Comparison of channel catfish virus and thymidine kinase negative recombinant channel catfish virus latency and recrudescence

The ability of channel catfish virus (CCV) and recombinant thymidine kinase negative CCV (CCVlacZ) to establish latency and to recrudesce from latent infection was evaluated. Channel catfish (Ictalurus punctatus ) juveniles exposed by immersion with a dose of virus that caused less than 20% mortality were sampled at 0, 30, 60, and 90 days post-challenge (DPC) with CCV or CCVlacZ at 3 × 105 PFU per 10-liter tank. Using conditions shown by others to induce viral recrudescence, infected fish were subsampled at 2, 4, 7, and 10 days post-treatment. The immunological profile of CCV and CCVlacZ exposed fish was evaluated to determine which antigenic determinants were more prevalent during an intermittent viral expression. In addition, the pattern of viral gene expression was also investigated during the latent phase of infection and following an attempted reactivation using dexamethasone (DEX) treatment (0.55 mg/kg/body weight). Recrudescence was monitored by co-cultivation assays using peripheral blood leukocytes (PBL) and charnel catfish ovary cells. Latent CCV-DNA was confirmed in 50% of PBL from CCV-infected fish by the presence of 136 by viral DNA product using specific CCV primers in a standard polymerase chain reaction (PCR). Despite the presence of CCV-DNA in PBL (75%), anal fin (75%), gill (66.6%), spleen, anterior kidney, posterior kidney, and caudal fin (50%) of latently infected fish four days post-DEX treatment, no infectious virus was observed after three consecutive blind-passages using PBL or any tissue sampled. However, a uniform increase in CCV-DNA was detected in caudal fin samples collected one week post-DEX treatment. Western immunoblot data showed that CCV specific antibodies were directed against high molecular weight proteins ranging from approximately 90 to greater than 200 kiloDaltons (kDa). Fish exposed to CCV produced higher specific primary response at 30 DPC and a stronger secondary immune response after DEX treatment than those exposed to CCVlacZ. Protective humoral immunity assessed by neutralizing index (NI) values ranged from 1.12 to 3.95 and 1.28 to 2.37 at 60 DPC with CCV and CCVlacZ, respectively. Regardless of the treatment, seropositve fish exposed to CCV had higher neutralizing activity than these exposed to CCVlacZ. Limited mRNA expression by ORF3 and ORF12 genes was evident in PBL and caudal fin samples from latently infected fish exposed to either CCV or CCVlacZ at 90 DPC. Following the attempted reactivation, ORF3 transcripts were more frequently detected in PBL than in caudal fin samples. Early and late gene expression was rarely detected. (Abstract shortened by UMI.)...