parasitism Intravascular Altered humoral signaling Immune-mediated Hemoparasitism Epo Endocrinopathies Toxicity Enzymatic Ineffective erythropoiesis Hypophosphatemia Immune-mediated dz Feline leukemia virus Myelodysplastic syndrome Primary production problem Bone marrow toxicity Myelophthisis Pure red cell aplasia Pancytopenic disorders Iron deficiency* Iron deficiency* Notes Evaluate in concert with history, Evaluate in concert with history, physical exam, other lab data physical exam, other lab data Página 213

12 Fluid shift occurs within hours of acute hemorrhage Bone marrow evaluation may be indicated Regeneration takes apx. 3-4 days to *Iron deficiency anemia may be become evident in blood, apx regenerative or non-regenerative days to reach maximum response Appendix B: Manual method of counting reticulocytes Mix anticoagulated whole blood and NMB (1:1) Incubate for 15 minutes Make a blood smear from the mixture Review the smear microscopically and count the % of reticulocytes o In cats, punctate reticulocytes should not be counted o The more cells counted, the more accurate the estimate (ideally, count 1,000 cells) Multiply the reticulocyte % by the RBC count to calculate the reticulocyte concentration: reticulocytes (#/ul) = reticulocyte % x RBC count (#/ul) o The RBC count can be determined either by an automated instrument or using a hemacytometer The disadvantage of expressing the reticulocyte count as a percentage is that the value needs to be corrected for the magnitude of the patient s anemia: Página 214

13 corrected retic % = uncorrected retic % x (patient Hct average normal Hct) Here s hypothetical data to illustrate the point the reticulocyte percentage (uncorrected) is the same in both examples: Example: Example: Reference RBC parameter Regenerative Non-regenerative values RBC (x 10 6 /ul) Hct (%) Reticulocytes concentration (x 10 3 /ul) 144 ( ) 72 (not ) < 80 %(uncorrected) 3 3 %(corrected to Hct = 40%) Página 215

14 Common Leukogram Abnormalities We will limit our discussion of leukogram abnormalities to some of the most common patterns seen in dogs and cats: Categorized by cell type Categorized by process Neutrophils Inflammation Mature neutrophilia Left shift Glucocorticoid-mediated ( stress leukogram ) Toxic change Epinephrine-mediated ( physiologic leukocytosis ) Lymphocytes Lymphopenia Immunologic response to antigenic stimulation Lymphocytosis Reactive lymphocytes Of course, there are many other leukogram abnormalities. First, a little bit of background: Overview of leukocyte production and function Granulocytes (neutrophils, eosinophils, basophils) and monocytes have key immunologic functions, including phagocytosis and microbicidal activity (neutrophils and monocytederived macrophages); parasiticidal activity and participation in allergic reactions (eosinophils and basophils); and antigen processing and presentation, and cytokine Página 216

15 production (macrophages). Many molecules influence production of these cell types, including cytokines such as granulocyte and granulocyte-monocyte colony- stimulating factors (G-CSF and GM-CSF, respectively) and interleukins (such as IL-3 and IL-6). Inflammatory mediators stimulate production of cytokines promoting granulopoiesis and monocytopoiesis. The earliest granulocytic or monocytic precursor identifiable by routine light microscopy is the myeloblast, which undergoes maturational division to produce 16 to 32 progeny cells. The normal transit time from myeloblast to mature neutrophil is approximately 5 days. In addition, a reserve of neutrophils is maintained in the bone marrow. The size of this so-called storage pool is species-dependent. Neutrophils normally are the predominant leukocyte type in blood of most domestic species. Neutrophils are only in circulation for a short time (less than 12 hours). T lymphocytes and B lymphocytes are the key effectors of cell-mediated and humoral immunity, respectively. T cells originate in the bone marrow and migrate to the thymus, where they undergo differentiation, selection, and maturation processes before migrating to the peripheral lymphoid tissue as effector cells. B-cell development occurs in two phases, first an antigen-independent phase in the bone marrow and ileal Peyer s patches (the site of B-cell development in ruminants), then an antigen-dependent phase in peripheral lymphoid tissues (such as spleen, lymph nodes, and mucosal associated lymphoid tissue or MALT). Trafficking of lymphocytes occurs under the direction of chemokine (chemoattractant cytokine) signals. Once they have migrated to the peripheral lymphoid tissue, lymphocytes may undergo clonal expansion in response to antigenic stimulation. Unlike other hematopoietic cells, which are unidirectional in the blood vessels, lymphocytes travel in both blood and lymphatic vessels, continually recirculating between the two systems. In most species, the majority of lymphocytes in blood circulation are T lymphocytes. What is normal? Veterinary laboratories typically provide species-specific hematology reference values based on data from a representative population of clinically normal adults. However, it is important to remember that what is normal may vary not only between species, but also as a function of other factors, including: Age Breed Página 217

16 Environment/husbandry Geographic location Differences in laboratory methodology. Página 218