Antioxidant enzyme activity and pathophysiological responses in the freshwater walking catfish, Clarias batrachus Linn under sub-chronic and chronic exposures to the neonicotinoid, Thiamethoxam®

The hydrophilic nature and resultant persistence of neonicotinoids in aquatic systems increase the exposure duration for non-target organisms. The sublethal toxicity of the neonicotinoid Thiamethoxam (R) spanning sub-chronic and chronic durations was investigated in Clarias batrachus, a non-target freshwater fish species. 96 h LC50 value of Thiamethoxam (R) on Clarias batrachus was 138.60 mg L-1. Pre-determined exposure concentrations of Thiamethoxam (R) (6.93 and 13.86 mg L-1) were used and effects were assessed at days 15, 30, and 45 exposure intervals. Biomarkereffects were evaluated using antioxidant enzyme responses (CAT, SOD) neurotransmission (acetylcholinesterase activity), haematological and serum biochemistry changes (including haemoglobin content, total erythrocyte count, and serum albumin total leukocyte count, total scrum protein, serum globulin, triglyceride, cholesterol, high-density lipoprotein, very low-density lipoprotein, low-density lipoprotein, phospholipid, and total serum glucose), histopathological alterations (gill and liver). 'l'hiamethoxam (R)-exposed fish showed a marked reduction in haemoglobin content, total erythrocyte count, and serum albumin levels compared to control fish. Similarly, gill and liver antioxidant enzyme activity (CAT, SOD) and neurotransmission (acetylcholinesterase) also showed altered responses between sub-chronic exposure on day-15 and chronic responses on day-45. Histopathological observations in gill tissue revealed alterations ranging from vacuolation, hypertrophy, disruption of primary lamellar architecture, haemorrhage, the fusion of secondary lamella, and sloughing of outer epithelia. For liver tissue of exposed fish histopathological observations included increased sinusoidal spaces (ISS), necrosis of hepatocytes (NOH), nuclear degeneration (ND), disruption of architecture (DOA), macrophage infiltration of the central vein, vacuolation (V), hypertrophied hepatocytes, and haemorrhages. The gradients of toxic responses across exposure concentrations and depictions of impaired fish health with increasing thiamethoxam (R) exposure duration portend lowered physiological capacity for survival in the wild.

Automated summary

The hydrophilic nature and persistent characteristics of neonicotinoids can lead to prolonged exposure for non-target organisms. In this study, the sublethal toxicity of the neonicotinoid Thiamethoxam (R) was investigated in the non-target freshwater fish species Clarias batrachus. The results showed that exposure to Thiamethoxam (R) affected the antioxidant enzyme responses, neurotransmission, and haematological and serum biochemistry of the fish, as well as causing histopathological alterations in the gill and liver tissues.