Tehran University of Medical Sciences Iranian Journal of Parasitology 1735-7020 21 01 2026 05 19 76 84 EN Vahid Dokhaei 1MSc student of parasitology, Faculty of Veterinary Medicine, Semnan University, Semnan, Iran Mahmood Ahmadi-hamedani Department of Clinical Science; Faculty of Veterinary Medicine, Semnan University; Semnan, Iran. Mohammad Reza SALIMI BEJESTANI 3Department of Pathobiology, Faculty of Veterinary Medicine, Semnan University, Semnan, Iran 2025 05 19 Background: Theileria are apicomplexan parasites that infect both domestic and wild animals. Methods: This study represents the first investigation into platelet indices (PI) and Hemoglobin (Hb), Albumin, Lymphocyte, and Platelet (HALP score) in sheep infected with Theileria. Blood samples were collected from 100 sheep. Microscopy and PCR were used to diagnose Theileria infection, and nested PCR was used for species identification. Hematocrit (Hct), Hb, RBC, lymphocyte counts, platelet counts (PLT), plateletcrit (PCT), mean platelet volume (MPV), platelet distribution width (PDW), and serum albumin were measured. The HALP score was calculated by multiplying hemoglobin in albumin by lymphocyte count and dividing by PLT. Results: Microscopic analysis revealed 21 out of 100 samples were Theileria-infected, while PCR identified 27 species. Results showed T. ovis in 52%, T. lestoquardi in 26%, T. annulata in 11%, and a co-infection of T. lestoquardi and T. ovis in 11% of the samples. A significant difference was observed in the PLT and PCT in sheep infected with T. lestoquardi compared to other species of Theileria. A significant difference was found in the PDW in sheep infected with T. ovis and T. lestoquardi than in those infected with T. annulata. Statistical differences in the HALP score were noted for T. lestoquardi (P = 0.03), T. annulata (P = 0.01), and co-infections of T. ovis and T. lestoquardi (P = 0.05) versus T. ovis. Conclusion: These findings suggest that PI and HALP scores can improve diagnostic processes, and future studies should use them alongside other methods for diagnosing Theileria species. https://ijpa.tums.ac.ir/index.php/ijpa/article/view/4611 https://ijpa.tums.ac.ir/index.php/ijpa/article/download/4611/1481