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The aim of this study was to determine the concentrations of oxidative stress parameters and DNA damage in horses infected by Theileria equi. Initial screening of 110 horses with duplex PCR enabled the selection of 30 infected horses with T. equi and 30 free of infection (control). Specimens from the 60 horses were further analysed by determining the following oxidative stress parameters: extent of haemolysis (EH), plasma free haemoglobin (PHb), catalase (CAT), Cu,Zn superoxide dismutase (SOD1), paraoxonase (PON1), nitrite (NO 2 - ), total nitrate and nitrite (NOx), malondialdehyde (MDA) and free thiol groups (-SH). In addition, relative distribution of lactate dehydrogenase (LDH 1 -LDH 5 ) activity and the DNA-damaging effects of T. equi infection were evaluated. Compared to control horses, horses infected with T. equi had significantly higher SOD1 activities (P <0.05) and PHb (P <0.01), NO 2 - (P <0.001), NOx (P <0.05) and MDA concentrations (P <0.001), and significantly lower EH (P <0.001), CAT (P <0.01) and PON1 (P <0.001) activities, and thiol group concentrations (P <0.05). The comet assay demonstrated significantly increased DNA damage in T. equi infected cells compared to non-infected cells (P <0.001). Infected horses had significantly increased LDH 5 isoenzyme activities (P <0.05). There was higher production of ROS/RNS in T. equi-infected horses, which resulted in changes in osmotic fragility, damage to lipids, proteins and DNA, haemolysis and hepatocellular damage. Oxidative stress in horses naturally infected with T. equi could contribute to the pathogenesis of the infection., (Copyright © 2016 Elsevier Ltd. All rights reserved.)