Equine herpesviruses 2 and 5 are gamma-herpesviruses

Equine herpesviruses 2 and 5 (EHV-2 and EHV-5) have biological properties and genome structures that support their classification as members of the Betaherpesvirinae. In order to investigate whether this is supported by genetic data, we analysed the sequences of random DNA fragments and identified 25 EHV-2 and 28 EHV-5 genes that encode amino acid sequences with significant homology to proteins from other herpesviruses. Greatest similarity was to proteins specified by the gamma-herpesviruses Epstein-Barr virus (a gamma 1-herpesvirus) and herpesvirus saimiri (a gamma 2-herpesvirus), and the level of similarity was marginally greater to the latter. Also, like other gamma-herpesviruses, the EHV-2 and EHV-5 genomes are deficient in the CG dinucleotide, suggesting that latent genomes are methylated. EHV-2 and EHV-5 are related to each other more closely than they are to other herpesviruses, but are clearly distinct gamma-herpesviruses. The data support the establishment of at least one more subdivision of the gamma-herpesviruses (the gamma 3-herpesviruses).