Re-emergence of the apicomplexan Theileria equi in the United States: elimination of persistent infection and transmission risk.
Authors: Ueti Massaro W, Mealey Robert H, Kappmeyer Lowell S, White Stephen N, Kumpula-McWhirter Nancy, Pelzel Angela M, Grause Juanita F, Bunn Thomas O, Schwartz Andy, Traub-Dargatz Josie L, Hendrickson Amy, Espy Benjamin, Guthrie Alan J, Fowler W Kent, Knowles Donald P
Journal: PloS one
Summary
Theileria equi, a tick-borne protozoan parasite, establishes lifelong persistent infections in horses with minimal clinical signs, creating a significant biosecurity challenge for international trade and disease control programmes. This study evaluated whether imidocarb dipropionate could clear T. equi infection and eliminate transmission risk in 25 naturally infected horses, using nested PCR to detect parasitaemia and splenectomised recipient horses to confirm infectivity. Treatment successfully reduced parasite levels from a mean of 10⁴·⁹ organisms/ml to undetectable levels in 24 of 25 horses, and crucially, blood from these treated animals no longer transmitted infection to naive recipients—demonstrating genuine clearance rather than suppression alone. One horse required a second treatment course for complete elimination, highlighting individual variation in treatment response, though antibody persistence in most treated horses underscores the distinction between serological conversion and actual pathogen eradication. For equine professionals involved in international horse movement, competition, breeding programmes, and export certification, these findings provide evidence-based support for using imidocarb dipropionate as a practical tool to remove transmission risk and facilitate the safe relocation of infected horses from endemic to non-endemic regions.
Read the full abstract on PubMed
Practical Takeaways
- •Imidocarb dipropionate offers a practical chemotherapeutic option for eliminating T. equi from persistently infected horses and removing transmission risk, facilitating safe international movement and trade.
- •Nested PCR is the appropriate diagnostic method to confirm treatment success rather than relying on antibody serology, which remains positive even after pathogen elimination.
- •A small percentage of horses may require retreatment; if PCR remains positive after initial treatment, a second course should be considered rather than assuming treatment failure.
Key Findings
- •Imidocarb dipropionate treatment eliminated T. equi to undetectable levels by nested PCR in 24 of 25 naturally infected horses (96% success rate).
- •Blood from treated horses that tested PCR-negative failed to transmit infection to naive splenectomized horses, confirming elimination of transmission risk.
- •T. equi-specific antibodies persisted in most treated horses despite pathogen elimination, indicating serological markers do not confirm active infection post-treatment.
- •One treatment-resistant horse required a second round of imidocarb to achieve elimination, while the remaining 24 achieved single-treatment success.