Avian Leukocyte Differential and Morphology

Granulocytes

Heterophils
Heterophilia occurs frequently in the abnormal avian hemogram. Absolute heterophilias often are the primary contributors to leukocytosis. A "stress" heterophilia occurs for the same reasons as "stress" leukocytosis. Expect to see heterophilia in acute inflammatory and infectious processes, including chlamydial, bacterial, and fungal infections.
Relative heteropenia may be a normal finding in species with lower heterophil/lymphocyte ratios, such as Amazon parrots and canaries. Artifactual heteropenia can follow poor blood smear techniques, resulting in smudge cells. Differential diagnosis for absolute heteropenia includes bacterial sepsis and severe viral disease. In bacterial sepsis, immature heterophils with toxic and degenerative changes may be seen. Intracellular bacteria indicates bacterial sepsis. Extracellular bacteria may be artifactual. In viral disease, the patient commonly presents hours from death. The typical hemogram shows the virtual absence of heterophils, with lymphocytes predominating in number.
Heterophil morphology exerts an important influence when evaluating the hemogram. A fair amount of variability occurs in color tones depending on the staining protocol used, so morphologic changes are not universally agreed upon. Artifactual cytological damage abounds, mainly due to poor technique which is characterized by free nuclei, ruptured cells, and "smudges." Toxic changes, however, are common in worsening infectious disease, particularly bacterial, fungal and chlamydial in origin. These cellular differences can include basophilic cytoplasmic inclusions, nuclear hypersegmentation, and cytoplasmic vacuolization. Toxic heterophils can be the only abnormal hematologic finding, but are very significant clinically. Granulocytic leukemia occurs in avian patients, although recognition can be difficult and may require bone marrow cytology and special stains for definitive diagnosis.
Immature heterophils occasionally occur in the peripheral blood.
Bands and mesomyelocytes usually imply a worsening bacterial infection and may provide for a more guarded prognosis.

Eosinophils

Eosinophils are uncommon in the hemogram of many avian species, common in others. The neophyte is challenged when trying to differentiate this cell from other granulocytes. In most cases, eosinophils are distinguished by their round shape, distinct nuclear/cytoplasmic contrast, and uniform cytoplasmic color. Granule shape and color varies greatly. The clinician should question the avian hemogram showing a high eosinophil count, as the inexperienced technologist can easily mistake the pinkish heterophil for the avian eosinophil. The function of the avian eosinophil is poorly understood, but clinical observations can provide guidelines.
An eosinophilia is typically a relative change, that is an increase in the percentage, but not necessarily the absolute number of circulating eosinophils. Eosinophilia can be observed in a variety of alimentary tract parasitisms including giardiasis, ascaridiasis, and cestodiasis, but eosinophilia is not a consistent finding in with parasitism Allergic, non-parasitic conditions such as allergic dermatitis or respiratory hypersensitivity demonstrate highly suggestive histopathologic changes, but are not associated with peripheral eosinophilia. Tissue sporozoa (Apicomplexa including Atoxoplasma, Toxoplasma, Sarcocystis) and malarial forms (Plasmodium, Hemoproteus) don't typically present with eosinophilia. Resolving tissue damage cases (trauma, organ damage) can sometimes show increases in peripheral eosinophils. The mechanism for this observation is unknown. Passerine birds with air sac mites (Sternostoma) variably can present with eosinophilia. It is not known whether this is an allergic or "tissue damage" response.
Eosinopenia is not well-documented in birds. In normal birds, peripheral eosinophils are rare. This is particularly true in new world psittacines. Abnormal eosinophil morphology is not generally recognized.

Basophils
Basophils are uncommon in avian peripheral blood. In most cases, avian basophils resemble their mammalian counterparts but variability in appearance does occur among avian species. It is important to differentiate basophils from toxic heterophils, which often have large amounts variably sized basophilic cytoplasmic inclusions.
Basophilia is observed with avian respiratory infections and resolving tissue damage Basophilia is a common presentation in active chlamydial infections, particularly in budgerigars and Amazon parrots.
Basopenia is not well-documented, but many normal avian hemograms show no basophils. Abnormal morphology is limited to degranulation; the clinical significance is unknown. It is not clear how to determine degranulation from preparation and staining artifacts.

Mononuclear Cells
Lymphocytes occur at a higher frequency than all other leukocytes, except heterophils. Some avian species are "lymphocytic," including Amazon parrots and passerine birds. While there are two to three unique populations of lymphocytes, based on size differences, these groups are not differentiated by the clinical laboratory.
Lymphocytosis is not common. An apparent relative lymphocytosis is normal for some avian species with a low heterophil/lymphocyte ratio, such as Amazon parrots and canaries. Absolute lymphocytosis should lead the clinician to consider lymphocytic leukemia, particularly if the total count is very high and the morphologic changes are also suggestive. In some stages of viral and chlamydial infections, absolute lymphocytosis may occur.
A relative lymphopenia appears to occur with marked heterophilias. It is best to consider that the heterophils are really just present in such large numbers to outnumber the lymphocytes. Absolute lymphopenias can occur in association with the pancytopenia observed in terminal viral infections, such as a juvenile African Grey Parrot infected with circovirus.
Abnormal lymphocyte morphology should be pondered by the clinician. Lymphocytes can appear reactive. While these changes are not standardized, they include cytoplasmic blebbing and vacuolization, nuclear changes and deeply bluish cytoplasm. Marked reactive changes are usually associated with severe viral infections. Reactivity may also be associated with chlamydial infections and blood parasites.
Monocytes are generally scarce in peripheral blood. The actual incidence of monocytes in avian blood remains unsettled and will require standardized cytochemical methods to resolve. It is sufficient to say that in many avian hemograms, monocytes are rare to non-existent.
Relative or absolute monocytosis is a hallmark for chronic infection. In the avian patient this can indicate chlamydial, mycobacterial, fungal, and granulomatous or organ-reactive bacterial infections. In budgerigars, a relative monocytosis and basophilia may be the only abnormal hematologic finding, suggesting chlamydiosis. The hemogram of aspergillosis and tuberculosis may be similar and includes leukocytosis and monocytosis. The stage of infection and host response in patients infected with Aspergillus or mycobacteria may result in little or no hematologic changes.
Monocytopenia is not documented, but a low or zero monocyte count is normal for most species. Morphologic changes can include visualization of intracellular bacteria in septicemic birds. Monocytes can become macrophagic in appearance in some serious chronic infections.