Old World Aviaries

The Complete Blood Count


by Scott McDonald, DVM

Reprinted with permission from the Spring 1996 issue of The Avian Quarterly

The complete blood count (CBC) is a good, sensitive indicator of the bird’s general health. A change in the hemogram can be noted when no other abnormalities are detected. It is the single most important test to perform.

If all parameters are normal on the CBC, then this is a good indication that the bird is not currently fighting some type of generalized infection or inflammatory disease process. However, a normal CBC does not rule out the possibility that the bird is an asymptomatic carrier of psittacosis, a viral disease, a gram negative bacteria, or has a localized infection.

There are three types of cells which are evaluated on the CBC; red blood cells (RBCs or erythrocytes), white blood cells (WBCs), and thrombocytes. In addition, the plasma (non-cellular part of blood) is examined for color, protein, and the presence of parasites.

The following is a list of parameters measured on the CBC with a range of normal values. The range of normal values can vary greatly because there are so many different kinds of psittacine birds. There are also differences between juvenile and adult birds.

Parameter

Range of Normal Values

RBC (millions of cells/mm3)

2.54.5

PCV (packed cell volume)

3555%

polychromasia

slt (slight)

anisocystosis

slt (slight)

WBC (cells/mm3)

500015000

het (heterophils)

4075%

lymphs (lymphocytes)

2050%

baso (basophils)

05%

eos (eosinophils)

02%

monos (monocytes)

03%

buffy coat

1% or less

Thrombocytes

present

Plasma

color clear or pale yellow

T.P. (total protein) gm/dl

2.55.5

Red blood cells: Examination of the erythrocytes provides the veterinarian with information concerning anemia-related problems and the bone marrow response (which is where RBCs are produced).

In a healthy bird, most RBCs should be mature but a small percentage of immature erythrocytes is expected, which indicates a normal replenishment from the bone marrow. Immature RBCs are slightly larger than mature RBCs, and the cytoplasm is lightly-stippled which produces a difference in color.

Anisocytosis (size variation between RBCs) and polychromasia (color variation between RBCs) are terms used to indicate the presence of immature RBCs. On the form used to indicate the results of the CBC a slt (slight) response is normal while mod (moderate) or max (maximum) indicates a strong bone marrow response to some type of anemia problem. If a bird is anemic and shows slight to no polychromasia or anisocytosis over a period of several weeks, this can indicate that the bone marrow is depressed or non-regenerative which is a grave situation.

Anemia means a decrease in the normal amount of red blood cells. It can be due to either blood loss or lack of production of new RBCs from the bone marrow.

The ratio of the volume of RBCs to the volume of whole blood is called the packed cell volume (PCV). This number is expressed as a percentage. It is determined by putting a sample of blood in a tiny collecting vial called a microhematocrit tube. At the laboratory this tube is spun in a centrifuge which separates the red blood cells from the plasma.

In a normal, healthy bird, the PCV ranges between 35 – 55%. Small birds, such as budgies and cockatiels, average 50% while the values in larger species are usually lower, around 42%.

Any value less than 35% indicates anemia and obviously the lower the value, the more critical the condition. Values higher than 55% are most often caused by dehydration.

The actual number of RBCs can also be counted, using either automated cell counting machines (which are very expensive) or by manual techniques which provide good estimates. The number of RBCs is recorded as a number times 10 to the sixth power which means millions of cells per cubic millimeter (mm3) of whole blood.

The normal number of RBCs varies depending on whether the bird is a juvenile or adult and the kind of bird being examined. In general, most adult, healthy psittacines average around 2.5 – 4.5 million RBCs/mm3 of blood. As a bird becomes anemic, the number of RBCs decreases.

White blood cells: White blood cells (WBCs) are part of the body’s defense or immune system.

There are five types of WBCs found in birds. Heterophils (hets), eosinophils (eos) and basophils (baso) are known as granulocytes because they all contain colored granules (tiny spindle shaped bodies) in their cytoplasm.  These cells are all produced in the bone marrow. Lymphocytes (lymphs) and monocytes (monos) are known as mononuclear WBCs. These two types of cells have a single nucleus and no granules in their cytoplasm. They originate in the spleen and in tiny collections of lymphoid tissue found throughout the body as opposed to the bone marrow.

During the collection of blood for a CBC, a tiny drop is placed on a glass microscope slide. A second slide is placed on top and the two are pulled apart creating a blood smear one cell layer thick on each slide.

Back at the laboratory, this smear is stained with a blue dye which colors the different cells so they can be identified. By looking at the smear under the microscope, the total white blood cell count can be estimated as well as determining the number and relative proportions (percentage) of the five different types of WBCs.  In addition, the WBCs can be examined for abnormal characteristics. All these findings are significant in determining whether a bird is fighting some type of disease and how serious it is.

White blood cell count The total WBC count can also be determined by actually counting all of the granulocytes in a dilution of whole blood that is stained and put in a cell counting chamber called a hemacytometer. The cytoplasmic granules of these cells are specifically highlighted by the dye which makes them easy to count. By also knowing the percentage of mononuclear WBCs present from the blood smear, one can arrive at a fairly accurate total WBC count.

The normal WBC count can vary from laboratory to laboratory. It is dependent on the staining quality of the smear and the proper identification of cells.

In our hospital, the normal WBC count in adult psittacines varies between 5000 – 15000 cells/mm3 of blood. In smaller species (i.e. budgies, cockatiels) the normal range tends to be lower (5000 – 10000 cells/mm3) than in larger species such as Amazons and Macaws (12000 – 15000 cells/mm3). Juvenile psittacines usually have higher WBC counts than adults. It is not uncommon to see counts in the 20000 – 25000 cells/mm3 range in large psittacines that are near the weaning age.

A crude, rapid technique for estimating if the WBC count is high is by measuring the buffy coat. When the microhematocrit tube is spun at the laboratory there is a tiny layer of white cells that separates out at the junction between the red cells and plasma. This is called the buffy coat. In a normal, healthy psittacine bird, the buffy coat should be 1% or less of the total volume of blood. A buffy coat of 2% or higher indicates a high WBC count. Six percent is the highest our lab has ever measured.

An increase in the WBC count is called leukocytosis.  A common cause of leukocytosis is the actual transporting and handling of a bird at the clinic. This is a physiological change due to stress. Disease causes include bacterial and fungal infection, psittacosis, avian TB, trauma, toxicities, and cancers such as leukemia.

A decrease in the WBC count is called leukopenia. The most common causes of this include acute viral infections and the end stages of overwhelming septicemia (blood poisoning) in which the body literally runs out of WBCs with which to fight with.

The percentage of the different types of WBCs is called the differential. As stated before, alterations of the differential from the normal range and changes in cell characteristics are significant for various disease conditions.

Heterophils: The heterophil is the most common granulocyte found in psittacine birds. The normal percentage of heterophils is between 40 – 75%. It is considered the “window to the bird’s state of health,” since even subtle changes in numbers and characteristics can occur with problems, such as stress, low grade infection, and mild inflammation. These cells are instrumental in body defense with incredibly large numbers available in acute and/or overwhelming infection or inflammation as well as their ability to phagocytize bacteria or other infectious agents.

Heterophils exhibit toxic changes in stress or disease conditions that can be seen microscopically. A low degree of toxicity is called degranulation. This is seen as a decrease in the number and size of cytoplasmic granules and a change in color of the cytoplasm from colorless or light blue to a muddy pink or red color. If degranulated hets are seen on the blood smear, this will be noted under the COMMENTS section. If nothing is written, then this means all the hets appear normal.

Mild degranulation of hets can be considered normal in birds under stress. However, this finding should alert the veterinarian that something is going on, especially if the total WBC count is higher than normal.

In severe, generalized illnesses, the WBC count may rise very high above normal with a varying number of the hets appearing dark and shrunken. In addition to degranulation, these cells may appear foamy (the presence of vacuoles in the cytoplasm), or to have ruptured. The presence of bacteria may also be evident in the cytoplasm.

Such hets are described as being toxic. The degree of heterophil toxicity usually increases with the severity of the bird’s condition. Birds having a high degree of toxicity in most or all of their heterophils often have a grave prognosis. This is especially true if leukopenia is present.

Eosinophils and Basophils: Eosinophils (eos) are similar in appearance to heterophils but can be differentiated by the color and shape of their cytoplasmic granules and the darker staining nucleus.

Eosinophils occur in very small numbers with the normal range considered to be 0 – 2%.  The function of the avian eosinophil is unclear; however, increased numbers are typically associated with parasitic infections, allergic reactions, and significant tissue damage.  Changes in cell morphology have not proven helpful.

Basophils (baso) are easy to identify because of their dark-staining (basophilic) cytoplasmic granules. They also occur in small numbers with a normal range of 0 – 5%.

The exact function of the basophil is not known. Increased numbers are often associated with chronic, long tern illnesses. They also appear to play a role in the early stages of inflammation.

Lymphocytes: Lymphocytes and heterophils are the most numerous WBCs seen in psittacine birds. The normal proportion of lymphocytes is 20 – 50% but it varies among species. Some species, such as cockatiels and Amazon parrots, are naturally lymphocytic, which means more lymphs than hets are seen normally.

Lymphocytes are a very important part of the bird’s immune system. Lymphocytes increase in numbers (lymphocytosis) in certain types of chronic infections and lymphoid leukemia. They decrease in numbers in acute viral infections. In a disease state, lymphocytes will become reactive.

Microscopically, the normally pale blue cytoplasm becomes multicolored with light blue, clear, and pink areas, and the cytoplasm becomes vacuolated (the presence of tiny “bubble-like” structures). In chronic conditions lymphs with azurophilic (purple) granules in the cytoplasm have been noted.

A few reactive lymphs on a blood smear is normal but if many are reactive then this is a significant finding that the body is responding to an infection of some sort. If reactive lymphs are present, this will be noted under the COMMENTS section. Reactive lymphs synthesize antibodies and other agents involved in the host immune system.

Birds with a marked lymphocytosis in which most or all of the lymphocytes appear abnormal or immature is consistent with leukemia. The WBC count in such birds can be greater then 100,000 cells/mm3 of which most are lymphocytes.

Monocytes: These are the largest WBCs found in avian blood, and they are very similar in appearance to lymphocytes. They occur in small numbers with a normal range of 0 – 3%. An increase in the percentage of monocytes (up to 10%) is seen with certain chronic diseases such as psittacosis, systemic fungal infections, tuberculosis, and massive tissue destruction. Changes in cell morphology are not significant.

The evaluation of the WBC count is most useful in the assessment of the progress of an avian patient. For example, a bird with an initial leukocytosis, increased heterophils which appear toxic, and reactive lymphocytes would be showing a favorable response to therapy if the total WBC and heterophil counts were decreasing, the toxic heterophils had disappeared, and the reactive lymphocytes returned to normal. The WBC count is thus an important monitor for effective therapy.

Thrombocytes: Thrombocytes are the third type of cells found in avian blood, and these are active participants in blood coagulation or clotting. Birds do not have platelets (cell cytoplasmic fragments) which perform this function in mammals.

Thrombocytes are produced in the bone marrow, are slightly smaller than erythrocytes, and are often clumped together in small numbers when viewed on a blood smear under the microscope. In addition to blood clotting, these cells may have the ability to phagocytize or eat foreign material (such as bacteria) like certain WBCs, and they may be able to carry oxygen like RBCs if an extreme anemic condition exists.

During the CBC evaluation, the presence of thrombocytes is noted on the form and if the numbers seem decreased or increased this is also indicated. Increased numbers of reactive thrombocytes can indicate a chronic disease condition.

Plasma: Plasma is the fluid portion of blood in which particulate components are suspended. Plasma is distinguished from serum which is the cell-free portion of the blood from which fibrinogen (blood clotting factors) has been separated in the process of clotting.

When blood is drawn for a CBC, the cell components need to be evaluated; therefore, the blood must not clot.  Heparin is the anticoagulant of choice in birds. The microhematocrit tubes or larger vials used to collect blood for a CBC are heparinized to prevent coagulation.

Color: After the microhematocrit tube is spun at the laboratory, the plasma is visually examined for color. In most birds it should be clear or pale yellow in color. The yellow color is due to the presence of carotenes, which are yellow pigments found in a variety of plant materials including some seeds. Yellow-colored plasma should not be misinterpreted as icteric plasma. In mammals, icterus or jaundice is due to increased amounts of bilirubin pigments in plasma as a result of liver disease. Birds do not have bilirubin, therefore they do not become icteric if they have liver disease.

Sometimes the plasma appears pink in color. This is due to hemolysis which is the breaking apart of RBCs and the subsequent release of red hemoglobin molecules into the plasma. Hemolysis is most commonly caused by improper handling of blood samples, such as forcibly expelling blood through the needle of the syringe, shaking rather than simply inverting the tube to mix blood with the anticoagulant, over centrifugation, or freezing and thawing of the sample.

Some toxicities can cause lysis of blood cells, the most common example being lead poisoning. Occasionally an overwhelming infection causing septicemia will also cause hemolysis.

The plasma can also appear white or milky in color, which is due to the presence of fat.  This is called lipemia.

Lipemic plasma is most commonly seen if a blood sample is drawn shortly after the bird eats a meal high in fat (i.e. sunflowers or other oily seeds). Sometimes if the bird is excessively overweight, the serum will always appear mildly lipemic. Liver or pancreatic disorders can cause lipemia. Lastly, in female birds that are reproductively active, the plasma will be lipemic due to yolk globules (fats and protein) which are synthesized in the liver and transported via the plasma to the ovary where they are incorporated in the oocyte.

Total Protein (T.P.): While plasma is mainly water, it does contain a substantial amount of protein. Most plasma proteins, with the exception of antibodies and hormones, are synthesized in the liver. Proteins form the basis of organ and tissue structure, operate as catalysts (enzymes) in biochemical reactions, are regulators (hormones), and are the transport and carrier compounds for most of the constituents of plasma. The total protein level in the plasma is often used as an indicator for the health status of the patient.

The measurement of total protein is a simple test to run in the laboratory. Most practitioners use an instrument called a refractometer which gives a good estimate of the protein level as long as the plasma is clear. Hemolysis or lipemia will give a falsely elevated level.

The normal value for psittacine species is in a range between 2.5 – 5.5 grams/deciliter (gm/dl). Low total protein can reflect chronic disease (especially liver and kidney disorders), cancer, parasitism, long-term stress, and starvation or malnutrition. Increased values may indicate dehydration, chronic infection, or leukemia.

Parasites: Blood parasites are often found in the blood smears of wild-caught, imported birds. However they are extremely rare in domestic-raised pet birds that are kept indoors.

Blood parasites are transmitted by bloodsucking insects (flies, mosquitoes, or mites) and whether they cause clinical signs of disease is determined by the type of parasite found and the species of bird involved. In general, most blood parasites found in psittacine birds are incidental, cause no clinical disease, and treatment is usually not recommended.

Parasites that invade red blood cells include Hemoproteus, Plasmodium (avian malaria), Leukocytozoon, Atoxoplasma, and Aegyptianella. Of these, only Hemoproteus is commonly seen in wild-caught psittacines, usually in cockatoos.

Microfilaria are microscopic, immature forms of filarial roundworms (related to heart worms in dogs) that are found in plasma. They have been seen in a variety of psittacines, especially cockatoos. The adult filarial worms are usually undetected and may occur in the air sacs, thoracic and abdominal cavities, or joints. Surgical removal of adult worms and ivermectin are the treatments of choice.


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