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.