Yes, there is a correlation among parasitism and oxidative stress in fish. The reason is that different stressful factors in water bodies such as anthropogenic activity, heavy metal pollution etc, cause an alteration in blood cell formula. What happens is that stress affect fish immunity probably through cortizol route, resulting in lowering the number of white blood cells and making animals prone to infectious diseases or parasites.

Thanks a lot Valvona.

Would you give me a reference, please?

You can also check the SOD (superoxide dismutase)  levels on both infected and non-infected fish.