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.