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Title: Cross-protection elicited in channel catfish (Ictalurus punctatus Rafinesque) immunized with a low dose of virulent Edwardsiella ictaluri strains

Author
item Panangala, Victor
item Shoemaker, Craig
item Klesius, Phillip
item MITRA, AMITAVA - AUBURN UNIVERSITY
item Russo, Riccardo

Submitted to: Aquaculture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2008
Publication Date: 5/1/2009
Citation: Panangala, V.S., Shoemaker, C.A., Klesius, P.H., Mitra, A., Russo, R. 2009. Cross-protection elicited in channel catfish (Ictalurus punctatus Rafinesque) immunized with a low dose of virulent Edwardsiella ictaluri strains. Aquaculture Research. 40:915-926.

Interpretive Summary: Enteric septicemia in channel catfish (ESC) is the most important bacterial disease in warm water aquaculture in the southeastern USA where catfish are farmed on an intensive commercial scale. Cross- protection of fingerling channel catfish with six different Edwardsiella ictaluri strains showed that significant (P is equal to or less than 0.001) protection is conferred by immersion immunization with low-doses of the bacterium irrespective of exposure to the strain that is used for immunization (homologous) or another strain (heterologous) of the same genospecies. Our studies further substantiate that E. ictaluri is a clonal bacterial species and that protection against ESC disease could be readily achieved by immunization with a single strain of E. ictaluri that possess all the antigenic determinants that are relatively conserved among most, if not all presently known E. ictaluri strains. These findings are important from the perspective of developing vaccines for protection of channel catfish against ESC. Prophylactic vaccines are the most efficacious and economical means by which large stocks of fish could be protected against disease and consequently make the aquaculture industry lucrative and sustainable.

Technical Abstract: Cross-protection of fingerling channel catfish (Ictalurus punctatus Rafinesque) by immersion immunization and challenge exposure to six E. ictaluri strains (AL-93-58, AL-93-75, S-94-1051, NJ #3, EILO and ATCC-33202) originating from out breaks of disease in disparate geographic locations was examined in four trials. The relative percent survival among the immunized, then challenged, fish ranged from 27.7% to 100%. With the exception of type strain ATCC-33202, there was no significant (P> 0.05) difference in the protection conferred by immunization with a given E. ictaluri strain and challenge with either homologous or heterologous field strain. However, when compared with the placebo (BHI)-treated controls challenged with any one of the five field strains, there was significant (P is equal to or less than 0.001) protection among the immunized groups in all 4 trials. The antibody titers of the pooled serum collected on day 22 (at termination of the experiments) from surviving fish and examined by ELISA ranged from 1: 40 to 1:320, but no differences were apparent among the different vaccinated groups. The protein profiles of the six E. ictaluri strains examined by SDS-PAGE showed a relatively homogeneous pattern. The immuno-blots probed with pooled serum from immunized and challenged fish showed a broad banding pattern within the higher molecular mass region similar to the LPS-reaction patterns observed with E. ictaluri in other studies. Based upon presently known immune responses documented in the literature on channel catfish, a possible scenario in the host immune response to E. ictaluri is suggested. Since our present studies further corroborate that E. ictaluri is a clonal bacterial species with no apparent antigenic differences, it is possible that immunization with any particular E. ictaluri field strain could confer protection against any other strain.