Transient marked reactive lymphocytosis in a puppy suspected to be secondary to recent vaccination.

Reactive lymphocytosis can mimic lymphoproliferative disease; monitor for spontaneous resolution.

A 3-month-old male Bernese Mountain dog presented with gastrointestinal and abdominal signs following vaccination. Clinical examination revealed peripheral lymphadenopathy. Hematological analysis demonstrated borderline anemia, thrombocytopenia, and moderate lymphocytosis with increased reactive lymphocytes. Splenic and lymph node cytology confirmed lymphoid hyperplasia. Molecular testing via PCR for antigen receptor rearrangement revealed a polyclonal lymphocyte population, excluding clonal rearrangements characteristic of lymphoproliferative disease. No therapeutic intervention was implemented. The patient experienced spontaneous complete resolution of hematological abnormalities and normalization of cytological findings within 30 days. While definitive causality cannot be established, transient antigenic stimulation secondary to vaccination represents a plausible temporal association. This case emphasizes the clinical importance of distinguishing benign reactive lymphocytosis from pathological lymphoproliferative disorders in young dogs, preventing unnecessary aggressive diagnostic and therapeutic approaches.

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