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Research Article: Acute infection with the U.S. isolate of Theileria orientalis genotype Ikeda is associated with decreased hematocrit and erythrocyte counts in experimentally infected cattle

Date Published: 2026-04-10

Abstract:
The emergence of the tick-borne Ikeda genotype of Theileria orientalis poses a significant threat to the U.S. cattle industry. Although information has been gathered on parasite distribution, data regarding the pathogenicity of the U.S. isolate of Ikeda remain to be elucidated. In this study, we evaluated the effect of the U.S. Ikeda isolate on cattle experimentally infected with sporozoites, the parasite stage naturally transmitted by ticks. Six spleen-intact Holstein calves, aged 6–12?months, were subcutaneously inoculated with 1-mL stabilate containing 3.8?×?10? of T. orientalis Ikeda sporozoites. Parasite load, temperature, hematocrit, blood cell counts, and serology were monitored. For this study, we defined acute infection as occurring up to 8?weeks post-inoculation (WPI) and chronic infection as occurring from weeks 10–20. Parasite load was quantified by qPCR targeting the major piroplasm surface protein (MPSP) gene. Hematological parameters were evaluated using a veterinary hematology analyzer. IgM, total IgG, IgG1, and IgG2 profiles were determined by an MPSP-based indirect ELISA. Parasite load in peripheral blood varied markedly among animals and peaked at 8 WPI (average of 27,078 parasites/?l). A substantial number of parasites were also detected in peripheral blood during the chronic phase (average of 931 parasites/?l). The results demonstrated a significant inverse correlation during the acute phase between parasite load and both hematocrit ( p =?0.004; r =??0.8703) and total erythrocyte count ( p =?0.001; r =??0.9205). However, no significant correlation was found between parasite load and changes in hematocrit or erythrocyte count during the chronic phase. Lymphocyte, neutrophil, and monocyte counts remained within physiological ranges throughout infection. Anti-MPSP IgM was detected at 2 WPI and peaked at 4 WPI, while total IgG appeared at 2 WPI, peaked at 6 WPI, and persisted during the chronic phase. Anti-MPSP IgG1 and IgG2 were detectable from 4 WPI onward, with no significant differences between subclasses. Acute infection with the U.S. isolate of T. orientalis Ikeda was correlated with reduced hematocrit and erythrocyte count in peripheral blood. Infected cattle exhibited robust anti-MPSP antibody responses during the acute phase, which maintained throughout chronic infection.

Introduction:
The emergence of the tick-borne Ikeda genotype of Theileria orientalis poses a significant threat to the U.S. cattle industry. Although information has been gathered on parasite distribution, data regarding the pathogenicity of the U.S. isolate of Ikeda remain to be elucidated. In this study, we evaluated the effect of the U.S. Ikeda isolate on cattle experimentally infected with sporozoites, the parasite stage naturally transmitted by ticks.

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