Edwardsiella ictaluri has recently emerged as a significant pathogen responsible for high mortality rates in cultured tilapia. However, the relationship between disease outbreaks and environmental conditions remains poorly understood. This study examines the growth capacity and pathogenicity of E. ictaluri under varying temperature and salinity conditions. Three bacterial isolates obtained from diseased tilapia were used in in vitro growth experiments, followed by challenge tests using an LD70 bacterial dose to assess pathogenicity in tilapia. The results indicate that E. ictaluri exhibits optimal growth at temperatures between 25°C and 30°C, with significantly reduced growth at 15°C, 20°C, and 35°C. Optimal bacterial growth occurred at salinities of 0–5‰, while growth declined sharply at 10‰, 15‰, and 20‰, with no viability detected at salinities of 25‰ or higher. Challenge experiments demonstrated that E. ictaluri induced Edwarsiellosis within a temperature range of 17.5°C to 27.5°C, with the highest mortality rates recorded at 20°C and 25°C. In comparison, dead or dying fish accounted for significantly lower mortality rates at 17.5°C and 27.5°C. Clinical signs were most severe at 25°C and 27.5°C, moderate at 20°C, and mild at 17.5°C. Regarding salinity, the pathogen caused disease across all tested levels, with 100% mortality observed at 15‰ and 20‰, reduced mortality at 0‰ and 5‰, and the lowest mortality rate at 10‰. Lesion severity was highest at salinities of 0–10‰, moderate at 15‰, and mild at 20‰. These findings provide critical insights into the environmental factors influencing E. ictaluri outbreaks, contributing to developing effective strategies to control tilapia disease.
Keywords: Edwardsiella ictaluri, tilapia, temperature, salinity, growth, pathogenicity