Infection dynamics of Theileria equi and Theileria haneyi, a newly discovered apicomplexan of the horse

Authors: SEARS, KELLY - Washington State University; Kappmeyer, Lowell; WISE, LAUREN - Saint Georges University; SIVA, MARTA - Washington State University; Ueti, Massaro; White, Stephen; REIF, KATHRYN - Kansas State University; KNOWLES, DONALD - Washington State University

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2019
Publication Date: 7/10/2019
Citation: Sears, K.P., Kappmeyer, L.S., Wise, L.N., Siva, M., Ueti, M.W., White, S.N., Reif, K.E., Knowles, D.P. 2019. Infection dynamics of Theileria equi and Theileria haneyi, a newly discovered apicomplexan of the horse. Veterinary Parasitology. 271:68-75. https://doi.org/10.1016/j.vetpar.2019.06.009.
DOI: https://doi.org/10.1016/j.vetpar.2019.06.009

Interpretive Summary: Tick-transmitted equine protozoal parasite Theileria equi is the subject of surveillance at the U.S. border, and is the parasite responsible for an outbreak of the the disease recently in Texas. Because of enhanced surveillance, a new but related protozoan species was identified at Eagle Pass, Texas, and designated Theileria haneyi. Horses infected with Theileria haneyi are not diagnosable with molecular or serological tools developed for Theileria equi. This study investigate the ability of the two species to co-infect horses, and their comparable disease progression after experimental infection. Both species produced limited disease manifestations in healthy horses, though Theileria equi infection led to lower numbers of the white blood cell called neutrophils. Another observation is that anti-Theileria haneyi antibodies took longer to develop in the blood of infected horses compared to anti-Theileria equi antibodies. Both Theileria haneyi and Theleria equi could infect horses already first infected with the other species. There was found to be limited cross reactivity of antibodies from horses infected with one species to protein antigens of the other species, though there is evidence for Theileria haneyi antibodies binding to Theileria equi parasite membrane proteins designated EMA-1 and EMA-2. The emergence of T. haneyi raises concerns of its global prevalence and eventual impact on movement of horses as it is undetectable clinically, or with T. equi tools.

Technical Abstract: 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, which warranted additional investigation due to the inability to detect by PCR targeting of the T. equi ema-1 gene and EMA-1-cELISA validated for T. equi. Infection dynamics of T. haneyi were evaluated, including the ability to superinfect in the presence of T. equi (Texas isolate), 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 (Florida isolate) revealed a different antibody binding profile to T. haneyi, T. equi and T. equi merozoite antigens and limited shared antigen/cross-reactive antibody(s). Affinity purified merozoite surface antigens EMA-1 and EMA-2 from T. equi (Florida isolate) were shown as targets for horse antibodies resulting from T. haneyi infection. 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.