Are reactive or atypical lymphocytes present?

Figure 5. Atypical lymphocytes in a dog (modified Wright's stain; 100X). Figure 6. Mild neutrophil toxicity in the form of cytoplasmic basophilia in a cat (modified Wright's stain; 100X). Figure 7. Moderate neutrophil toxicity in the form of Döhle bodies (arrow) in a cat (modified Wright's stain; 100X). Figure 8. Marked neutrophil toxicity in a dog. Note the cellular giantism and basophilic, foamy, vacuolated cytoplasm (modified Wright's stain; 100X).

Atypical lymphocytes (Figure 5) are abnormal lymphocytes characterized by increased size with indented or cleft nuclei and large azurophilic cytoplasmic granules. These cells are not disease-specific and may be present with infectious and neoplastic conditions. Atypical lymphocytes require additional investigation by a clinical pathologist. Bone marrow examination and detailed cytologic and possible histologic evaluation of peripheral and internal lymphoid tissues may also be recommended.

Toxic neutrophils are a result of an accelerated rate of neutrophil production in response to inflammatory signals received by the bone marrow. Toxic neutrophil changes include retention of cytoplasmic features of immaturity and include foamy basophilic cytoplasm (Figure 6) and small basophilic precipitates known as Döhle bodies (Figure 7). Döhle bodies are often present in low numbers in normal cats, so, by themselves, they are a relatively equivocal finding; but Döhle bodies indicate serious toxicity in dogs.⁵ Other toxic changes include bizarre nuclear shapes and cellular giantism (Figure 8).

Systemic toxemia is often associated with bacterial endotoxins, but noninfectious causes also occur. Infectious diseases commonly accompanied by severe neutrophil toxicity include feline pyothorax, pyometra, and severe canine prostatitis. Noninfectious causes associated with toxemia include immune-mediated hemolytic anemia, acute pancreatitis, tissue necrosis, zinc and lead toxicosis, and cytotoxic drug therapy.⁶

Conclusion

Evaluating the five different WBC types is critical when you develop a differential diagnosis list. Increases or decreases in neutrophils, eosinophils, basophils, lymphocytes, and monocytes should alert you to investigate certain conditions. For example, neutrophilic left shifts, persistent eosinophilia, and monocytosis are indicators of inflammation: Left shifts (increased numbers of immature, or band, neutrophils in circulation) indicate increased turnover and tissue use of neutrophils. Persistent peripheral eosinophilia indicates a systemic allergic or hypersensitivity reaction. And monocytosis is seen in peripheral blood when a demand for phagocytosis is present.

Fred L. Metzger Jr., DVM, DABVP (canine and feline practice)
Metzger Animal Hospital
1044 Benner Pike
State College, PA 16801

Alan Rebar, DVM, PhD, DACVP
Department of Veterinary Pathobiology
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907

ACKNOWLEDGMENT

The authors gratefully acknowledge the technical assistance and images provided by Dennis DeNicola, DVM, PhD, DACVP.

REFERENCES

1. Rebar, A.; Metzger, F.: The Veterinary CE Advisor: Interpreting Hemograms in Cats and Dogs. Vet. Med. (suppl.) 96 (12):1-12; 2001.

2. Duncan, J. et al.: Erythrocytes, leukocytes, hemostasis. Veterinary Laboratory Medicine, 3rd Ed. Iowa State University Press, Ames, 1994; pp 75-93.

3. Willard, M. et al.: Leukocyte disorders. Textbook of Small Animal Clinical Diagnosis by Laboratory Methods, 3rd Ed. W.B. Saunders, Philadelphia, Pa., 1999; pp 53-79.

4. Rebar, A.H. et al.: Laboratory methods in hematology. A Guide to Hematology in Dogs and Cats. Teton New Media, Jackson, Wyo., 2002; pp 10-28.

5. Willard, M. et al.: The complete blood count and bone marrow examination. Textbook of Small Animal Clinical Diagnosis by Laboratory Methods, 3rd Ed. W.B. Saunders, Philadelphia, Pa., 1999; pp 11-30.

6. Duncan, J. et al.: Erythrocytes, leukocytes, hemostasis. Veterinary Laboratory Medicine, 3rd Ed. Iowa State University Press, Ames, 1994; pp 75-93.