Genetic variability of equid γ-herpesviruses detected in thoroughbred mares and their foals in Poland

Abstract

Background: Equid herpesvirus 2 (EHV-2) and 5 (EHV-5) are two γ- herpesviruses that are commonly detected in horses worldwide. A considerable sequence variability has been demonstrated among glycoproteinB sequences of EHV-2, and to a lesser extent for EHV-5.Objectives: To characterise the genomic diversity of the glycoprotein B gene of EHV-2 and EHV-5 in the nasal secretions of a cohort of Thoroughbred mares and their foals.Study design: Cross-sectional study.Methods: Viral DNA from selected EHV-2 and/or EHV-5 positive swabs was used as a template in conventional PCR assays targeting a1222 bp (EHV-2) and 1339 bp (EHV-5) fragment of the gB gene. Amplified products were subjected to electrophoresis, purified and cloned into pCR™4-TOPO vectors and transformed into Escherichia coli competent cells. At least five clones from each successful amplification were sequenced. Phylogenetic trees were constructed using the maximum likelihood method with 1000 bootstrap replicates in MEGA7. Sequence identity among sequences analysed in the study was calculated using the identity matrix in BioEdit software.Results: Long PCR products were amplified from 24 EHV-2 and 29 EHV-5 PCR positive swabs tested, from which 82 and 94 clones were obtained, respectively. Sequence alignments of partial gB genefrom 77 EHV-2 clones analysed showed 65.4 to 100% identity at the nucleotide level and 58.9 to 100% at the amino acid level. The genetic variability of EHV-2 sequences from mares and their foals was at thesame nucleotide level (65.3%–100%). The degree of identity between sequences of 94 EHV-5 clones was higher and ranged from 89.6 to 100% and 86.7 to 100% at the nucleotide and amino acid level, respectively. Based on the phylogenetic tree, Polish EHV-2 sequences clustered within two main branches. Most of them (n = 64) clustered with international sequences (including Australian reference strain EHV-2.86/67) within the main branch, defined as group 1. The 94 EHV-5 sequences clustered within at least three main lineages.Main limitations: A limited number of EHV-2/EHV-5 positive swabs from the same mare-foal pairs sampled over several months.Conclusions: EHV-2 sequences from mares and their foals showed similar, high genetic variability. Weaning does not seem to have an impact on the genetic variability of equid gammaherpesviruses among Thoroughbreds.