Effects of exposure to a therapeutic level of methylene blue on antioxidant capacity, haemato-immunological responses and resistance of goldfish, Carassius auratus to Aeromonas hydrophila

摘要

Methylene blue (MB) can have inhibitory effects against protozoan and fungi in fish. In this investigation, fish were first exposed to different MB concentrations (0 and 2 mg L-1) for 21 days. Then, the fish were challenged with Aeromonas hydrophila for 7 days. Some haemato-immunological and biochemical parameters, oxidative stress biomarkers in blood, growth performance and fish resistance to A. hydrophila were examined once a week. The results showed that erythrocyte, haemoglobin, leucocytes, haematocrit, mean corpuscular volume., mean corpuscular haemoglobin concentration and lymphocyte percentage significantly reduced in the fish after exposure to 2 mg L-1 MB on the 14(th) and 21(st) days and challenged period on 28(th) day. Although the superoxide dismutase, catalase, and glutathione peroxidase, lysozyme, phagocytic activities and a quantitative nitroblue tetrazolium, total protein and cellular total antioxidant levels in the serum decreased significantly, the percentage of neutrophils, aspartate aminotransferase, alanine aminotransferase and lactate dehydrogenase activities increased significantly in fish exposed to 2 mg L-1 MB (p < 0.05). There was a significant difference in the growth performance and the percentage of monocytes exposed to 2 mg L-1 of MB compared to the reference group. The mortality rate was declined in the treated fish with 2 mg L-1 of BM after challenge with A. hydrophila compared with the control group. Overall, the present results indicate some negative impacts of MB application in goldfish even at the therapeutic level. From these, the immunosuppressive effects of MB may be potentially disruptive for a health condition.