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

A 3-month-old male Bernese Mountain dog was presented with a 10-day history of vomiting, vocalisation and abdominal discomfort following vaccination. Physical examination revealed subtle peripheral lymphadenopathy. Haematological testing revealed borderline anaemia, thrombocytopenia and moderate lymphocytosis with a high proportion of reactive or atypical lymphocytes. Cytology of the spleen and lymph nodes showed lymphoid hyperplasia, and PCR for antigen receptor rearrangement confirmed a polyclonal lymphocyte population with no evidence of clonal rearrangements of immunoglobulin or T-cell receptor genes. No treatment was initiated. Follow-up testing demonstrated spontaneous resolution of haematological abnormalities and normalisation of cytology within 30 days. Although causality cannot be established, a transient antigenic stimulation temporally associated with recent vaccination was considered among the possible explanations. This report highlights the importance of recognising benign reactive lymphocytosis that may mimic lymphoproliferative disease in young dogs.