Channel catfish virus disease : detection of latent virus and correlation with serology

Vita.The purpose of this study was to detect channel catfish virus in adult seropositive catfish through explant culture and molecular hybridization and to develop methods for the detection of channel catfish virus specific antibodies in previously exposed channel catfish (Ictalurus punctatus, Raf.). Channel catfish virus(CCV) could not be isolated from the explanted tissues of seropositive catfish by direct culture, co-cultivation with cell lines permissive for the replication of the virus, or through the use of prolonged culture and chemical induction. Serology conducted in conjunction with the explant studies revealed that not all survivors of CCV epizootics were seropositive several years subsequent to an outbreak. It was also found that members of a large population of hatchery reared catfish had high levels of serum neutralizing antibodies to CCV without record of exposure or disease activity. The enzyme labelled immunosorbent assay was tested for its abilty to detect antibodies to channel catfish virus in fish serum. Although several methods were tested for reducing high background levels with the enzyme labelled immunosorbent assay, none were effective. Higher positive values could be obtained in this assay using catfish serum which was positive for anti-CCV activity by serum neutralization than with negative serum, but the high background levels obtained made the test difficult to interpret. Catfish immunoglobulin isolated in conjunction with the development of the enzyme labelled assay was analyzed. This immunoglobulin was found to be a multimeric protein consisting of heavy and light chains with molecular weights of 74,000 daltons and 21,500 daltons respectively. When isolated from CCV positive serum, the purified immunoglobulin preparation formed a line of id entity with the unfractionated serum from which it was prepared in the agar gel diffusion immunodiffusion test using CCV as the antigen. Additionally, the purified immunoglobulin preparation neutralized infectious CCV in vitro. Preliminary experiments indicated that the agar gel diffusion test might be an inexpensive and technically simple alternative to the use of serum neutralization for the detection of anti-CCV antibodies in catfish sera. ..