• Adhesion to exposed subendothelial collagen

• Secretion of bioactive granule contents (including calcium, adenosine diphosphate [ADP], fibrinogen, vWF, histamine, serotonin, and others)

• Aggregation with other platelets

• Eventual fusion with other platelets (a process known as viscous metamorphosis) to form a platelet plug

Anemia: Anemia refers to subnormal red blood cell mass or hemoglobin concentration. Anemia causes clinical signs referable to decreased oxygen-carrying capacity (pallor of mucous membranes, lethargy, weakness, exercise intolerance) and may also result in detectable abnormalities because of 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.

Classifying anemia as regenerative or nonregenerative is clinically useful because it provides information about the mechanism of disease (Table 13-1). The hallmark of regenerative anemias, except in horses, is reticulocytosis (i.e., increased numbers of circulating reticulocytes [immature erythrocytes]), which is evident as polychromasia on a routinely stained blood smear. In ruminants, reticulocytosis is often accompanied by basophilic stippling (Fig. 13-7).

Reticulocytosis indicates increased bone marrow erythropoiesis (Fig. 13-8) and release of erythrocytes before they are fully mature (further back in the production “pipeline”). Reticulocytosis is an appropriate response to anemia. A strong regenerative response may produce an increased mean cell volume (MCV) and decreased mean cell hemoglobin concentration (MCHC) on the complete blood count (CBC), because reticulocytes are larger and have a lower hemoglobin concentration than mature erythrocytes. Horses are an exception to this classification scheme because they do not release reticulocytes into circulation even when their marrow is producing increased numbers of erythrocytes. Horses with a regenerative response may have an increased MCV and red cell distribution width (an index of variation in cell size). But definitive determination of regeneration in a horse requires either bone marrow examination, in which case the evidence of regeneration is erythroid hyperplasia, or sequential CBCs, in which case the evidence of regeneration is an increase in red cell mass over time. Punctate reticulocytes in cats are evident when blood is stained with new methylene blue, but they are not counted as reticulocytes on the CBC and have the same appearance as mature erythrocytes on routine blood smear examination.

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 regeneration 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 a regenerative response to anemia, they should be in low numbers relative to the numbers of reticulocytes.

Recall that the stimulus for increased erythropoiesis is increased secretion of 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, for example, it typically takes 3 to 4 days until reticulocytosis is evident on the CBC, and several more days until the regenerative response peaks. A similar lag occurs in cases of acute hemolysis. 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.

Regenerative anemia occurs because of 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 or internal or external. Causes of hemorrhage include trauma, abnormal hemostasis, certain forms of parasitism, ulceration, and neoplasia. Thus regenerative anemias usually occur not because of a problem with erythropoiesis but because of a process affecting erythrocytes that have already been released into the blood circulation. 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 later in this chapter.

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. The next sections discuss basic mechanisms of intravascular and extravascular hemolysis in health and disease in more detail.

Normal turnover of erythrocytes occurs mainly by extravascular hemolysis, in which senescent erythrocytes are phagocytosed by macrophages in the spleen and to a lesser extent in other organs such as liver (Kupffer cells) and bone marrow. The exact controls are not clear, but factors that likely play a role include the following:

Macrophages degrade erythrocytes into reusable constituents, such as iron and amino acids, and the waste product bilirubin. Intravascular hemolysis normally occurs at only extremely low levels. Hemoglobin is a tetramer that, when released from the erythrocyte into the blood, splits into dimers that bind to a plasma protein called haptoglobin. The hemoglobin-haptoglobin complex is taken up by hepatocytes and macrophages. This is the major pathway for handling free hemoglobin. However, free hemoglobin may also oxidize to form methemoglobin, which dissociates to form metheme and globin. Metheme binds to a plasma protein called hemopexin, which is taken up by hepatocytes and macrophages in a similar manner to hemoglobin-haptoglobin complexes. Free heme in the reduced form binds to albumin, from which it is taken up in the liver and converted into bilirubin.

In hemolytic anemia, erythrocytes are destroyed at an increased rate. Whether the mechanism is intravascular or extravascular, or a combination, depends on the specific disease process (specific diseases are discussed later in this chapter). A classic sequela of hemolytic anemias in general is hyperbilirubinemia, which is an increase in the plasma concentration of bilirubin. Bilirubin is a yellow pigment, which explains why hyperbilirubinemia, if severe enough, causes icterus—the grossly visible yellowing of serum or tissue (Fig. 13-9). Icterus of mucous membranes, skin, and other tissue is known as jaundice. Icterus is usually detectable when the plasma bilirubin concentration exceeds 2 mg/dL. Hemolysis is a cause of prehepatic hyperbilirubinemia, but it is important to note that hyperbilirubinemia can also occur as the result of hepatic or posthepatic conditions causing impaired bile flow (cholestasis).

Laboratory findings and clinical observations may point to a specific mechanism of hemolysis. In patients with extravascular hemolytic anemia, increased destruction of erythrocytes by splenic macrophages often results in splenomegaly (Fig. 13-10). Splenomegaly may also occur because of other conditions, as discussed elsewhere in this chapter. Intravascular hemolysis is grossly evident as hemoglobinemia (pink-tinged plasma or serum), if the concentration of extracellular hemoglobin is greater than approximately 50 mg/dL. Haptoglobin is saturated with dimeric hemoglobin at a concentration of approximately 150 mg/dL. When haptoglobin is saturated, any remaining free hemoglobin has a low enough molecular weight to pass through the renal glomerular filter into the urine (hemoglobinuria), imparting a pink or red discoloration to the urine. Thus extracellular hemoglobin can cause gross discoloration of the plasma, where it is bound to haptoglobin, before becoming grossly visible in urine. The half-life of haptoglobin is markedly decreased when bound to hemoglobin, so when large amounts of haptoglobin-hemoglobin complex are formed, the concentration of haptoglobin in the blood decreases and hemoglobin can pass through the glomerulus at even lower concentrations. Hemoglobinuria is a contributing factor in the renal tubular necrosis (hemoglobinuric nephrosis) that often occurs in cases of acute intravascular hemolysis (see Chapter 11). A similar lesion occurs in the kidneys of individuals with marked muscle damage and resulting myoglobinuria.

Hemoglobinuria cannot be distinguished grossly from hematuria (RBCs in the urine) or myoglobinuria, and both hemoglobin and myoglobin cause a positive reaction for “blood protein” on urine test strips. Comparing the colors of the plasma and the urine may be informative. In contrast to hemoglobin, myoglobin causes gross discoloration of the urine before the plasma is discolored. This is because myoglobin is a low-molecular-weight monomer, freely filtered by the glomerulus, and does not bind plasma proteins to a significant degree. Hematuria can be distinguished from hemoglobinuria on the basis of microscopic examination of urine sediment (RBCs are present in cases of hematuria).

CBC data are often useful in illuminating hemolytic mechanisms. Total hemoglobin concentration is conventionally measured by lysing all the erythrocytes and measuring the hemoglobin in solution spectrophotometrically. The value for mean cell hemoglobin (MCH) is calculated based on the total hemoglobin concentration and the concentration of erythrocytes. The value for MCHC is calculated based on the MCH and MCV. Thus although erythrocytes do not actually contain supranormal concentrations of hemoglobin, an excess of extracellular hemoglobin may cause an artifactual increase in the calculated MCH and MCHC. It is important to remember that similar increases may also occur artifactually because of positive interference with spectrophotometric measurement of hemoglobin such as occurs with lipemia.

Extravascular hemolysis also often produces characteristic CBC abnormalities that may reflect the mechanism of disease. For example, spherocytosis and autoagglutination are hallmarks of IMHA. Spherocytes form when macrophages (mainly in the spleen) phagocytose part of an erythrocyte plasma membrane bound with autoantibody (Fig. 13-11). The remaining portion of the RBC assumes a spherical shape, thus preserving maximal volume. This change in shape results in decreased deformability of the cells. Erythrocytes must be extremely pliable to traverse the splenic red pulp and sinusoidal walls (Fig. 13-12); spherocytes therefore tend to be retained in the spleen in close association with macrophages with risk of further injury and eventual destruction. In the dog, spherocytes appear smaller than normal and have uniform staining (Fig. 13-13, A), in contrast to normal RBCs, which have a region of central pallor imparted by their biconcave shape. This difference in staining between spherocytes and normal RBCs is not consistently discernible in many other domestic animals (including horses, cattle, and cats), whose erythrocytes differ from those of the dog in that they are smaller and have less pronounced biconcavity and therefore less pronounced central pallor. Autoagglutination occurs because of cross-linking of antibodies bound to erythrocytes. Autoagglutination is evident microscopically as clusters of erythrocytes, and macroscopically as blood with a grainy consistency (Fig. 13-13, B). Autoagglutination may also result in a falsely increased MCV and decreased RBC concentration when clustered cells are mistakenly counted as single cells by automated hematology analyzers.

Oxidative damage to erythrocytes occurs when normal antioxidative pathways that generate reducing agents (such as NADH, NADPH, and reduced glutathione) are compromised or overwhelmed and can result in hemolytic anemia, abnormal hemoglobin function, or both. Hemolysis caused by oxidative damage may be extravascular or intravascular, or a combination (predominant forms of hemolysis are discussed further in the section on specific diseases). Evidence of oxidative damage to erythrocytes may be apparent on examination of a blood smear, or even on gross physical examination. Heinz bodies are foci of denatured globin that interact with the erythrocyte membrane. They are usually subtly evident on routine Wright’s-stained blood smears as pale circular inclusions or blunt, rounded protrusions of the cell margin and are readily discernible on smears stained with new methylene blue (Fig. 13-14). Cats are particularly susceptible to Heinz body formation and may have low numbers of small Heinz bodies normally. There is no unanimity of opinion, but some clinical pathologists believe that the presence of Heinz bodies in up to 10% of all RBCs in cats is within normal limits. This predisposition is believed to reflect unique features of the feline erythrocyte, whose hemoglobin has more sulfhydryl groups (preferential sites for oxidative damage) than do erythrocytes of other species and may also have lower intrinsic reducing capacity. It is also possible that the feline spleen, which is nonsinusoidal, does not have as efficient a “pitting” function as does a sinusoidal spleen, such as that of the dog (splenic structure and function are discussed in more detail later in this chapter). Eccentrocytes, evident as erythrocytes in which one side of the cell has increased pallor, are another manifestation of oxidative damage. They form because of cross-linking of membrane proteins, with adhesion of opposing areas of the cell’s inner membrane leaflet, and displacement of most of the hemoglobin toward the other side (Fig. 13-15). Oxidative insult may also result in conversion of hemoglobin (iron in the Fe²⁺ state) to methemoglobin (iron in the Fe³⁺ state), which is incapable of binding oxygen. Methemoglobin is produced normally in small amounts but reduced back to oxyhemoglobin by the enzyme cytochrome-b5 reductase (also known as methemoglobin reductase). Methemoglobinemia results when methemoglobin is produced in excessive amounts (because of oxidative insult) or when the normal pathways for maintaining hemoglobin in the Fe²⁺ state are impaired (as in cytochrome-b₅ reductase deficiency). When present in sufficiently high concentration (approximately 10% of total hemoglobin), methemoglobin imparts a grossly discernible chocolate color to the blood.

Nonregenerative anemias are characterized by a lack of reticulocytosis on the CBC; however, reticulocytosis does not occur in horses even in the context of regeneration. Most often this is a result of decreased production in the marrow (i.e., erythroid hypoplasia). Erythrocytes circulate for a long time, so anemias caused by decreased production tend to develop slowly. The most common form of nonregenerative anemia is known as anemia of inflammation or anemia of chronic disease. In this form of anemia, RBCs are decreased in number but are typically normal in size and hemoglobin concentration (so-called normocytic, normochromic, anemia). It has long been known that patients with inflammatory or other chronic disease often become anemic and that this condition is associated with increased iron stores in the bone marrow. Sequestration of iron may be a bacteriostatic evolutionary adaptation because many bacteria require iron as a cofactor for growth. In recent years, investigators have begun to elucidate the molecular mechanisms underlying anemia of inflammation. Hepcidin, an acute phase protein synthesized in the liver that was first identified as an antimicrobial peptide, is a key mediator that acts by limiting iron availability. Hepcidin expression increases with inflammation, infection, or iron overload and decreases with anemia or hypoxia. Hepcidin exerts its effects by causing functional iron deficiency. It binds to and causes the degradation of the cell surface iron efflux molecule, ferroportin, thus inhibiting both absorption of dietary iron from the intestinal epithelium and export of iron from macrophages and hepatocytes into the plasma (Fig. 13-16).

Anemia of inflammation involves factors besides decreased iron availability. Inflammatory cytokines are likely to inhibit erythropoiesis by oxidative damage to and triggering apoptosis of developing erythroid cells, by decreasing expression of Epo and stem-cell factor, and by decreasing expression of Epo receptors. In addition, experimentally induced sterile inflammation in cats resulted in shortened erythrocyte survival, indicating that anemia of inflammation is likely also a function of increased erythrocyte destruction.

True iron deficiency has long been recognized as a cause of anemia. In domestic animals, iron deficiency occurs most commonly because of chronic blood loss, and thus loss of iron in the form of hemoglobin, and less frequently because of nutritional deficiency. Although iron deficiency often results in nonregenerative anemia, it is not always so, especially when nutritional iron is not a limiting factor. Many underlying conditions can cause hemorrhage-induced iron deficiency anemia, including primary or secondary gastrointestinal (GI) disease (e.g., hookworms, neoplasia, ulceration) and marked ectoparasitism. The classic hematologic picture with iron deficiency is microcytic, hypochromic anemia. Microcytosis and hypochromasia are indicated on the CBC by abnormally low MCV and MCHC values, respectively. Microcytosis and hypochromasia may also 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. Low hemoglobin concentration is believed to contribute to microcytosis because one of the feedback mechanisms signaling erythroid cells to stop dividing is reaching a threshold hemoglobin concentration. Abnormally low hemoglobin concentration during erythropoiesis thus results in extra cell divisions and smaller erythrocytes. Microcytosis usually develops before hypochromasia.

Other causes of decreased erythropoiesis include the following:

Specific examples of diseases causing nonregenerative anemia by these mechanisms are discussed later in this chapter. It is important to point out that nonregenerative anemia is not always caused by decreased erythropoiesis. For example, 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 produced at abnormal or increased rate but are destroyed, presumably because of an immune-mediated mechanism, before they enter the circulation.

Neutropenia: Neutropenia refers to a decrease in the concentration of neutrophils in circulating blood. Neutropenia may be caused by decreased production, increased destruction, altered distribution, demand for neutrophils in inflamed tissue that exceeds the rate of granulopoiesis, or inherited disease. Decreased production is evident on bone marrow examination as granulocytic hypoplasia. This usually results from an insult that affects multiple hematopoietic lineages, such as chemical insult, radiation, neoplasia, infection, and fibrosis, but may also be caused by a process that preferentially targets granulopoiesis. Immune-mediated neutropenia is a rare but recognized condition in domestic animals. Bone marrow findings range from granulocytic aplasia or hypoplasia to hyperplasia, depending on where the cells under immune attack are in their differentiation programs. Neutropenia with no evidence of decreased production and in which other causes of neutropenia have been excluded may be a result of destruction of neutrophils before they leave the bone marrow, a condition known as ineffective granulopoiesis. Like other forms of ineffective hematopoiesis, this condition is often presumed to be immune mediated; in cats this condition may occur as a result of infection of hematopoietic cells with the feline leukemia virus (FeLV).

In marked contrast to erythrocytes, neutrophils have a very short lifespan in circulation. Once released from the bone marrow, a neutrophil is only in the bloodstream for hours before migrating into the tissues. The fate of neutrophils after they leave the bloodstream in normal conditions (i.e., not in the context of inflammation) is poorly understood. They migrate into the GI and respiratory tracts, liver, and spleen and may be lost through mucosal surfaces or undergo apoptosis and be phagocytosed by macrophages. When neutrophil production ceases, a reserve of mature neutrophils in the bone marrow (discussed in more detail later) may be adequate to maintain normal numbers of circulating neutrophils for a few days; however, after the bone marrow storage pool is depleted, neutropenia rapidly ensues.

Neutrophils within the blood vasculature are in two compartments: a circulating pool, consisting of those cells flowing freely in the blood, and a marginated pool, consisting of those cells transiently associated with the endothelial surface. (In reality, neutrophils are constantly shifting between these two pools, but the proportion of cells in either pool normally remains fairly constant in any given species.) Circulating neutrophils are part of the blood sample collected during routine venipuncture and are thus counted in the CBC, whereas marginated neutrophils are not. Pseudoneutropenia refers to the situation in which there are an increased proportion of neutrophils in the marginated pool. This may occur because of decreased blood flow or in response to stimuli, such as endotoxemia, that increase expression of molecules promoting interaction between neutrophils and endothelial cells.

Neutropenia may also result from increased demand for neutrophils in the tissue. How rapidly such a situation develops depends not only on the magnitude of the inflammatory stimulus but also on the reserve of postmitotic neutrophils in the bone marrow. The size of this reserve, or storage pool, is species dependent. In dogs, this pool contains the equivalent of 5 days normal production of neutrophils. Cattle represent the other extreme in that they have a small storage pool and thus are predisposed to becoming neutropenic more easily than are dogs. Horses and cats are somewhere between the two extremes, closer to cattle and dogs, respectively. It stands to reason that the clinical significance of neutropenia because of a supply and demand imbalance is also species dependent. In dogs, neutropenia as a result of inflammation is an alarming finding because it is evidence of a massive tissue demand for neutrophils that has exhausted the patient’s storage pool and is exceeding the rate of granulopoiesis in the bone marrow, whereas in cows, neutropenia is commonly noted in a wide range of conditions involving acute inflammation and does not necessarily indicate an overwhelming demand.

Eosinopenia/Basopenia: In many laboratories, CBC reference values for eosinophils and basophils are as low as zero cells per microliter, precluding detection of eosinopenia or basopenia. When detectable, eosinopenia is often part of a stress (glucocorticoid-mediated) leukogram.

Thrombocytopenia: Thrombocytopenia refers to a decrease in the concentration of circulating platelets. Mechanisms of thrombocytopenia include decreased production, increased destruction, increased consumption, altered distribution, and hemorrhage. Decreased production may occur because of a condition affecting cells of multiple hematopoietic lineages, including platelets, or to one specifically depressing thrombopoiesis. In either case, decreased thrombopoiesis is evident on examination of a bone marrow sample as megakaryocytic hypoplasia. General causes of decreased hematopoiesis outlined earlier in the sections on anemia and neutropenia also apply to thrombocytopenia. Specific diseases causing thrombocytopenia are covered later in this chapter. Immune-mediated thrombocytopenia is a fairly common disease in dogs and may also occur in other species. Increased consumption of platelets is a hallmark of disseminated intravascular coagulation (DIC), a syndrome in which hypercoagulability leads to increased consumption of both platelets and coagulation factors in the plasma, with subsequent hypocoagulability and susceptibility to bleeding. The spleen normally contains a significant proportion of total platelet mass (up to one-third in some species), and abnormalities involving the spleen may result in changes in the number of circulating platelets. For example, splenic congestion may result in thrombocytopenia, and splenic contraction may result in thrombocytosis. Acute hemorrhage may result in mild to moderate thrombocytopenia. Potential mechanisms of thrombocytopenia caused by hemorrhage include loss and consumption. Of course, thrombocytopenia can also be a cause of hemorrhage. In the absence of other complicating factors, marked to severe thrombocytopenia (less than 50,000 platelets/µL) is more likely to be the cause of rather than the result of bleeding. Megakaryocytic hyperplasia on examination of a bone marrow sample is evidence of a regenerative thrombopoietic response, and an increase in the mean platelet volume (MPV) value on the CBC often accompanies such a response.

Lymphopenia: Lymphopenia refers to a decrease in the concentration of lymphocytes in blood circulation. It is a common CBC finding in sick animals. Usually the precise mechanism of lymphopenia is not clear. It is often presumed to be mediated at least in part by endogenous glucocorticoid excess. Lymphopenia may occur because of various mechanisms, including altered distribution of lymphocytes (increased trafficking of lymphocytes to, and decreased egress from, lymphoid tissues), lymphotoxicity (direct damage to lymphocytes or suppression of lymphopoiesis) of therapeutic or infectious agents, loss of lymphocyte-rich lymphatic fluid, or congenital disorders. Normal lymphocyte trafficking may be altered because of disruption of the normal architecture of lymphoid tissue (e.g., because of neoplasia or inflammation), or in response to cytokine signals. Glucocorticoid excess may cause lymphopenia via redistribution from the blood to lymphoid tissue, or via direct lymphotoxic effects. Anticancer treatments (chemotherapy or radiotherapy) and immunosuppressive drugs may also be lymphotoxic. Some hereditary immunodeficiencies (such as severe combined immunodeficiency or thymic aplasia) can cause lymphopenia.

Erythrocytosis: An increase in the measured red cell mass above the normal range (i.e., the opposite of anemia) is known as erythrocytosis. The term polycythemia is often used interchangeably with erythrocytosis, but technically and for the purposes of this chapter, polycythemia refers to a specific type of leukemia called primary erythrocytosis or polycythemia vera. Relative erythrocytosis occurs most frequently because of dehydration, when the decreased proportion of water in the blood results in hemoconcentration. A similar situation may occur as a result of epinephrine-mediated splenic contraction. Erythrocytosis caused by splenic contraction occurs to the most pronounced degree in the horse and may occur to a lesser extent in other species. In both of these cases, the total RBC mass is not in fact increased but appears to be so because of other factors. Secondary erythrocytosis refers to a true, Epo-mediated increase in red cell mass, either as an appropriate response to hypoxemia (such as may be seen in patients with cardiopulmonary disease and severely impaired oxygenation, or cardiovascular defects causing right-to-left shunting) or in rare cases an Epo-secreting tumor (inappropriate secondary erythrocytosis). Polycythemia vera is diagnosed based on a marked increase in red cell mass (hematocrit in normally hydrated dogs ranges from 65% to >80%), absence of hypoxemia, absence of other tumors, and normal or decreased concentration of plasma erythropoietin concentration. Absolute erythrocytosis, whether primary or secondary, causes increased viscosity of the blood and resulting impairment of blood flow and distention of the microvasculature. Affected individuals are at increased risk of tissue hypoxia and thrombosis or hemorrhage. Related clinical signs (hyperviscosity syndrome) may include erythematous mucous membranes (Fig. 13-17) and prolonged capillary refill time, prominent scleral vessels, evidence of thrombosis or hemorrhage, and secondary signs (e.g., neurologic and cardiovascular) related to specific organ systems affected.

Neutrophilia: Neutrophilia occurs in response to a number of different stimuli, which are not mutually exclusive. Major mechanisms of neutrophilia are shown in Fig. 13-18. Understanding the CBC findings characteristic of these responses is an important part of clinical veterinary medicine. Inflammation can result in neutropenia, as discussed earlier, or neutrophilia, as discussed next. However, before moving on to a discussion of inflammatory neutrophilia and the so-called left shift, it is important to mention two other common causes of neutrophilia.

Glucocorticoid excess, either because of endogenous production or exogenous administration, results in a CBC pattern known as the stress leukogram, characterized by mature neutrophilia (i.e., increased concentration of segmented neutrophils), lymphopenia, and, especially in dogs, monocytosis. Eosinopenia is another feature of the stress leukogram, although in many situations this is inapparent because the normal reference values for eosinophils are so low (in some laboratories, the lower reference value is zero). Mechanisms contributing to glucocorticoid-mediated neutrophilia include the following:

The magnitude of neutrophilia tends to be species dependent, with dogs having the most pronounced response (up to 35,000 cells/µL) and in decreasing order of responsiveness, cats (30,000 cells/µL), horses (20,000 cells/µL), and cows (15,000 cells/µL) less marked responses. With long-term glucocorticoid excess, neutrophil numbers tend to normalize, whereas lymphopenia persists.

Epinephrine release results in a different pattern, known as physiologic leukocytosis, characterized by mature neutrophilia (like the glucocorticoid response) and lymphocytosis (unlike the glucocorticoid response). This phenomenon is short lived (<1 hour). Neutrophilia occurs primarily because of a shift of cells from the marginated to the circulating pool. Physiologic leukocytosis is common in cats (especially when they are highly stressed during blood collection) and horses, less common in cattle, and uncommon in dogs.

Of course, neutrophilia may also indicate inflammation, and inflammatory stimuli of varying magnitude and duration produce different patterns of neutrophilia. A classic hematologic finding in patients with increased demand for neutrophils is the presence of immature forms in the blood, known as a “left shift.” Not all inflammatory responses have a left shift, but the presence of a left shift almost always signifies active demand for neutrophils in the tissue. The magnitude of a left shift is assessed by the number of immature cells and their degree of immaturity. The mildest form is characterized by increased numbers of band neutrophils, the immediate predecessor to the segmented neutrophil normally found in circulation, with increasingly severe forms involving progressively immature predecessors. A left shift is considered orderly if the number of immature neutrophils in circulation decreases as they become progressively immature. The term degenerative left shift is sometimes used to describe cases where the number of immature forms exceeds the number of segmented neutrophils. As with glucocorticoid-mediated neutrophilia, the typical magnitude of neutrophilia caused by inflammation varies by species, with dogs having the most pronounced response.

Marked neutrophilia may also occur because of an inherited condition known as leukocyte adhesion deficiency (LAD), in which leukocytes lack normal expression of an adhesion molecule required for migration from the bloodstream into the tissue. This condition is described further in the section on specific diseases.

It might be useful to think of neutrophil kinetics in terms of a producer-consumer model in which the bone marrow is the factory, and the tissues (where the neutrophils eventually go) are the customers. The bone marrow storage pool is the factory inventory, and the neutrophils in the bloodstream are in delivery to the customer. Within the blood vessels, circulating neutrophils are on the highway, and marginated neutrophils are temporarily pulled off to the side of the road. During health, there is an even flow of neutrophils from the factory to the customer. Thus the system is in steady state, and neutrophil numbers remain relatively constant and within the normal range. However, disease states may perturb this system at multiple levels. Decreased granulopoiesis is analogous to a factory working below normal production level. Ineffective granulopoiesis is analogous to goods produced at a normal to increased rate but that are damaged during manufacturing and never leave the factory. A left shift is analogous to the factory meeting increased customer demand by shipping out unfinished goods. Cases of persistent, established inflammation are characterized by bone marrow granulocytic hyperplasia and mature neutrophilia, analogous to a factory that has had time to adjust to increased demand and is meeting it more efficiently by increasing its output.