49 AVIAN HEMATOLOGY

1 How are avian blood cells different from the blood cells of mammals?

Avian blood cells are similar to mammalian blood cells in some aspects but differ significantly in others, particularly morphology. As in mammals, avian blood cells include erythrocytes, leukocytes, and thrombocytes (platelets), all of which are easily evaluated by examination of air-dried, Romanowsky-stained blood smears.

2 What are some important principles to understand about the identification of avian leukocytes by blood smear evaluation?

a. Proper identification of leukocytes depends on familiarity with the morphology of leukocytes and the species of bird, prompt processing of the blood sample, quality of the blood smear, and staining techniques used to visualize the leukocytes.

b. Fresh blood (no anticoagulant) and ethylenediaminetetraacetic acid (EDTA) or heparin anticoagulated blood samples are both satisfactory for use in the identification of leukocytes, but fresh blood smears are preferred. When cell enumeration is required, it is best to keep samples at a cool temperature when shipping. Long transit times may result in inaccurate leukocyte counts, abnormal morphology, or hemolysis (e.g., ostrich erythrocytes in EDTA).

c. Gaining familiarity with leukocyte morphology requires frequent examination of blood samples from normal and sick patients of various species. Staining affinity of cytoplasmic granules may vary between species of birds.

d. Use of consistent staining techniques and methods is important to avoid excessive variation in cell staining and to ensure consistent and accurate leukocyte identification.

e. Formalin fumes, even if the source is a few feet away, will alter cellular proteins in such a way that the staining intensity and affinity of leukocytes will be altered. Therefore, blood smears should be kept away from formalin sources and stained promptly.

f. If a blood smear must be stored for longer periods, either for later examination or for archiving, immersion oil should be removed with appropriate solvents and the slide permanently coverslipped.

3 Describe the various avian leukocytes.

Avian leukocytes include the heterophil, basophil, monocyte, lymphocyte, and eosinophil. Descriptions are based on Romanowsky-stained (e.g., Wright-Giemsa, Diff-Quik) blood smears.

a. The heterophil is the predominant leukocyte in health in many avian species and predominates in many disease conditions (Figure 49-1). It has a lobulated, condensed nucleus, although the degree of lobulation is less than that of mammals. The heterophil of most bird species has a cytoplasm that is filled with elongate, rod- to spindle-shaped, orange to brick-red granules.

b. The eosinophil has prominent small, round cytoplasmic granules that vary in color from bright red to pink depending on the species (Figure 49-2). In a few species the granules may appear lavender blue using Romanowsky-type stains. The eosinophil is not seen as frequently as the heterophil, and the granules of the eosinophil will stain differently than those of the heterophil within the same blood smear. Compared with other bird species, raptors tend to have slightly greater numbers of eosinophils in circulation.

c. The basophil has prominent small, round, deep-magenta to purple cytoplasmic granules and, unlike the mammalian basophil, has a round to oval, nonlobed nucleus (Figure 49-3). The contents of basophil granules can be dissolved by Romanowsky-type stains, giving the basophil a foamy, vacuolated appearance with few granules, if any. Careful examination of these cells in a blood smear may reveal a few characteristic granules, particularly over the nucleus, helping to identify them as basophils.

d. The monocyte is similar to its mammalian counterpart. It is a large mononuclear cell with abundant cytoplasm and a round to oval to indented nucleus (see Figure 49-1). Monocytes lack the prominent cytoplasmic granules of the granulocytes but may contain fine, dustlike, pink granulation and small, clear vacuoles.

e. The lymphocyte is also morphologically similar to that of mammals and is the predominant leukocyte in some species of birds (see Figure 49-1). It is a mononuclear cell, with low to scant amounts of blue cytoplasm, high nuclear/cytoplasmic (N/C) ratio, and a round nucleus. Birds normally have small and larger lymphocytes in circulation. Care must be taken not to confuse the small lymphocytes with thrombocytes. Thrombocytes may be smaller, more oval, with a darker, more condensed nucleus, indistinct pale to nonstaining cytoplasm, and in contrast to lymphocytes, may readily clump in blood smears.