Clinical progression of Theileria haneyi in splenectomized horses reveals decreased virulence compared to Theileria equi

Authors: SEARS, KELLY - Washington State University; KNOWLES, DONALD - Washington State University; Fry, Lindsay

Submitted to: Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2022
Publication Date: 2/16/2022
Publication URL: https://handle.nal.usda.gov/10113/7684737
Citation: Sears, K.P., Knowles, D.P., Fry, L.M. 2022. Clinical progression of Theileria haneyi in splenectomized horses reveals decreased virulence compared to Theileria equi. Pathogens. 11(2). Article 254. https://doi.org/10.3390/pathogens11020254.
DOI: https://doi.org/10.3390/pathogens11020254

Interpretive Summary: Theileria haneyi is a recently discovered, tick-transmitted apicomplexan blood parasite of horses and one of the three causes of a syndrome known as equine piroplasmosis, which is characterized by variable levels of anemia, weakness, and exercise intolerance in affected horses. Equine piroplasmosis can be fatal. Due to its resistance to imidocarb diproprionate (ID), and its interference with T. equi clearance by ID in co-infected horses, this organism presents significant regulatory concerns for the U.S. equine industry. Although closely related to Theileria equi, another cause of equine piroplasmosis, comparison of the genomes of these parasites revealed that T. haneyi has a significantly smaller genome than T. equi. Initial data from horses infected with T. haneyi suggested that this parasite may cause less severe disease than T. equi. To confirm these observations, we performed a controlled study using experimental infection of spleen-intact and splenectomized horses. While splenectomized horses infected with T. equi always develop fatal disease, eight of nine splenectomized, T. haneyi-infected horses survived. Further, in six horses co-infected with T. equi and T. haneyi, only horses cleared of T. equi infection by ID survived subsequent splenectomy and developed asymptomatic, persistent infection. The naturally reduced disease severity of T. haneyi compared to T. equi provides a foundation for defining virulence mechanisms of theileriosis in the horse and of apicomplexan parasites in general.

Technical Abstract: Theileria haneyi is a recently discovered apicomplexan hemoparasite of equids. This parasite, and the closely related organism, T. equi, cause equine theileriosis in horses throughout tropical and subtropical regions of the world. Due to its resistance to imidocarb diproprionate (ID), and its interference with T. equi clearance by ID in co-infected horses, this organism presents significant regulatory concerns for the U.S. equine industry. Genomic comparison of T. haneyi to T. equi revealed significant genomic reduction in the former, and, while T. equi causes variable degrees of hemolytic anemia in spleen-intact horses, T. haneyi causes mild, often undetectable clinical signs. Strikingly, while T. equi is invariably fatal in splenectomized horses, splenectomized horses in initial studies of T. haneyi did not develop severe disease. Survival of infected, splenectomized horses led to the hypothesis that T. haneyi is less virulent than T. equi, and a controlled study was conducted to test this hypothesis. Eight of nine splenectomized, T. haneyi-infected horses survived. Further, in six horses co-infected with T. equi and T. haneyi, only horses cleared of T. equi infection by ID survived subsequent splenectomy and developed asymptomatic, persistent infection. This naturally occurring reduction of virulence in a natural host provides a foundation for defining virulence mechanisms of theileriosis in the horse and apicomplexan parasites in general.