Development and Characterization of a Novel Live Attenuated Vaccine Against Enteric Septicemia of Catfish

Edwardsiella ictaluri is a Gram-negative intracellular pathogen causing enteric septicemia of channel catfish (ESC). Type six secretion system (T6SS) is a sophisticated nanomachine that delivers effector proteins into eukaryotic host cells as well as other bacteria. In the current work, we in-frame deleted the E. ictaluri evpB gene located in the T6SS operon by allelic exchange. The safety and efficacy of Ei Delta evpB as well as Aquavac-ESC, a commercial vaccine manufactured by Intervet/Merck Animal Health, were evaluated in channel catfish (lctalurus punctatus) fingerlings and fry by immersion exposure. Our results showed that the Ei Delta evpB strain was avirulent and fully protective in catfish fingerlings. The Ei Delta evpB strain was also safe in catfish fry, and immersion vaccination with Ei Delta evpB at doses 10 6 and 10 7 CFU/ml in water resulted in 34.24 and 80.34% survival after wild-type immersion challenge compared to sham-vaccinated fry (1.79% survival). Catfish fry vaccinated with Ei Delta evpB at doses 10(6), 10(7), and 10(8) CFU/ml in water exhibited dose-dependent protection. When compared with Aquavac-ESC, Ei Delta evpB provided significantly higher protection in catfish fingerlings and fry (p < 0.05). Results indicate that the Ei Delta evpB strain is safe and can be used to protect catfish fingerlings and fry against E. ictaluri.