The malignant ovine theileriosis is caused by Theileria lestoquardi, which is highly pathogenic in sheep. Theileriosis involves different organs in ruminants. Little is known about role of proinflammatory cytokines in the pathogenesis of T. lestoquardi infection, the aim of the study is to measure concentration changes of proinflammatory cytokines and immunoglobulin G -IgG- during an ovine experimental theileriosis and correlate it with clinical and hematological parameters. During an experimental study, seven healthy Baluchi sheep -4 females and 3 males- about 6-8 months were infected with T. lestoquardi by feeding of infected unfed ticks on the sheep ears. The infected sheep were clinically examined during the study and blood samples were collected on days 0, 2, 5, 7, 10, 12, 14, 17 and 21. The hematological parameters were analyzed by an automatic veterinary hematology cell counter and the inflammatory cytokines; interleukin-6 -IL-6-, tumor necrosis factor-α -TNF-α-, interferon-γ -IFN-γ- and IgG were measured by ELISA. All infected sheep showed a fever above 40°C on days 3-4 post infection -PI- and reached maximum temperature at day 7 and remained high until day 21. The parasitaemia of T. lestoquardi infection increased from 0.01% -day 7 PI- to 3.3% -day 21 PI-. The Mean white blood cell -WBC-, red blood cell -RBC-, lymphocyte, neutrophil and platelet values slightly increased on day 2 PI and decreased until days 17 and 21 PI. The percentage parasitaemia and fever had a negative correlate with the numbers of WBCs, RBCs, lymphocytes, neutrophils and platelets. The serum concentration of IL-6, TNF-α and IFN-γ cytokines increased and peaked on day 12 and thereafter decreased decreased to levels lower than 0. Out of all tested cytokines, the concentration of IL-6 was significantly higher, as early as day 2 PI. No significant changes were observed for the IgG levels during the course of disease. A significant and strong correlation was observed between IL-6, TNF-α and IFN-γ values and a moderate correlation between IL-6 and the numbers of lymphocytes in the present study. A high correlation was determined between the percentage parasitaemia and hematological parameters in T. lestoquardi infected sheep. In addition, preliminary results indicate that the measurement of the serum concentrations of IL-6 in combination with haematological parameters could be considered a good marker to estimate the pathogenicity of T. lestoquardi strain.

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