Genomic diversity and natural recombination of equid gammaherpesvirus 5 isolates.

Genomic analysis of equid gammaherpesvirus 5 (EHV5) reveals significant genetic diversity and evidence of natural recombination events among virus isolates, highlighting recombination as a key driver of EHV5 genomic variation.

Background and Purpose

• Equid gammaherpesvirus 5 (EHV5) is a herpesvirus related to equid gammaherpesvirus 2 (EHV2) and is commonly found in horse populations worldwide.
• Although EHV5 is often detected without causing notable symptoms, it has been linked to respiratory illnesses, including equine multinodular pulmonary fibrosis (EMPF).
• Previous studies noted genetic variation within small regions of the EHV5 glycoprotein B (gB) gene, with multiple viral genotypes sometimes present within the same horse.
• Comprehensive full genome sequence data for EHV5 isolates had been limited before this study.
• The primary goal of the research was to evaluate the overall genomic diversity and identify natural recombination events across full-length EHV5 genomes.

Methods

• Researchers analyzed the genome sequences of six EHV5 strains—one prototype strain and five isolates cultured specifically for this study.
• Four of the isolates were collected from the same individual horse, enabling intra-host diversity assessment.
• Genome sequence sizes were measured, along with variations in terminal repeat regions.
• Comparisons of nucleotide sequence identity and diversity were conducted among the six genomes.
• Gene-level nucleotide diversity was examined to identify variable regions within the genome.
• Rates of nonsynonymous substitutions (Ka values) were calculated to detect genes under higher selective pressure or with high diversity.
• Specialized software tools, specifically RDP4 and SplitsTree, were used to analyze potential recombination events in the viral genomes.

Key Findings

• The genome sizes ranged from approximately 181,929 to 183,428 base pairs, with terminal repeat size variations from 0 to 10 base pairs.
• The nucleotide identity among the six EHV5 genomes varied between 95.5% and 99.1%, with an average nucleotide diversity of about 1%.
• Individual genes showed a wide range of nucleotide diversity, from complete conservation (0%) up to 19% difference.
• Eight genes exhibited particularly high diversity, indicated by nonsynonymous substitution rates exceeding 0.025, suggesting functional or adaptive changes.
• Recombination analysis revealed evidence of past natural recombination events occurring between different EHV5 isolates.
• Regions of the genome serving as recombination hotspots were identified, indicating locations prone to genetic exchange.

Implications and Conclusion

• Recombination serves as an important mechanism driving genetic diversity in EHV5, especially valuable for a virus with a relatively low mutation rate.
• This natural recombination contributes to the genetic heterogeneity observed within and between viral isolates.
• The study enhances understanding of EHV5 genome variability, which is crucial for interpreting virus evolution, epidemiology, and potential impacts on horse health.
• Such insights may aid in developing diagnostic tools, vaccines, or treatments by considering the diverse genomic landscape of EHV5.
• Further research may explore how recombination influences viral pathogenicity and transmission dynamics in equine populations.