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Lymphocytes are second only to the heterophil in frequency in most species. Size and shape varies, with small, medium and large cells that may be round or moulded around neighbouring cells being seen in the same smear. Occasional reactive lymphocytes are a normal finding, but large numbers indicate marked antigenic stimulation as seen in severe infections (e.g. severe viral infections, chlamydiosis, Aspergillosis, salmonellosis and tuberculosis).
Lymphocytosis is seen in:
· Chronic infectious or inflammatory conditions.
· Lymphoid leukaemia.
· Normal finding in Amazons and canaries. Lymphopaenia is seen in:
· Viral infections and diseases that cause bursal damage or bone marrow suppression.
· Relative to a marked increase in heterophils. Monocytes
Monocytes are the largest of the mononuclear leucocytes, but are rarely seen in peripheral blood smears. They spend only a short time in circulation before passing into tissues and becoming macrophages. The eccentric nuclei are either round, elongated or indented, and the cytoplasm typically stains a blue–grey colour with a reticular or finely granular appearance, with occasional vacuoles. Care must be taken not to confuse them with large lymphocytes.
Monocytosis is most commonly associated with chronic granulomatous infections
Mature azurophils are similar in size to heterophils and vary in shape from round to monocytoid in appearance. The nuclei are usually eccentric and the cytoplasm is bluish grey with azurophilic granules. Azurophils occur at relatively low numbers in healthy reptiles, but are increased in bacterial infection and cellular necrosis.
Basophils are uncommon in peripheral blood smears of birds. They appear as small cells with clear cytoplasm and spherical basophilic granules. The nucleus stains
a light blue colour. Care must be taken not to confuse them with immature heterophils. Basophilia has been reported in respiratory disease (e.g. air sac mite in canaries), chlamydiosis and tissue trauma more than 48 hours old. In birds and reptiles basophils appear to play an important role in early inflammatory and immediate hypersensitivity reactions, but differ from those in mammals by not contributing to delayed hypersensitivity.
This releases intracellular enzymes, which then reach elevated levels in the blood. These so-called ‘leakage enzymes’ include:
· Aspartate aminotransferase. This cytosolic enzyme
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Thrombocytes are small, oval, nucleated cells that can be differentiated from erythrocytes by their size (they are smaller than erythrocytes) and their nucleus, which is larger, more rounded and darkly basophilic-staining. The cytoplasm is colourless or a faint blue colour with one to two small basophilic inclusions at the poles.
Total counts are difficult and not routinely performed
as the thrombocytes tend to clump. However, there
are typically 1–2 cells seen per high-power field. Their function is unclear; they contain little thromboplastin,
so it is unlikely that they initiate clotting. With bacterial infections they tend to increase in numbers and become activated (pseudopodial formation and vacuolation) and tend to aggregate in clumps. They appear to have some phagocytic activity.
Thrombocytosis is rarely reported and may arise as
in response to thrombocytopenia. Thrombocytopenia may occur due to bone marrow suppression or disease processes causing an excessive demand (e.g. viral diseases such as circovirus, reovirus or polyomavirus).
Clinical biochemistry
Clinical biochemistry involves the measurement of specific groups of chemicals within the body and the interpretation of the results obtained. These chemicals include:
· Metabolites. Those chemicals that are produced as the end-products of various metabolic processes within the body.
· Tissue enzymes, which catalyse chemical reactions within the body without being altered themselves.
· Electrolytes, including sodium, potassium and chlo- ride.
· Minerals, such as calcium, phosphorus and magne- sium.
· Bile acids, produced in the liver from cholesterol and used in the emulsification of dietary fats.
· Lipids
Liver enzymes
The detection of liver disease through biochemistry is complicated by the fact that there are no specific ‘liver enzymes’ that can be evaluated conclusively in each and every case. Liver disease can be broadly classified into three conditions: hepatocellular rupture, decreased hepatic function and cholestasis. These conditions can occur either separately or concurrently.
Hepatocellular rupture
is found in many tissues in the body, but the highest concentrations are found in skeletal muscle and liver. Significant elevations usually represent either muscu-

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