Chlorpyrifos mediated oxidative damage and histopathological alterations in freshwater fish Oncorhynchus mykiss in Northern Pakistan

The intensive applications of chlorpyrifos (CPF) in agricultural production have resulted in lethal effects on the environment, particularly on aquatic fauna. Thus, present study was aimed to assess the haematological responses, antioxidant enzymes (superoxide dismutase [SOD] and catalase [CAT]) activity, acetylcholinesterase enzyme (AChE) inhibition and histopathological alterations in liver and gill tissues of cold‐water fish rainbow trout (Oncorhynchus mykiss) in their native habitat, when exposed to CPF. In total, 240 fingerlings were subjected to various CPF concentrations (0, 2, 4 and 6 μg/L) for acute and sub‐lethal toxicity tests. Results revealed that increasing CPF concentration and exposure time have resulted in reduced erythrocyte count, haemoglobin concentration and haematocrit level; however, leucocyte count was increased. Additionally, decrease in AChE activity was also observed with increasing concentration. CPF concentrations at lowest exposure duration induced slight increase of SOD and CAT activity while the high exposure durations (14 and 21 days) exhausted the activity of antioxidant enzymes. Structural damages produced by CPF in liver tissues were hyperaemia, dilated central veins, sinusoids and degenerative changes, while the lesions observed in gill tissue were curling of secondary lamella, cell degeneration, cell necrosis, narrowing of water channels, hyperplasia, oedematous changes, fusion of secondary lamella and sloughing.