Comparative disinfection efficacy in aquaculture: novel methylene bis(thiocyanate) vs. conventional trichloroisocyanuric acid against Aeromonas hydrophila

<i>Aeromonas hydrophila</i> is a major pathogen responsible for significant economic losses in global aquaculture. Inhibitory and bactericidal activities of seven disinfectants were tested against 10 aquatic pathogens, including <i>A. hydrophila</i>, <i>A. veronii</i>, <i>A. salmonicida</i>, <i>A. sobria</i>, <i>Edwardsiella tarda</i>, <i>E. ictaluri</i>, <i>Pseudomonas aeruginosa</i>, and <i>Yersinia ruckeri</i>. Minimal effective concentrations were determined via quantitative suspension tests, while a scale-trauma zebrafish model assessed <i>in vivo</i> protection. Methylene bis(thiocyanate) (MBT) exhibited the strongest antimicrobial activity, with minimum inhibitory concentrations and minimum bactericidal concentrations of 0.1-0.4 mg/L and 0.2-0.8 mg/L, respectively. At 0.01 mg/L, MBT achieved a 3-log pathogen inactivation, equivalent to 0.2 mg/L Trichloroisocyanuric acid (TCCA). <i>In vivo</i>, 0.01 mg/L MBT provided 100% protection in zebrafish after 6 and 12 h exposure, whereas 0.2 mg/L TCCA yielded 95.83 ± 3.61% survival after 1 h. The safe concentrations of MBT and TCCA for zebrafish were 0.0364 and 0.0677 mg/L, respectively. The results showed that both TCCA and MBT effectively controlled <i>A. hydrophila</i> infection; however, MBT demonstrated greater potential for aquaculture applications due to its lower effective concentration (0.01 mg/L) and reduced sensitivity to interference from organic matter. In addition, this study presents a systematic protocol for evaluating the efficacy of disinfectants.