Immunization with bacterial antigens: edwardsiellosis

Enteric septicaemia of catfish (ESC), caused by Edwardsiella ictaluri, is the most serious disease affecting commercial catfish culture in the United States. ESC is generally an acute septicaemia that develops very quickly, especially in the temperature range of 22-28 degrees C, with a more chronic disease presentation outside this range. The ability of E. ictaluri to avoid the host's immune system and proliferate into a systemic infection is impressive. Catfish kidney tissue cultured positive for E. ictaluri as soon as 15 minutes following gastric lavage and signs of disease are observed microscopically within two days of immersion challenge, with reported mortalities as early as five days following immersion challenge. Analysis of E. ictaluri antigens by several investigators using SDS-PAGE and colorimetric western blotting with immune catfish has identified as many as 15 immunogenic bands. Analysis using two-dimensional SDS-PAGE and chemiluminescent western blotting identified 14 bands and 25 spots as consistently immunogenic. The strongest immunodominant antigens were reported as 34-37 KD and 60 KD, depending on the study. Lipopolysaccharide is the only purified component of E. ictaluri tested for fish vaccination, and results indicated that very poor protection was induced unless Freund's Complete Adjuvant was used. Because E. ictaluri strains are serologically homogeneous, most studies on vaccination have emphasized killed whole cell preparations and have delivered equivocal results. Although antibodies are produced to a variety of preparations, a positive antibody response does not correlate with protection unless very high titres are achieved. Efficacy of killed products has been demonstrated in field trials, and an orally delivered product has been licensed. However, protection probably relies on booster exposure of the host to E. ictaluri during non-permissive temperatures. As a facultative intracellular pathogen, further studies on vaccination of catfish against E. ictaluri should target products and delivery methods that favour induction of cell mediated immunity.