The Leukogram - The Merck Veterinary Manual

The Leukogram

Corticosteroid-induced or Stress Leukogram

Physiologic Leukocytosis:

Leukocytosis may occur as a result of exercise or excitement; this response is mediated by increased epinephrine. Epinephrine centralizes the marginal pool; thus, the effect of excitement may double the total WBC count 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. A lymphocytosis also may be present, especially in young horses or cats.

Corticosteroid-induced or Stress Leukogram:

Treatment with exogenous corticosteroids or endogenous release results in a typical leukogram with lymphopenia as a consistent feature and invariably accompanied by neutrophilia. The net effect on the total WBC count is a function of the species and its normal differential WBC count. In dogs, in which neutrophils are the predominant circulating WBC, corticosteroids produce leukocytosis. In cattle, in which lymphocytes predominate, the WBC count is variable depending on the degree of neutrophilia and lymphopenia. There usually is a reversal of the neutrophil to lymphocyte ratio. Stress leukograms have a characteristic WBC differential count consisting of a mature neutrophilia, lymphopenia, and eosinopenia. Neutrophilia is due to decreased adherence to the vascular endothelium, which prolongs circulating time and shifts the marginating cells to the circulating pool, and increased marrow release of neutrophils. Lymphopenia is due to redistribution or lysis of lymphocytes. Monocyte numbers are variable; however, a monocytosis often occurs in steroid or stress reactions in dogs and horses.

Other Leukogram Alterations:

Although the specific features of leukocytosis and leukopenia vary with species, there are some generalities. Radiation and radiomimetic drugs (eg, many antineoplastic drugs) produce leukopenia. Lymphocytes are extremely sensitive to radiation, but mature granulocytes—unlike their precursors in the bone marrow—are not particularly radiosensitive. Pancytopenias, which include leukopenias, may result from drug toxicities (eg, estrogen toxicity in dogs, bracken fern toxicity in cattle), nutritional deficiencies, stem-cell alterations (possibly immune-mediated), and space-occupying lesions of bone marrow (myelophthisis). Generally, viral infections, particularly acute infections not associated with extensive tissue necrosis, produce leukopenia as neutropenia and may have mild lymphocytosis or lymphopenia. In viral infections of longer duration (2-3 wk) with secondary bacterial complications, the total WBC count may be increased or within normal limits due to concurrent neutrophilia and lymphopenia. Ehrlichiosis in dogs is associated with thrombocytopenia, but persistent leukopenia may be seen, especially in chronic cases.

Species-specific Leukograms:

In dogs, the normal neutrophil to lymphocyte ratio is ~3.5:1. A variety of infectious and noninfectious diseases can induce changes in the leukogram. Inflammation, corticosteroid (stress) reactions, and physiologic (epinephrine) responses are common causes of leukocytosis. A leukocytosis is typically present at parturition in the bitch. Cyclic hematopoiesis of gray Collies (see Inherited Leukocytic Disorders) produces peripheral neutropenia at 11- to 14-day intervals. Severe panleukopenia with total WBC counts <1 × 10³ /µL often characterizes parvovirus infection, while in coronavirus infection and in infectious hepatitis, neutropenia may be more moderate. In the later stages of canine distemper with secondary bacterial infection, the total WBC count may be normal with neutrophilia, lymphopenia, and sometimes increased band neutrophils and toxic changes.

In cats, the neutrophilic response can be strong, although less marked than in dogs. Feline infectious peritonitis generally does not produce a leukopenia but more commonly a neutrophilia and lymphopenia. Feline leukemia virus has a variable effect on peripheral blood cells. In the absence of leukemia or lymphoma (see below), it may produce a panleukopenia with marked neutropenia, toxic changes, and mild lymphopenia sometimes with atypical cells that may resemble the leukogram induced by feline panleukopenia virus.

In horses, the normal neutrophil to lymphocyte ratio is ~1.1:1. The magnitude of neutrophilic response is much less than in dogs or cats. The highest absolute lymphocyte values are normally present in yearlings. In horses, corticosteroids induce neutrophilia, lymphopenia, eosinopenia, and frequently monocytosis. In equine viral arteritis, an initial leukopenia during the febrile episode is followed by lymphopenia and mild neutrophilia. Leukopenia occurs in equine herpesvirus infection, influenza, and sometimes in the early febrile phase of equine infectious anemia. In equine ehrlichiosis, there may be leukopenia, sometimes accompanied by thrombocytopenia, anemia, or both.

In cattle and most ruminants, the normal neutrophil to lymphocyte ratio is ~0.5:1, and the neutrophilic response is weaker than in any other common domestic species. Leukopenia occurs early in bacterial infections in ruminants and persists longer than in other species due to their low bone marrow neutrophil reserve (storage) pool and lymphopenia secondary to endogenous corticosteroid release. A degenerative left shift frequently develops in ruminants with acute inflammatory disease; however, it does not carry the same poor prognosis as in other species unless the WBC count fails to increase in 2-3 days. Cattle with septic infections, including septic mastitis or peritonitis, characteristically have leukopenia, lymphopenia, and neutropenia the first few days. A return to normal of absolute lymphocyte and neutrophil numbers or neutrophilia are good prognostic signs. Persistent leukopenia or toxic changes in the neutrophils indicates an unfavorable prognosis.

In pigs, the normal neutrophil to lymphocyte ratio is 0.7:1. Neutrophilic responses may be marked, possibly even more so than in dogs. The stress of venipuncture may result in a doubling of the circulating neutrophil count in 30 min, which persists for 8 hr; this may impair studies that require frequent serial blood sampling. During parturition, sows manifest neutrophilia, lymphopenia, and eosinopenia. Leukopenia is marked (<10×10³ /µL) in classical swine fever and African swine fever.