AVIAN AND REPTILE HEMATOLOGY
Anne Barger
Small Animal Track
2012 ISVMA Annual Conference Proceedings
EDTA is the most common anticoagulant used for mammalian hematology however it is
known to cause hemolysis in some birds, so heparin is used most commonly as the
anticoagulant for avians and reptiles. This is also useful because hematology and
chemistry can be performed on the same sample. The hematology is done first and then
the sample can be centrifuged and biochemical analyses can be performed on the
plasma.
Avian and reptile erythrocytes are nucleated. Avian erythrocytes have an oval nucleus
but some reptiles, like turtles have a round nucleus. The cell shape is oval with orange
cytoplasm. Similar to mammals, reticulocytes have more of a purple tint to the cytoplasm
also called polychromasia. The hematocrit can be influenced by age, gender, disease
state and reproductive status. As a general rule, the hematocrit increases with age and
estrogens can depress erythropoiesis so the hematocrit is slightly higher in males.
Anemia is diagnosed as a decreased hematocrit, hemoglobin and red blood cell count.
Reticulocyte counts can be performed similar to mammals with new methylene blue
staining of erythrocytes. The presence of erythrocytosis is usually associated with
dehydration though primary erythrocytosis has been reported with hypoxia or
cardiopulmonary disease.
White blood cell morphology is drastically different from mammals. Leukocyte counts are
also more challenging due to the nucleated red blood cells. White blood cell counts are
performed manually on a hemocytometer. The cells are stained with a Natt and Herrick’s
like reagent and incubated on a hemocytometer. All 9 squares are counted and
multiplied by 1.1 x 16 x 100 and then divided by the percentage of heterophils and
eosinophils. Therefore the 100 cell differential count must be performed prior to the
calculation of the actual white blood cell count. Most birds and reptiles have the
heterophil as the primary white blood cell. These cells are similar to neutrophils in
function but not quite as adept at phagocytosis and bacteriacidal activity. In the majority
of birds and reptiles the granules are rod shaped. The nucleus in birds is lobulated but in
reptiles such as turtles, the nuclei are round. Toxic changes can be seen in heterophils
association with inflammation. Degranulation, vacuoles, presence of purple primary
granules and cytoplasmic basophilia are identified as toxicity.
Small lymphocytes are the second most predominant cell type. Small, intermediate and
large lymphocytes can be seen in circulation. These cells are round with a scant rim of
basophilic cytoplasm and round to cleaved nuclei. The basophilia of the cytoplasm is
important because lymphocytes can be difficult to distinguish from thrombocytes.
Eosinophils can look very similar to heterophils only the granule shape is round rather
than rod shaped. Additionally, species that have round heterophil nuclei have lobulated
eosinophil nuclei making them easy to distinguish. Basophils are very different in
morphology from mammals. The cells are small and round with round nuclei and
abundant amounts of purple granules. These cells are very easy to identify.
Monocytes vary greatly between birds and reptiles. Birds have similar monocyte
morphology to mammals. The cells are round with round to reniform to lobulated nuclei
with greyish-blue cytoplasm. Vacuolization is commonly observed especially in birds with
granulomatous inflammation. Reptiles have a subset of azurophilic monocytes or
azurophils. These cells can have varying numbers of purple granules and round to lobed
nuclei. The morphology of azurophils can vary greatly between different reptile species.
A leukocytosis can be associated with several different processes including
inflammation, stress, toxicity and neoplasia. The differential count helps greatly in
distinguishing different disease processes. Inflammatory leukons primarily consist of a
heterophilia with toxic changes and in birds, band heterophils. A stress leukon is
characterized by lymphopenia early followed by a mild heterophilia. No toxic changes
are noted in the heterophils. Granulomatous inflammation can result in a significant
monocytosis, particularly in birds infected with Mycobacterium sp., aspergillosis or
chlamydiosis. Leukopenia can also be an indication of severe inflammation and has
been identified in Pacheco disease in parrots. Lymphocytosis is indicative of antigenic
stimulation and often reactive lymphocytes with deeply basophilic cytoplasm are
identified with regularity on the blood smear.
The normal thrombocyte count varies greatly with the species but is 20,000-30,000
thrombocytes/microliter of blood. Thrombocytes can be estimated from the peripheral
blood smear. Like all other cells in avian and reptile blood, thrombocytes are nucleated.
These cells are round with a scant rim of pale, colorless cytoplasm. In birds
thrombocytes are usually round but reptiles can have more oval thrombocytes
occasionally containing one to two distinct cytoplasmic vacuoles. Thrombocytes can be
difficult to distinguish from small lymphocytes but the cytoplasm color can help greatly.
Lymphocytes have basophilic cytoplasm and thrombocyte cytoplasm is colorless. This
subtle difference is very important. Often thrombocyte clumps are noted on the smear.
Suggested Reading:
Schalm’s Veterinary Hematology, 6th edition, 2010, Wiley-Blackwell.
Campbell TW and Ellis CK. Avian and exotic animal hematology and cytology, 3rd
edition, 2007, Blackwell publishing.