Page 315 - The Veterinary Laboratory and Field Manual 3rd Edition

284 Susan C. Cork and Roy Halliwell

3 Wash off excess stain with buffer. The blood smear should also be exam-
4 Flood with excess buffer and leave until differ- ined for the presence of abnormal white cells,
entiation is complete (usually for 1–2 min). for example, unusual cell nuclei or staining
5 Dry carefully with blotting paper or leave characteristics and for haemoparasites (for
upright to dry in a rack. example, Babesia spp. Theileria spp. and so on)
and inclusion bodies (dense staining material
in the nucleus or cytoplasm). Inclusion bodies
5.3 Cell counts and white cell indices in the nucleus or cytoplasm of lymphocytes or
monocytes may indicate viral or toxic insult
The white cells (or leukocytes) in the blood or nutritional deficiencies and so on. In the neu-
are especially important for the body’s defence trophils the presence of cytoplasmic change such
against infectious disease. There are different as a foamy appearance, a basophilic cytoplasm
types of white cells which can be identified or excessive granulation may also indicate toxic
morphologically in a blood smear (see Figures change. The size and shape of the leukocytes,
5.8–5.10). The total number of white cells, along as well as their granules, varies a little between
with the relative proportion of each type of cell, species but some of the common characteristics
can be used to identify the nature of the disease are outlined in section 5.1. In Giemsa stained
process in an animal. Leukocytes, which include smears the red cells appear pink and the nuclei
granulocytes (neutrophils, eosinophils and of white cells appear purple or blue. The nuclei of
basophils) and mononuclear cells (monocytes the granulocytes (neutrophils, basophils and
and lymphocytes), are produced in the bone eosinophils) are usually multi-lobular with three
marrow and lymphoid tissue, along with red to five segments (variable). Granulocytes are
blood cells (erythrocytes) and platelets, and are important in the acute inflammatory response
released into the circulation in greater numbers and their granules contain digestive enzymes
in response to some diseases and physiological to kill bacteria and to remove cellular debris.
states. A high number of white cells in the circu- Precursor PMN have a band shaped nucleus and
lation (leukocytosis) most commonly indicates are referred to as band cells. If these are pres-
that an inflammatory process is occurring such ent in large numbers (that is, a proliferative
as in response to infectious diseases. In viral dis- response) it is interpreted as being evidence of
eases, the proportion of lymphocytes may rise a strong immune response to an infectious dis-
(lymphocytosis) or fall (lymphopaenia) whereas ease, for example, a neutrophilia with a left shift.
in bacterial diseases, the proportion of neutro- In Romanowsky stained smears, the cyto-
phils usually increases (neutrophilia). In allergic plasm of eosinophils, in most species, contains
and some parasitic diseases, the number of pink granules and that of basophils contains
eosinophils increases (eosinophilia). However, blue granules. The nuclei of monocytes may be
this is not always the case and in very debilitated lobulated and in lymphocytes are usually round
animals the total number of white cells may fall in shape.
(leukopaenia). In animals with chronic intra-cel- Monocytes are generally larger than lym-
lular bacterial diseases, such as tuberculosis and phocytes but in blood or tissue smears it is not
brucellosis, the white cell count may be normal always easy to tell these two cell types apart.
although an increase in monocytes (monocyto- Lymphocytes are present in the spleen, lymph
sis) may be evident. Evaluation of the white cell nodes and in other reticulo-endothelial tissues
population involves performing a total white of the body, such as the liver. The lymphoid
cell count (TWCC) and a differential count. system acts as the defence system for the blood

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