Investigating IcHV-1 Variants in Channel, Blue and Channel x Blue Hybrid Catfish

 

1,2Arun Venugopalan, 1Larry A. Hanson, 2,3David J. Wise, 4Andy D. Perkins, 5Thomas B. Waltzek, 6Geoffrey C. Waldbieser and 1,2Matt J. Griffin. 

1College of Veterinary Medicine, Mississippi State University; 2Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University; 3Mississippi Agriculture and Forestry Experiment Station, Mississippi State University; 4College of Engineering, Mississippi State University; 5Emerging Pathogens Institute, University of Florida; 6Warmwater Aquaculture Research Unit, USDA-ARS

ABSTRACT

Channel Catfish Virus Disease (CCVD) is the principal viral disease of US farm-raised catfish.  The causative agent is Ictalurid herpesvirus 1 (IcHV-1).  It primarily infects juvenile catfish (< 1 yr old; <15 cm) during the summer months, typically in the first few months post-stocking.  Environmental stressors strongly influence disease outbreaks and transmission can occur both vertically and horizontally. Mortality rates may approach 100% in catastrophic outbreaks. Previous research has identified three distinct genetic group variants based on RFLP analysis, namely IcHV-1A, IcHV-1B and the recently described Blue catfish alloherpesvirus (BcAHV).  Herein, the differential susceptibility of channel, hybrid and blue catfish against three genetic variants of IcHV-1 was investigated. Channel and hybrid catfish were exposed to three, discrete representative isolates of IcHV-1A and IcHV-1B at dosages equating the median lethal dose of the ATCC type strain (IcHV-1 Auburn). Differences in mortality between channel and hybrid catfish or between RFLP group were insignificant (p>0.05).  Mortality for channel and hybrid catfish exposed to IcHV-1A was 30.6% (±7.3%) and 32.8% (±9.4%), respectively.  Comparably, exposure of channel and hybrid catfish to IcHV-1B yielded mortality of 36.7% (±7.0%) and 46.1% (10.35%).  After 28 days, survivors were re-exposed to IcHV-1A and IcHV-1B representative isolates. Previous exposure to IcHV-1 resulted in significantly improved survival compared to naïve controls (p<0.05).  Relative percent survival (RPS) ranged from 53.8-100% for channels and 82.8-100% for hybrids, suggesting a cross-protective immune response develops in IcHV-1 survivors. Similarly, channel, hybrid and blue catfish were exposed to BcAHV, resulting in 51.6% mortality in blue catfish.  No mortality was observed in either channel or hybrid catfish.  Twenty-eight days post challenge, channel and hybrids surviving BcAHV exposure were exposed to IcHV-1A, resulting in 100% RPS for both groups.  In addition to these infectivity trials, catfish fry were collected from commercial catfish hatcheries in Mississippi to estimate prevalence of IcHV-1 RFLP groups in US farm-raised catfish.  Validation of qPCR assays to differentiate between IcHV-1 RFLP groups is ongoing and will be used to assess IcHV-1 prevalence in hatchery samples.  Further, in collaboration with researchers at the University of Florida and the USDA-ARS, the genomes of representative IcHV-1 field isolates were sequenced to probe inter- and intra-species genomic diversity. Comparative genome analysis between BcAHV and other alloherpes virus revealed BcAHV possesses distinct genome characteristics indicating a unique viral “species.” Lastly, a command line based, customizable genome data analysis pipeline was developed with Snakemake and Python workflows to analyze IcHV-1 genome data. Infectivity trials, hatchery surveys and IcHV-1 comparative genomics will lead to better understanding of the molecular drivers of IcHV-1 transmission dynamics, immunogenicity, pathogenicity and latency.  Furthermore, data indicates BcAHV has potential as a vaccine candidate against IcHV-1 in channel and hybrid catfish.