Differential Diagnoses of Lymphocytosis in Veterinary Medicine

Following are differential diagnoses for patients presented with lymphocytosis.

• Age-related cause (eg, dogs and cats <6 months of age often have mild lymphocytosis due to vaccination or exposure to novel antigens)

• Antigenic stimulation

  • Immune-mediated disease (rare; eg, immune-mediated hemolytic anemia in cats) 

  • Infection (most commonly, Ehrlichia canis; rarely, protozoal [eg, Leishmania infantum], Spirocerca lupi, FIV)

• Endocrine disease

  • Hyperthyroidism (cats; usually mild; can be seen prior to diagnosis [possibly epinephrine-related] or secondary to methimazole treatment)

  • Hypoadrenocorticism (primarily dogs; lack of a stress leukogram in a sick patient can indicate disease)

• Lymphoid neoplasia

  • Acute lymphoblastic leukemia

  • Chronic lymphocytic leukemia (± small cell lymphoma)

• Nonlymphoid neoplasia (eg, thymoma)

• Physiologic (eg, epinephrine-induced) response (primarily cats)

Editor’s note: This article was originally published in September 2020 as “Lymphocytosis.”