Avian Leukocyte Counts

Leukocytosis
Leukocytosis occurs in birds as a result of disease states or "stress." Stress leukocytosis appears to occur in a variety of avian species, including macaws, cockatoos, African greys, and ratites. The avian spleen does not appear to be source of blood pooling. Clinical observations indicate that leukocyte counts can increase markedly in excited birds compared to samples taken when the patient is "at rest." Stress hemograms can also be observed in patients who have been recently treated with corticosteroids.
Juvenile birds can demonstrate a greater variability in total leukocyte counts until four to six months of age. Elevated counts are common and must be interpreted with caution, as the bird may in fact be normal. The clinician should always carefully review the differential and cell morphology to determine if illness seems likely. Our experience, young birds residing in pet stores before sale to the public, tend to run higher leukocyte counts in the absence of documentable disease. This may be the "stress hemogram" effect.
Disease-caused leukocytosis in birds occurs mostly due to infections. Degenerative and neoplastic disorders can inconsistently present with moderate elevations in the total leukocyte counts.
The clinician cannot easily differentiate diseases based solely on leukocytosis, but certain trends emerge. Mild leukocytosis correlates with bacterial, fungal, and chlamydial infections. Moderate elevations can be caused by yolk peritonitis, granulomatous disease, and some phases of septicemia.
Very high leukocyte counts (over 60,000) can relate to active chlamydiosis (especially macaws), aspergillosis, or tuberculosis. Of course these counts are rated differently in various species. Leukocyte counts rarely exceed 20,000 in budgerigars, 30,000 in cockatiels, and 80,000 in Amazon parrot species, they can exceed 150,000 in the macaw. Leukemia must be included in the differential diagnosis of very high leukocyte counts, but cannot be easily ruled out without repeated morphologic examination of peripheral and bone marrow elements.

Leukopenia
Leukopenia must be interpreted in conjunction with knowledge of typical reference ranges for the species examined. Smaller birds tend to have lower normal leukocyte counts. In any species, a total leukocyte count which is less than 3000 should create concern.
The main rule-out for avian leukopenia involves sample artifacts. Whole blood that clots before placement in anticoagulant will yield reduced counts. Lysis of leukocytes before analysis, due to excessive shipping and storage times can result in pseudoleukopenia. Poor quality blood films often display a high percentage of ruptured leukocytes or "smudge cells" which will artifactually indicate a leukopenia when slide assessment is used to quantitate leukocytes.
True leukopenia usually stems from an overwhelming bacterial infection, severe viral disease, or occasionally toxic substances. In bacterial sepsis, a degenerative left shift becomes a heteropenia to only a few lymphocytes seen. Depending on the length of the sepsis, this leukopenia may present with non-regenerative anemia. Another inconsistently seen feature of bacterial leukopenia concerns the presence of intracellular bacteria, which can be visualized in the cytoplasm of heterophils or monocytes. The classic presentations include cat-bite septicemia (Pasteurella) sepsis from an abscess focus (Staphylococcus, Pseudomonas, Salmonella, coliforms), and sepsis secondary to apparent viral immunosuppression. The prognosis for recovery is poor, even with aggressive intravenous therapy.
Viral leukopenia may present similarly, except that intracellular bacteria are uncommon. Very low leukocyte counts can be seen in neonatal and pediatric psittacine circovirus (PBFD) infection, characterized by the presence of only a few lymphocytes seen in a blood film and accompanied by severe non-regenerative anemia. In the rare occasion which a psittacine yields samples before dying of psittacine herpesvirus (Pacheco's Disease), leukopenia may be present. Psittacine polyomavirus infections present variably with leukopenia or leukocytosis. Psittacine viral serositis, a syndrome of unproven etiology, does not typically present with leukopenia. Proventricular dilation syndrome (PDS, macaw wasting disease) is believed to be viral in origin, but a causative agent has not been confirmed. PDS does not usually produce leukopenia.
Normal leukocyte counts can occur in the medically-compromised bird. Chronic, low-grade infections can result in a normalizing of the leukocyte count. Degenerative and neoplastic disorders may have a normal leukocyte count. Sick birds with a non-specific infection may have a normal leukocyte count. The clinician should assess changes in the leukocyte morphology and differential.