Tafenoquine succinate inhibits the growth of the equine piroplasmosis hemoparasites Theileria equi and Babesia caballi

Background Equine piroplasmosis (EP) is a tick-borne disease of equids caused by intraerythrocytic apicomplexan parasites Theileria equi , Babesia caballi , and the recently identified Theileria haneyi . Acute cases can be severe, with anemia, jaundice, abortion, or sudden death. Survivors remain lifelong carriers, serving as reservoirs for tick-borne and iatrogenic transmission. No vaccines are currently available, and control strategies rely heavily on accurate diagnostics and chemotherapeutic intervention. Imidocarb dipropionate (ID) is the current standard of care for both acute treatment and radical cure. However, growing concerns regarding ID-resistant parasite strains and its associated toxicity have highlighted the urgent need for novel, safer, and more effective antiparasitic agents. Here, we assessed the in vitro efficacy of tafenoquine succinate (TFQ), a synthetic 8-aminoquinoline with broad antiparasitic activity, against T. equi and B. caballi as a potential treatment for equine piroplasmosis. Methods The effect of TFQ on T. equi and B. caballi was evaluated in vitro in parasite cultures. The percentage of parasitized erythrocytes was measured by flow cytometry, and the effect of TFQ on parasite growth was compared to that of ID. TFQ toxicity on horse peripheral blood mononuclear cells (PBMC) was assessed via a colorimetric metabolic assay. Results TFQ reduced T. equi parasitemia dose-dependently, matching ID efficacy at 72 hours. For B. caballi , TFQ had no effect at 5–10 µM but inhibited growth at 15 µM, similar results to ID. TFQ exhibited approximately threefold greater potency against T. equi [IC₅₀: 5.90 µM (95% CI: 4.99–5.96); IC₉₉: 60.74 µM (95% CI: 37.41–113.3)] compared to B. caballi [IC₅₀: 14.5 µM (95% CI: 13.81–15.23); IC₉₉: 20.44 µM (95% CI: 17.77–28.84)]. The narrower confidence intervals for T. equi suggest a more consistent antiparasitic response across replicates. Cytotoxicity assays showed no toxic effects on equine PBMCs at 2.5–5 µM (p > 0.05), while concentrations ≥ 10 µM indicated potential toxicity. These findings suggest TFQ selectively targets parasites over host cells, supporting its therapeutic potential. Conclusions TFQ significantly inhibited T. equi and B. caballi growth at doses tolerated by equine PBMCs, supporting its potential as an alternative treatment for EP and warranting further in vivo study.