Abnormalities of the leukogram include quantitative or numerical concentration abnormalities and WBC morphologic abnormalities.

Numerical Abnormalities

WBC concentration values are interpreted by comparison with species-specific reference ranges. Valid interpretations can be made only by considering the absolute numbers. For reference values for total WBC and differential WBC concentrations in absolute numbers for common domestic species, see Reference Guides: Hematologic Reference Ranges a. The total WBC concentration is more variable and often higher in neonates than in adults. Age-related reference values should be used to evaluate hemograms in young animals, especially species in which lymphocytes are more numerous (and neutrophils less numerous) in adults, typically the ruminants. Generally, differential WBC patterns of adults are reached at about the age of sexual maturity.

Abnormality in the total WBC concentration is useful only to alert the clinician to look for and interpret abnormalities in the cell distributions in the differential. When the total WBC is abnormal, one or more distributional abnormalities in the differential are likely. When the total WBC is normal, there still may be one or more distributional abnormalities in the differential. As a result, evaluation of the differential data is the most important component of the leukogram.

Leukocytosis is an increase in the total WBC concentration, while leukopenia is a decrease in the total WBC concentration. Changes in the concentrations of specific leukocyte types are more important for clinical interpretation purposes.

Neutrophilia or neutrophilic leukocytosis is an increase in neutrophil concentration. Lymphocytosis is an increase in lymphocyte concentration. Monocytosis is an increase in monocyte concentration. Eosinophilia and basophilia refers to an increase in eosinophil or basophil concentration, respectively. Metarubricytosis is an increase in nucleated RBC (nRBC) in blood. Mastocytosis is an increase in mast cells in blood.

Decreases in concentration of a cell type are indicated by the suffix “penia.” This is applied only to cell types in which a decrease is possible. It does not apply to cell types for which the concentration may be 0, such as monocytes, basophils, nRBC, and any other abnormal cell type. Hence, neutropenia is a decrease in neutrophil concentration, lymphopenia is a decrease in lymphocyte concentration, and eosinopenia is a decrease in eosinophil concentration. Cytopenia is a nonspecific term indicating a decrease in cell concentration(s), but the cell type is not specified. Pancytopenia indicates all cell types are decreased, usually to a severe level.

Terms used to describe or qualify abnormalities most often associated with inflammatory responses include various left shifts and leukemoid response. A left shift is an increase in concentration of immature, nonsegmented neutrophils, typically bands, but may also include metamyelocytes or even more immature forms. A regenerative left shift describes leukocytosis characterized by the combination of neutrophilia and a left shift. In this situation, the segmented neutrophils will be greater in concentration than bands and less mature forms. A degenerative left shift describes a neutrophil pattern characterized by normal to decreased total neutrophil concentration, but with a left shift in which the concentration of bands and less mature forms is greater than segmented neutrophils. This is an indication of maximal release from bone marrow in response to inflammation. A leukemoid response describes a marked neutrophilia of a magnitude sufficient to indicate chronicity of an inflammatory response. The magnitude is also such that myeloid leukemia becomes a diagnostic consideration. Guidelines for neutrophilic leukocytosis considered to indicate a leukemoid response are >70,000/μL for dogs, >50,000/μL for cats, >30,000/mL for horses, and >20,000/μL for ruminants.

Morphologic Abnormalities

Abnormalities of morphology may be associated with either acquired or inherited disease. Many of these are uncommon.

A toxic change is identified only in neutrophils. The term originates from historical observation that certain cell features were associated with general, usually overwhelming, toxic states, such as systemic bacterial infections and severe, acute inflammatory lesions. It is misleading in that it implies neutrophil injury. The cells are not injured and have normal function. Toxic change is best defined as a set of morphologic changes observed on the blood film that are a result of accelerated marrow production of neutrophils. The accelerated production is in response to relatively severe inflammatory states that maximally signal the bone marrow. Morphologic changes include (in order of frequency) diffuse cytoplasmic basophilia, Döhle bodies, and fine cytoplasmic vacuolation. More rare changes include increased prominence of cytoplasmic azurophilic granules, cellular gigantism, and binucleation. Toxic change is almost always associated with the concurrent presence of a left shift. It is graded as mild, moderate, or severe by subjective evaluation of the more common changes. Döhle bodies, blue-gray cytoplasmic inclusions, are aggregates of endoplasmic reticulum. They are unique in that they may occur in clinically healthy cats and therefore are not interpreted as toxic change unless accompanied by other features.

Döhle bodies, equine blood smear

Reactive lymphocytes have increased, distinctly basophilic cytoplasm and may have irregular or clefted nuclei. They may vary considerably in diameter. They have condensed chromatin and therefore are not blasts. They are interpreted as immunologically stimulated B cells.

Granular lymphocytes have condensed chromatin and increased pale blue-gray cytoplasm that contains several small pink or azurophilic granules. The nucleus may be round to clefted. These are large granular lymphocytes and may be either natural killer (NK) lymphocytes or T lymphocytes.

Blasts are usually an indication of hematopoietic cell neoplasia if they are reproducible or present in large numbers. Their lineage may be tentatively identified by morphologic criteria, but often cytometric analysis is required to identify lineage.

An autosomal recessive condition in Birman cats results in fine intracytoplasmic eosinophilic granules within neutrophils that may be mistaken for toxic granulation. Neutrophil function is normal and the anomaly is regarded as an incidental finding.

Chédiak-Higashi syndrome (see Hemostatic Disorders: Chédiak-Higashi Syndrome) described in Persian cats, people, mink, foxes, Hereford and Brangus cattle, mice, and killer whales is an autosomal recessive defect involving lysosomal granules. There is hyperfusion of granules resulting in large, eosinophilic cytoplasmic inclusions. There is an increased susceptibility to bacterial infections and an increased tendency to bleed due to both neutrophil and platelet function abnormalities, and partial oculocutaneous albinism due to abnormal melanin granule formation. However, cats may maintain reasonable health.

The mucopolysaccharidoses are a group of lysosomal storage disorders in which there is a defect in degradation of glycosamino-glycans. The neutrophils and lymphocytes contain accumulated mucopolysaccharide product in the form of purple or metachromatic intracytoplasmic granules. Lymphocytes may also be vacuolated. These disorders are associated with a variety of systemic clinical abnormalities and are seen in dogs and cats.

Another group of lysosomal storage disorders recognized in dogs and cats may result in cytoplasmic vacuoles predominantly in lymphocytes and occasionally in neutrophils. These disorders include gangliosidoses, α-mannosidosis, Niemann-Pick disease variants, acid-lipase deficiency, and fucosidosis. Most of these disorders result in severe, progressive neurologic disorders resulting from accumulated product in neuronal tissue.

Locoweed toxicity is regarded as an acquired form of lysosomal storage defect in large animals. It is due to toxic principle from the plant that inhibits one or more enzymes of oligosaccharide metabolism. This may result in vacuolation in lymphocytes.

Pelger-Huët anomaly is a nuclear hyposegmentation defect of granulocytes in people, cats, rabbits, and dogs that are heterozygous for the anomaly. Neutrophils have normal function and a near absence of segmented nuclear morphology. Most or all of the neutrophils appear as bands and metamyelocytes and may appear as a marked left shift in an otherwise normal leukogram. Affected heterozygote animals are clinically normal; the homozygous inheritance of the trait is lethal.

Pelger-Huët anomaly, canine blood smear

Hypersegmentation is an increased degree of nuclear segmentation resulting in multiple lobes connected by nuclear filaments. It is a nonspecific indication of increased time in blood and is normal aging of the cell. This may be seen with steroid leukograms.

Leukocyte agglutination may affect either neutrophils or lymphocytes. This is seen on low magnification as aggregates of 5–15 tightly clumped leukocytes. Avid agglutination may result in markedly false low total WBC concentration on some cell counting systems. This is likely due to the presence of a naturally occurring cold agglutinin that is operative only in vitro at laboratory temperature. There is no known clinical significance.

Infectious disease inclusions are occasionally recognized. Canine distemper inclusions may be seen in neutrophils, monocytes, and lymphocytes as well as in newly produced RBC. The ehrlichioses of various animal species and canine hepatozoonosis may have cytoplasmic inclusions of respective organisms of these tickborne diseases.

Interpretative Patterns for the Leukogram

The abnormal leukogram is typically interpreted into one of several patterns, each of which may consist of more than one abnormality in the differential. Some patterns may also be associated with concurrent changes in RBC and platelets. Important species differences in leukogram responses are described below.

Corticosteroid-induced or Stress Response

This is likely the most common leukocyte response. Endogenous steroid release or treatment with exogenous corticosteroids results in a leukogram with multiple changes. Lymphopenia is the most consistent change, and mature neutrophilia is usually present. Monocytosis and eosinopenia are expected changes but are more variable. Neutrophilia is due to decreased adherence to the vascular endothelium, which inhibits margination and increases circulating time. As a result, neutrophils may become hypersegmented. There may also be increased marrow release of neutrophils. This response is often misinterpreted as inflammatory.

Excitement or Epinephrine Response

Leukocytosis may occur as a result of exercise or excitement; this response is mediated by increased epinephrine. Epinephrine flushes cells from the marginal to the central pool. The effect may double the total WBC concentration within minutes. In addition, splenic contraction releases WBC and RBC into the peripheral circulation. The leukocytosis is usually due to a mature neutrophilia without a left shift. Lymphocytosis may be present, especially in young horses or cats. The effect in cats is often recognized as a prominent lymphocytosis—as much as double the upper reference limit. The excitement response is relatively uncommon in dogs.

Inflammatory Response

The blood neutrophil response to inflammatory disease is highly variable and dynamic. It is best viewed as a balance between tissue consumption and bone marrow production at all phases of the response. There are important species differences in this balance that are related to neutrophil storage reserve and marrow proliferative capacity.

At the beginning of an inflammatory process, the bone marrow responds by blood delivery from its reserve of late-stage maturing neutrophils, including left shift cells. If consumption exceeds marrow delivery during this acute stage, neutropenia with prominent left shift will develop. In dogs and cats, this is an indication of marked severity of the inflammatory lesion, which has historically been characterized as a degenerative left shift. However, strict classification of “degenerative” should be de-emphasized in the interpretation. Of greater importance is that neutropenia with left shift should prompt consideration of a severe inflammatory scenario in dogs and cats.

Subsequently, it takes 2–4 days for the marrow to accelerate neutrophil production by increased stem-cell entry and expansion of proliferative stages that feed the maturation stages and amplify neutrophil delivery to blood. In dogs, the acute stage of the inflammatory response that is mild to moderate neutrophilia is expected, with left shift being somewhat proportional to severity of demand.

After a few days, accelerated production adds to the picture. Neutrophilia may increase along with left shift and toxic change. As the process becomes chronic, the balance between increased marrow output and consumption may favor the development of higher magnitudes of neutrophilia. The most chronic form, present for weeks or months, is described as a “closed cavity” inflammatory process. The lesion becomes somewhat walled-off and therefore consumes fewer neutrophils, yet it still stimulates maximal marrow production. Good examples of closed cavity processes are pyometra in dogs and traumatic reticuloperitonitis (hardware disease) in cows. In these processes, the magnitude of the total WBC concentration, consisting of neutrophilia, may be as high as 100,000/μL of blood in dogs.

In contrast to the inflammatory response in dogs, cattle and most other ruminants have a relatively low reserve of marrow neutrophils and a lower capacity for accelerating granulopoiesis. This is reflected in the relatively low neutrophil concentration in normal ruminant blood. As a result, acute inflammation in cows is characterized by neutropenia that can be profound. Therefore, neutropenia in cattle does not reveal the degree of inflammatory severity. After several days, the marrow response may establish a return of blood neutrophils in modest concentration, characterized by marked left shift and toxic change. This may fit the definition of degenerative left shift but still cannot define severity in cattle. Chronic, closed cavity inflammatory lesions are associated with magnitudes of neutrophilia that rarely exceed 25,000/μL of blood.

Cats and horses are intermediate in these responses, with cats being more like dogs and horses being more like cattle in magnitudes of response. Pigs have an inflammatory response similar to that of dogs.

Monocytosis may occur in the inflammatory pattern at any stage of its progression. Monocytosis is more likely and of greater magnitude when the process becomes chronic.

Combined Steroid and Inflammatory Pattern

Inflammatory disease processes commonly induce a concurrent endogenous steroid response, recognized by the presence of lymphopenia in conjunction with an inflammatory neutrophil pattern. The neutrophil response to inflammation overrides and may be additive to the steroid influence on neutrophils.

Lymphocytosis

Modest lymphocytosis, in the range of 7,000–20,000/μL, should prompt consideration of a possible excitement response, particularly in cats. If that is excluded, then a lymphoproliferative disorder should be considered. If examination of lymphocyte morphology reveals prolymphocytes and/or blasts, then lymphocytic leukemia is the working interpretation. If the cells are all small with normal appearing chromatin, then chronic lymphocytic leukemia is a consideration requiring further workup. Chronic ehrlichiosis may result in lymphocytosis of this magnitude in dogs. At higher concentrations, the lymphocytosis may be regarded as conclusive evidence of leukemia.

Stem-cell Injury Pattern and Pancytopenia

A number of factors may cause reversible or irreversible stem-cell injury. These injuries affect erythroid cell, platelet, lymphocyte, and granulocyte production. Because of short circulation time, neutropenia is often the first abnormality observed. When chronic or irreversible, these injuries result in decreases in all 3 major blood cell lines, with the hemogram demonstrating leukopenia, nonregenerative anemia, and thrombocytopenia. Groups of causes include: 1) overdoses of radiation and antineoplastic drugs; 2) drug or plant toxicities (eg, estrogen toxicity in dogs, bracken fern toxicity in cattle, phenylbutazone toxicity in species other than horses); 3) hematopoietic cell neoplasia involving bone marrow (myelophthisis); and 4) viral infections that injure rapidly dividing cells and may cause transient neutropenia (see Leukocyte Disorders: Viral Infections that May Cause Transient Neutropenia).

Table 1
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Viral Infections that May Cause Transient Neutropenia Species Infection Dogs Parvovirus Canine distemper (acute phase) Cats Parvovirus (panleukopenia of the cat) Feline leukemia virus Horses Equine influenza Equine viral arteritis (acute phase) Equine herpesvirus Cattle Bovine viral diarrhea virus Pigs Classical swine fever virus African swine fever virus

Eosinophilia and Basophilia

Either eosinophilia or the combination of eosinophilia and basophilia prompts the consideration of the following processes in case management and assessment: allergic-based inflammation, parasitic infestation, subepithelial (skin, respiratory, GI) inflammation that is likely allergic in nature, paraneoplastic induction when common considerations have been excluded. Eosinophilia occurs in most dogs with heartworm disease and may occur in dogs and cats with flea infestation.

Prominent Metarubricytosis

Metarubricytes occasionally become a major component of the total nucleated cell fraction. The magnitude may be 10–50% of the nucleated cell population or more, with absolute numbers reaching 5,000–10,000/μL. This may occur rarely in early phases of an intense regenerative response. It may also be associated with endothelial injury (eg, heat stroke) resulting in abnormal release rate of nRBC from marrow. Most nRBC will be counted as lymphocytes on cell counters with differential capability. This may result in a preliminary result of lymphocytosis that is resolved only by examination of the lymphocyte population on the blood film.

Hematopoietic Cell Neoplasia and Leukemia

Most cases of hematopoietic cell neoplasia of either lymphoid or bone marrow origin will have some number of abnormal cells in blood. Sometimes, neoplastic cells are present in low numbers and are detected only by scanning the blood film under low magnification. Finding abnormal hematopoietic precursor cells in blood in small numbers prompts investigation of marrow and/or other hematopoietic tissues for possible neoplastic disease involvement.

The opposite extreme is marked leukocytosis with a predominance of the abnormal (neoplastic) cell population. In this situation, the blood is diagnostic for leukemia. If poorly differentiated, the cells are classified as blasts, with possible cell lineage based on morphologic appearance. If well differentiated, the cell lineage is usually clearer based on morphologic appearance.

Considerable progress is being made in the use of monoclonal antibody labeling and cytometric analysis to better establish cell lineage, particularly when the morphology is equivocal. This is particularly useful in poorly differentiated leukemias, in which morphology alone is unreliable. The distinction between well-differentiated or chronic myelogenous leukemia and extreme neutrophilic leukocytosis can be difficult.

Last full review/revision July 2011 by Glade Weiser, DVM, DACVP