Development Of Vaccines For Enteric Septicemia Of Catfish (ESC)     Craig A. Shoemaker and Phillip H. Klesius   Aquatic Animal Health Research Laboratory, United States Department of Agriculture-Agricultural Research Service, PO Box 0952, Auburn, AL 36830     Channel catfish culture has grown since the early 1980's to a $900 million dollar industry.  Edwardsiella ictaluri was first isolated from channel catfish and identified as the causative agent of enteric septicemia of catfish (ESC) in 1976.  This gram-negative pathogen is responsible for annual losses of about $60 million to the catfish industry.  First attempts at vaccination against E. ictaluri occurred in 1983 by injection with adjuvant LPS and formalin-killed cells. Early attempts at immersion vaccination with formalin killed or sonicated E. ictaluri cells were ineffective.  Two bacterins were produced and licensed for vaccination against E. ictaluri with protective effect believed to be antibody mediated.  Recent published results using these products in young fish (12 days post hatch) showed that they failed to provide protection. We demonstrated that bacterin vaccinated fish (antibody positive) and fish passively immunized with anti-E. ictaluri antibodies were not protected against ESC.  Acquired immunity was cell mediated and relied on the killing of E. ictaluri by macrophages.           Development of a successful vaccine against E. ictaluri required that the product be safe, easily administered and stimulated the correct immune response.  Modified live vaccines which stimulate cell mediated immunity appeared to be the solution.  We used rifampicin (an antibiotic) to screen E. ictaluri isolates because it was suggested in the attenuation of other Gram-negative bacteria (i.e., bacteria which grew on rifampicin had an altered LPS).  The modified isolate designated RE-33 was attenuated, had a modified LPS and did not cause disease in susceptible animals.  Reversion to virulence studies in vivo also demonstrated the isolate was safe. Field safety in 2.2 million channel catfish was demonstrated in state veterinarian and USDA-APHIS approved field trials in 1997.  Efficacy of the RE-33 was demonstrated in 3-9 month old channel catfish against a number of E. ictaluri isolates.  In 1998-99, we demonstrated the effectiveness of the modified live RE-33 E. ictaluri vaccine in 12, 14, 16, and 31 day post hatch channel catfish following immersion (2 or 10 min) vaccination.  In ovo vaccination of eyed channel catfish eggs with the modified live E. ictaluri vaccine was also accomplished in 1999-2000. Intervet, Inc marketed the modified live RE-33 E. ictaluri in 2000 under the trade name AQUAVAC-ESC.