OXIDATIVE STRESS BIOMARKERS, HAEMATOLOGICAL PARAMETERS AND HISTOPATHOLOGICAL CHANGES IN THE AFRICAN CATFISH, Clarias gariepinus EXPOSED TO AN ORGANOPHOSPHATE PESTICIDE, FENTHION
In the present study, specimens of Clarias gariepinus having mean weight and standard length of 197.38 ± 2.34 g and 27.36 ± 0.23 cm respectively were exposed to an organophosphate pesticide, fenthion. The median lethal concentration value (LC50) of fenthion and the effects of sub-lethal concentrations of the pesticide on the oxidative stress biomarkers, haematological parameters and histopathological changes in the blood and tissues of Clarias gariepinus were determined. The 24, 48, 72 and 96 hour LC50 values (with 95% confidence limits) estimated by probit analysis were found to be 61.987, 50.318, 45.056 and 39.968 mg/L, respectively. Significant differences (p < 0.05) at different durations of LC50 were observed. A concentration-dependent increase and time-dependent decrease was observed in mortality rate. Behavioural responses such as hyperactivity, erratic swimming, skin discoloration, jerky movement, followed by exhaustion and death were observed during the acute toxicity test. Fish were exposed to 2.0, 4.0 and 8.0 mg/L corresponding to 1/20th, 1/10th and 1/5th respectively, of 96 hour LC50 of fenthion for 21 days and 7 days recovery to evaluate the haematological parameters, enzyme activities and histopathological changes. There was no significant difference between the condition factors of the treated fish and the control (p > 0.05) but values of hepatosomatic index of the treated groups were reduced compared to the control though the reduction was not significant (p > 0.05) except on day 21 of the exposure. The liver and gill tissues were sampled on day 1, 7, 21 and during the 7 days recovery for analysis of the different parameters. Induction of oxidative stress in the liver and gill tissues of Clarias gariepinus were evidenced by increased lipid peroxidation (LPO) levels (p < 0.05), such that incrreased LPO was higher in fish exposed to the highest concentration of fenthion. The values of glutathione (GSH) activities were not significant (p > 0.05) except on day 1 of the exposure where there was significant increase (p < 0.05) in GSH activities in liver of fish exposed to the different concentrations of fenthion compared to the gills. Slight decrease in the activity of glutathione reductase (GR) was observed in the liver and gill tissues of the treated fish compared to the control but the reduction was not significant (p > 0.05). Increase in GR activities was observed in post-exposure (7 days recovery). There was a general reduction in glutathione peroxidase (GPx) activities but not significant (p > 0.05) except on days 1 and 7 where liver tissues exposed to the highest concentration of fenthion differed significantly compared to the gill tissues (p < 0.05). Increase in catalase (CAT) activities was both time and concentration dependent in all the tissues. CAT activity increased significantly in the liver of fish exposed to the highest concentration compared to gill tissues (p < 0.05) except on day 7. Superoxide dismutase (SOD) activities in the liver decreased from day 1 of the exposure to day 7 (though not significant). SOD activity slightly increased from day 14 of the exposure to post exposure (though not significant) except on day 21 where the liver tissues significantly increased compared to the gill tissues (p < 0.05). The exposure of fish to fenthion revealed that the pesticide had adverse effects on various blood parameters. Red blood cell (RBC) counts of the treated fish were lower compared to the control (p < 0.05). Packed cell volume (PCV) of the treated fish were lower than that of the control but the difference was not significant (p > 0.05) except in fish exposed to 8.0 mg/L on day 1 of the exposure (p < 0.05). Haemoglobin concentration (Hb) in fish decreased after the exposure though not significant (p > 0.05) but fish exposed to the highest concentration had significant increase in Hb at days 1 and 21 of the exposure. Significant increase in white blood cell (WBC) counts was also observed (p < 0.05). No significant difference (p > 0.05) in erythrocyte indices, mean corpuscular volume (MCV), mean corpuscular haemoglobin concentration (MCHC) and mean corpuscular haemoglobin (MCH) was observed in all the concentrations during the experiment. Changes in white blood cell differentials (neutrophils, lymphocytes, monocytes, eosinophils and basophils) revealed that no significant difference (p > 0.05) was observed in the parameters except in monocytes. The Histopathological effects of fenthion on the liver and gill tissues were also studied. The liver and gill tissues of the control exhibited quite a normal architecture. The main histopathological changes caused by fenthion in the liver of fish exposed to different concentrations were disarray of hepatic chords, vacuolation, sinusoid enlargement and necrosis. Liver suffered from toxicity as hypertrophy of hepatocytes and necrosis. These hepatic alterations were more evident in fish exposed to the highest concentration of fenthion. Alterations in gill structure exposed to the highest concentration were oedema, lifting of lamellar epithelia, destruction of gill architecture and lamellar fusion. Gill disorder and fusion of the secondary lamellar were pronounced in all treatments. The present study revealed that sub-lethal concentrations of fenthion resulted in the alterations of the studied parameters. Histopathogical changes observed during the period of the experiment as well as the recovery shows that fenthion-induced oxidative stress is irreversible.