Effectiveness of Five Common Chemicals Against Edwardsiella tarda Isolated from Ornamental Fish

Abstract

Edwardsiella tarda is a bacterial pathogen that causes edwardsiellosis in fish. The present study aimed to estimate the prevalence of E. tarda in diseased ornamental fish and to evaluate the antibacterial effect of five common aquaculture chemicals against E. tarda. A total of 27 diseased ornamental fish exhibiting red spots, ulcers, exophthalmia, and skin discolouration were collected. E. tarda was isolated only from ornamental pangasius (Pangasianodon hypophthalmus), while goldfish and Koi carp were negative. The prevalence of E. tarda in ornamental pangasius was 35.71%. The bacterial isolates were tested for antimicrobial resistance (AMR) against 13 classes of antimicrobials. Resistance was observed\ only to tetracycline. Tetracycline-resistant and intermediate-susceptible isolates harboured the tetA gene, as confirmed by PCR analysis. Five chemicals were tested for antibacterial activity against E. tarda at three concentrations: recommended dose, double dose, and half dose. The effect of the chemicals was assessed by measuring the reduction in total plate count (TPC). Acetic acid exhibited the strongest antibacterial activity, completely inhibiting E. tarda growth at >1000 ppm. Potassium permanganate (KMnO₄) demonstrated a 5.29 log reduction compared to the 3.96 log reduction with copper sulphate (CuSO₄) at the recommended dose of 2 ppm. However, povidone-iodine and MgSO₄ showed no reduction in TPC at any of the three doses. This study highlights the association between E. tarda and diseased ornamental pangasius and demonstrates the effectiveness of acetic acid, KMnO₄, and CuSO₄ against E. tarda.