Improved detection and further characterization of white sturgeon iridovirus: Insights into pathogenesis and exclusion from the family Iridoviridae

A comparative analysis of a newly developed conventional PCR and current histological methods for detecting white sturgeon iridovirus (WSIV) among infected juvenile white sturgeon (WS) (Acipenser transmontanus) determined sensitivity estimates as 98% and 64% for the two tests, respectively. Conventional PCR detected WSIV survivors 9 mo after exposure, whereas a real-time Taqman PCR assay further improved viral DNA detection in asymptomatic carriers among groups of captive and wild white sturgeon. The real-time Taqman PCR detected 100 fold lesser concentrations of serially diluted samples of a plasmid containing the target sequence of the WSIV MCP gene than the conventional PCR. The potential for vaccination of WS began with passive immunization trials using serum from WS hyperimmunized with white sturgeon iridovirus (anti-WSIV), normal serum, and serum of PBS injected WS. Similar survivorships were observed between groups when challenged with WSIV, suggesting a single injection of anti-WSIV WS serum is insufficient in protecting juvenile WS from a lethal challenge of WSIV. Immunization with formalin-inactivated WSIV (FI-WSIV), recombinant major capsid protein (rMCP), or PBS was evaluated in juvenile WS. Each vaccine was injected into a group of fish intraperitoneally and then challenged to WSIV 6 wk later. A second group were given a boost injection with FI-WSIV, rMCP, or PBS 10 wk after the initial vaccination, and then challenged to WSIV 4 wk later. Challenge doses did not induce mortality among any vaccinated or vaccinated plus boosted groups. However, evaluations of virus infection, which did occur, did not differ between vaccinates and control groups. Phylogenetic analyses of the major capsid and the DNA-directed RNA polymerase domain 5 proteins suggest WSIV is more similar to mimivirus and phycodnaviruses than to the iridoviruses. Immunocytochemistry assays using polyclonal anti-WSIV rMCP antibodies did not cross react with other sturgeon viruses and iridoviruses from each genera. Analysis of WSIV DNA with restriction enzymes Msp I and Hpa II determined WSIV DNA lacks internal methylation at CpG residues. The phylogenetic, serological, and methylation data provide compelling evidence that WSIV is not an iridovirus but likely represents a unknown viral specie, for which the name white sturgeon epivirus is proposed.