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 = 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 were evaluated, including ability to superinfect in the presence of T. equi-Texas (T. equi), the isolate responsible for the reemergence of T. equi in the U S. Experimental infection with T. equi or T. haneyi revealed minimal clinical disease however, T. equi infection led to significantly greater neutropenia. Comparison of time to antibody detection following inoculation revealed significantly greater time to detectable anti-T. haneyi antibody (26.67 days post-inoculation (DPI)) than T. equi (11.67 DPI). Regardless of initial infection with either T. equi or T. haneyi, superinfection was established. Comparative analysis of antibody responses from a splenectomized horse infected with T. haneyi to that of a spleen intact horse infected with T. equi revealed a different antibody binding profile to T. haneyi, T. equi and T. equi merozoite antigen and limited shared antigen/cross-reactive antibody(s). Affinity purified T. equi EMA-1 and EMA-2 from T. equi were shown as targets for horse antibodies against T. haneyi. Data presented here show (1) T. haneyi can superinfect in the presence of T. equi 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 and T. equi includes reactivity to EMA-1 and EMA-2.