Development and Evaluation of an Enzyme-Linked Immunosorbent Assay for Catfish Serum Antibody toEdwardsiella ictaluri

An indirect enzyme-linked immunosorbent assay (ELISA) was developed for the detection of serum antibodies in channel catfish Ictalurus punctatus against Edwardsiella ictaluri. The ELISA antigen was prepared by application of an antigen extract from E. ictaluri to an affinity column containing immobilized monoclonal antibody (MAB). The MAB had specific reactivity for the immunodominant epitope of E. ictaluri. No cross-reactivity was detected when the E. ictaluri. Of 349 sera from food fish, 83% showed exposure to E. ictaluri in an area where ESC is wardsiella tarda or Aeromonas hydrophila. Further, adsorption of sera from E. ictaluri-infected catfish with E. ictaluri reduced the ELISA values, whereas adsorption with A. hydrophila reduced the ELISA values only slightly from those of unadsorbed sera. Antibody to E. ictaluri was reliably measured by ELISA in culture-positive fish that were experimentally or naturally infected with E. ictaluri. Tests by ELISA of field sera from 926 channel catfish in areas with or without endemic enteric septicemia of catfish (ESC) caused by E. ictaluri confirmed differences in incidence of E. ictaluri infection. In areas where E. ictaluri is endemic, 53% of 160 fingerlings tested had antibodies, whereas in areas where E. ictaluri does not naturally occur, 9% (N = 305) had antibodies to E. ictaluri. Of 349 sera from food fish, 83% showed exposure to E. ictaluri in an area where ESC is endemic. About 25% of 102 brood fish tested had antibody to E. ictaluri whether the fish were from areas where the bacterium occurred naturally or not. The ELISA uses a stable antigen preparation and is a reliable tool for diagnosis and monitoring of E. ictaluri infection.