Neutropenia in dogs receiving vincristine for treatment of presumptive immune‐mediated thrombocytopenia

BACKGROUND: Neutropenia is an adverse effect of vincristine when used in multidrug chemotherapy protocols. OBJECTIVE: To determine the incidence of neutropenia, identify potential risk factors for neutropenia, and determine the effect of neutropenia on outcome, in dogs receiving vincristine for treatment of immune‐mediated thrombocytopenia (ITP). ANIMALS: One hundred twenty‐seven client‐owned dogs presumptively diagnosed with ITP. METHODS: In this retrospective cohort study, medical records were reviewed to identify dogs presumptively diagnosed with ITP, and treated with vincristine, over a 15‐year period. Logistic regression was used to identify risk factors for the development of neutropenia in dogs receiving vincristine. Time to platelet count ≥40 000 platelets/μL, survival, and duration of hospitalization were compared between neutropenic and non‐neutropenic dogs. RESULTS: Vincristine was administered to 127 dogs with presumptive ITP; 19 became neutropenic. Administration of cyclosporine was significantly (P < .001) associated with the development of neutropenia (odds ratio: 12.97, 95% confidence interval: 4.17, 40.35). There was no difference in median time to ≥40 000 platelets/μL between neutropenic dogs (4 days; range, 1‐14 days) and non‐neutropenic dogs (3 days; range, 0‐48 days). Percentage survival to discharge was 95% in both groups, but median duration of hospitalization was significantly longer in neutropenic dogs (6 days; range, 3‐22 days) compared to non‐neutropenic dogs (4 days; range, 2‐15 days). CONCLUSIONS AND CLINICAL IMPORTANCE: Cyclosporine administration was associated with the development of neutropenia in dogs receiving vincristine, which might be related to effects on metabolism of vincristine. Neutrophil counts should be monitored in dogs receiving vincristine treatment for ITP, particularly if administered in conjunction with cyclosporine.