Detrimental effects of metronidazole on selected innate immunological indicators in common carp (Cyprinus carpio L.).

The widely used antibiotic metronidazole (MTZ) was investigated for its toxic effects on the innate immunity in common carp (Cyprinus carpio L.). The fish were exposed to MTZ at nominal concentrations of 0.1, 0.5, and 2.5 mg L(-1) in water for 30 days, followed by a 5-days of cleanout period, after which certain innate immunity parameters were measured. The results showed that under the tested concentrations, MTZ-exposed fish exhibited decline in several humoral and cellular parameters, including complement activity, lysozyme activity, bactericidal activity, total serum protein levels, total WBC count, and the respiratory burst activity of kidney leukocytes. Except for total serum proteins, all of these parameters showed a significant difference in the 2.5 mg L(-1) MTZ group compared to control group (p < 0.05). The results clearly support the contention that MTZ suppresses the innate immunity of common carp.