Successful Use of Vincristine in a Horse with Immune-Mediated Thrombocytopenia

Successful Use of Vincristine in a Quarter Horse Gelding with Immune-Mediated Thrombocytopenia
J Equine Vet Sci. 2026 Apr 8:105889. doi: 10.1016/j.jevs.2026.105889. Online ahead of print.
Authors
K E Salewski 1 , M A Talavera 2 , G A Gonzalez 2 , B M Waldridge 2 , J M White 2 , M J Williams 2 , M L Williams 2
Affiliations

1 Department of Pathobiology and Population Medicine, Mississippi State University College of Veterinary Medicine, 240 Wise Center Drive, Starkville, MS 39762, USA. Electronic address: ksalewski729@gmail.com.
2 Department of Pathobiology and Population Medicine, Mississippi State University College of Veterinary Medicine, 240 Wise Center Drive, Starkville, MS 39762, USA.

PMID: 41962605
DOI: 10.1016/j.jevs.2026.105889

Abstract

This case reports the successful use of vincristine in a Quarter Horse with presumed immune-mediated thrombocytopenia. A 15-year-old Quarter Horse gelding presented for a pinch skin graft procedure on the left forelimb over the cannon bone. On day 10 of hospitalization, the horse developed a fever (102.6°F, RI: 99-101°F), and severe petechiae of the mucous membranes. A complete blood count (CBC) (Advia 2120i) revealed a severe thrombocytopenia (16,000 platelets/μL, RI:100,000-400,000/μL). Diagnostic testing was negative for Anaplasma phagocytophilum by PCR, Theileria equi and Babesia caballi by cELISA, and equine infectious anemia (EIA) virus by agar gel immunodiffusion. A sternal bone marrow aspirate was suggestive of megakaryocyte hypoplasia. However, the sample was not of good quality and was suspected not to be a good representation of the marrow. Over a 20-day treatment period, the horse received multiple immunosuppressive medications (dexamethasone, prednisolone, azathioprine) and vincristine in attempt to improve the thrombocytopenia. Following vincristine administration, the platelet count increased substantially and eventually normalized. Vincristine is thought to stimulate thrombopoiesis and increase megakaryocyte fragmentation within the bone marrow. Based on response to vincristine, exclusion of infectious agents, and lack of a consumptive process (e.g., disseminated intravascular coagulation [DIC], chronic hemorrhagic lesion), a presumptive diagnosis of immune-mediated thrombocytopenia (IMTP) was made. To the authors’ knowledge, this is one of only two reported cases in equine medicine describing successful treatment of IMTP with vincristine. This case report offers novel treatment information, previously well documented in domestic small animals.

Keywords: IMTP; Megakaryocyte hypoplasia; Petechiae; Thrombocytopenia; Vincristine.

Copyright © 2026. Published by Elsevier Inc.