Oxidative stress plays a dual role in parasitic infections: while host-derived ROS such as superoxide and hydrogen peroxide (H2O2) exert antiparasitic effects, H. contortus counteracts this stress through antioxidant defenses, including selenium-dependent glutathione peroxidase (GPx), catalase (CAT), and superoxide dismutase (SOD). Here, CAT is linked to parasitic infectious disease.