Deletion of thymidine kinase gene attenuates channel catfish herpesvirus while maintaining infectivity

A recombinant thymidine kinase (TK) gene deletion mutant of channel catfish herpesvirus (CCV) was constructed and compared to the parent virus for replicative ability in cell culture, for the ability to infect, replicate, and cause lethal disease in the channel catfish host, and for the ability to induce protective immunity in channel catfish fingerlings. There was no difference between parent CCV and the TK-negative mutant (CCVTK-) in viral production in cell culture. However, in immersion challenge trials, 100-fold more CCVTK- than parent CCV was required to kill similar numbers of channel catfish fingerlings. The attenuation was confirmed as a TK-associated characteristic in marker rescue; the revertant regained the pathogenic characteristic of the parent virus. When catfish were immersion challenged with equal amounts of CCVTK- or CCV, the levels of virus isolated from the posterior kidney were similar through the peak production period which occurred 4 days postinfection. Subsequently, CCVTK- levels decreased dramatically when compared to CCV levels. Exposure to CCVTK- induced protective immunity against challenge with a lethal dose of wild-type CCV. The degree of protection provided by CCVTK- was related to the dose of CCVTK- given. This study demonstrates that the genetically distant CCV retains recombination properties and TK-associated attenuation properties of herpesviruses of higher vertebrates. Additionally, the unique characteristic of CCV containing two copies of the TK gene did not prevent the use of this gene as a selectable site of homologous recombination.