Razi University

  Theileriosis is an important tick-borne disease of small ruminants that can cause considerable clinical and economic impacts in sheep and goat populations. Among Theileria species, Theileria ovis is a common infectious agent of small ruminants, and many infections may remain subclinical. Given the epidemiological importance of this parasite and the role of ticks in its transmission, the present study aimed to determine the prevalence of Theileria ovis infection and to investigate its association with selected host characteristics, clinical findings, geographical location, and tick species in Kermanshah Province, western Iran. In this cross-sectional study, 400 tick-infested sheep and goats were examined from eight districts of Kermanshah Province, including Kermanshah, Sarpol-e Zahab, Javanroud, Sonqor, Kangavar, Gilan-e Gharb, Islamabad-e Gharb, and Salas-e Babajani, from May to September 2025. Blood samples were examined by polymerase chain reaction (PCR) for the detection of Theileria DNA, followed by molecular identification of the parasite species. Data on host species, age, sex, abortion history, and clinical parameters, including heart rate, respiratory rate, rectal temperature, and mucous membrane color, were recorded. Ticks collected from the animals were morphologically identified, and associations between infection status and the investigated variables were evaluated. Of the 400 animals examined, 343 (85.8%) were positive for Theileria infection. All PCR-positive samples in the present dataset were identified as Theileria ovis. No significant association was observed between infection status and host species, age, or sex. In contrast, heart rate was significantly associated with infection status, and this association remained significant in the multivariable analysis. Rectal temperature and sampling location were also associated with infection status in the final multivariable model. A total of 800 ticks were identified, with the genus Rhipicephalus being the most frequently represented. Although the high prevalence of Theileria ovis coincided with the presence of diverse tick species, the findings alone were insufficient to establish any particular tick species as a competent vector of the parasite. Overall, the high prevalence of Theileria ovis among tick-infested small ruminants highlights the epidemiological importance of this parasite in Kermanshah Province. The observed associations between infection status and selected clinical and geographical variables further indicate heterogeneity in the distribution of infection. However, given the cross-sectional design of the study and the lack of molecular testing of individual ticks, longitudinal, seasonal, and transmission studies are warranted to identify competent vectors and clarify the factors influencing Theileria ovis transmission. Keywords: Theileria ovis, theileriosis, sheep, goats, ticks, PCR, Kermanshah