Clonal T Cell Proliferation Induced by Acute Anaplasmosis in a Dog.

A 4-year-old spayed female Golden Retriever was examined for hindlimb lameness, lethargy, poor appetite, and pyrexia 3 weeks after the removal of numerous ticks. Complete blood count revealed moderate thrombocytopenia and mild lymphopenia. A SNAP 4Dx test was negative for Borrelia burgdorferi, Ehrlichia spp., Anaplasma spp., and Dirofilaria immitis. Abdominal ultrasound identified moderate mesenteric lymphadenopathy and regionally hyperechoic mesentery. Joint fluid cytology raised concern for large cell lymphoma. Flow cytometry identified 37% aberrant large CD4+ T cells, suggesting lymphoma. PCR for antigen receptor rearrangement (PARR) testing on the joint fluid confirmed clonal expansion of T cells, further supporting T-cell lymphoma. Peripheral blood PCR was positive for Anaplasma phagocytophilum. The dog's clinical signs and laboratory abnormalities resolved after a 30-day treatment with doxycycline. After treatment, joint fluid analysis, including cytology and flow cytometry, was normal. This case highlights the importance of considering A. phagocytophilum as a differential diagnosis in dogs with a clonal expansion of CD4+ T cells, emphasizing the comprehensive diagnostic approach.