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• Iatrogenic inoculation with T. haneyi results in asymptomatic, persistent infection. • Initial infection with T. equiTX is associated with a marked neutropenia. • T. haneyi is undetectable with currently accepted diagnostics. • Superinfection regardless of the initial infecting species can be established. Theileria equi infection, exotic to the United States has reemerged through intravenous (iatrogenic) and tick-borne transmission. Surveillance at the US-Mexico border identified a new species, Theileria haneyi , (T. haneyi EP) (EP = Eagle Pass, Texas) which warranted additional investigation due to inability to detect by PCR targeting of T. equi ema-1 and EMA-1-cELISA validated for T. equi. Infection dynamics of T. haneyi EP were evaluated, including ability to superinfect in the presence of T. equi -Texas (T. equi TX), the isolate responsible for the reemergence of T. equi in the U S. Experimental infection with T. equiTX or T. haneyi EP revealed minimal clinical disease however, T. equiTX infection led to significantly greater neutropenia. Comparison of time to antibody detection following inoculation revealed significantly greater time to detectable anti- T. haneyi EP antibody (26.67 days post-inoculation (DPI)) than T. equi TX (11.67 DPI). Regardless of initial infection with either T. equi TX or T. haneyi EP, superinfection was established. Comparative analysis of antibody responses from a splenectomized horse infected with T. haneyi EP to that of a spleen intact horse infected with T. equi FL revealed a different antibody binding profile to T. haneyi EP, T. equi TX and T. equi FL merozoite antigen and limited shared antigen/cross-reactive antibody(s). Affinity purified T. equi EMA-1 and EMA-2 from T. equi FL were shown as targets for horse antibodies against T. haneyi. Data presented here show (1) T. haneyi EP can superinfect in the presence of T. equi TX infection and co-persists for minimally 25 months, (2) intravenous challenge with T. haneyi is subclinical, and (3) limited cross-reactive antibody between T. haneyi EP and T. equi includes reactivity to EMA-1 and EMA-2. [ABSTRACT FROM AUTHOR]