Clinical Progression of Theileria haneyi in Splenectomized Horses Reveals Decreased Virulence Compared to Theileria equi

Abstract

The global importance of the hemoparasite Theileria haneyi to equine health was recently shown by its resistance to imidocarb dipropionate (ID) and its interference with T. equi clearance by ID in some co-infected horses. Genetic characterization of T. haneyi revealed marked genomic reduction compared to T. equi, and initial experiments demonstrated reduced clinical severity in spleen-intact horses. Furthermore, in early experiments, splenectomized horses survived T. haneyi infection and progressed to an asymptomatic carrier state, in stark contrast to the high fatality rate of T. equi in splenectomized horses. Thus, we hypothesized that T. haneyi is less virulent than T. equi. To objectively assess virulence, clinical data from nine splenectomized, T. haneyi-infected horses were evaluated and compared to published data on T. equi-infected, splenectomized horses. Seven of eight splenectomized, T. haneyi-infected horses survived. Further, in six horses co-infected with T. equi and T. haneyi, only horses cleared of T. equi by ID survived splenectomy and became asymptomatic carriers. The reduced virulence of T. haneyi in splenectomized horses instructs why T. haneyi was, until recently, undetected. This naturally occurring comparative reduction in virulence in a natural host provides a foundation for defining virulence mechanisms of theileriosis and Apicomplexa in general.

Discovery of a Novel Species Infecting Goats: Morphological and Molecular Characterization of Babesia aktasi n. sp.

“…Experimental infections have been shown in splenectomized and dexamethasone immunodepression sheep and goats using the Babesia species. Because splenectomy reduces the host’s ability to control parasitemia, it allows for the large-scale expansion of previously undetected parasite populations to detectable and, in some cases, clinically significant levels [ 37 , 38 ]. High fever (42 °C), weakness, anorexia, anemia, and hemoglobinuria were observed in sheep with spleen-intact and splenectomized sheep during an experimental infection with B. ovis [ 39 ].…”

Diagnostic performance of a rapid immunochromatographic test for the simultaneous detection of antibodies to Theileria equi and Babesia caballi in horses and donkeys

“…A third species, Theileria haneyi, recently discovered in horses at the United States-Mexico border, further complicates the surveillance of equine piroplasmosis [6]. Although T.haneyi is less virulent than T.equi in horses, this third blood parasite must be considered when developing and applying methods for serodiagnosing piroplasmosis in horses [7,8].…”

Tulathromycin and Diclazuril Lack Efficacy against Theileria haneyi, but Tulathromycin Is Not Associated with Adverse Clinical Effects in Six Treated Adult Horses

“…Equine theileriosis causes varying degrees of anemia, leading to lethargy, exercise intolerance, and potentially death in severe cases [ 2 , 3 , 6 , 7 , 8 ]. Studies indicate that T. haneyi infections lead to milder clinical disease than T. equi , such that most splenectomized horses infected with T. haneyi survive, whereas T. equi infection of splenectomized horses is almost invariably fatal [ 9 , 10 ]. T. equi infection precludes the transport, sale, import and export of horses into T. equi -free countries such as the U.S., leading to significant economic costs to the equine industry [ 3 , 10 ].…”

“…Studies indicate that T. haneyi infections lead to milder clinical disease than T. equi , such that most splenectomized horses infected with T. haneyi survive, whereas T. equi infection of splenectomized horses is almost invariably fatal [ 9 , 10 ]. T. equi infection precludes the transport, sale, import and export of horses into T. equi -free countries such as the U.S., leading to significant economic costs to the equine industry [ 3 , 10 ]. While the U.S. is currently free of T. equi , T. haneyi has been detected in horses detained at the southern U.S. border, and studies are currently underway to determine whether T. haneyi is present within U.S. horse populations.…”