Equid herpesvirus 2 and equid herpesvirus 5 infections

Equid herpesvirus 2 and equid herpesvirus 5 infections

G P ALLEN AND M J MURRAY

Introduction

The prototype LK strain of equid herpesvirus 2 (EHV-2), the first gammaherpesvirus to be recovered from the horse, was isolated in 1962 from the respiratory mucus of a foal with ‘catarrh and coughing’.56 Since then, a great number of equid gammaherpesvirus isolates have been recovered from horses in all parts of the world.16, 19, 29, 35, 37, 41, 43, 64, 70, 73, 84 Twenty-five years elapsed before a subset of this large virus collection was recognized as sufficiently different to be classified as a second equid gammaherpesvirus type, equid herpesvirus 5 (EHV-5).2, 12 At this writing, isolates of EHV-5 have been recovered from domestic horses only in Australia and New Zealand;2, 12, 14, 24 however, the viral genome has been demonstrated by EHV-5 specific polymerase chain reaction (PCR) in leukocytes of horses from other countries.30 Characterized by extensive intratypic heterogeneity (both genetic and antigenic), a long replicative cycle, slow cell-to-cell spread of viral infectivity, and low extracellular titres of infectious virus, the equid gammaherpesviruses comprise a complex and technically challenging group of herpesviruses for both equine virologists and practitioners. 2, 3, 11-13, 70

A large proportion of the world’s horse population carries gammaherpesviruses as a life-long latent infection of circulating B lymphocytes as well as a fluctuating, persistent and productive infection of epithelial cells of the nasopharynx, conjunctiva and cornea.9, 13, 22, 28, 41, 46, 64, 65, 79, 84 Virus intermittently shed from these mucosal areas of mares is transmitted to foals soon after birth, even in the presence of colostrum-derived antibody.11, 32, 35, 49, 84 Although of low inherent pathogenicity, EHV-2 has been associated with several significant, clinically overt disease conditions in the horse, including syndromes in foals of upper and lower respiratory disease, keratoconjunctivitis, and chronic follicular pharyngitis, as well as a malaise and poor performance syndrome in young performing horses.5, 8, 17, 18, 21, 26, 32, 35, 37, 40, 51, 52, 65, 71, 72, 74, 77, 78

Aetiology

Described originally as ‘slowly cytopathic orphan herpesviruses’ of the domestic horse (Equus caballus) 38, 50 and, for a time, classified as cytomegalovirus-like betaherpesviruses,13, 29, 38, 67, 70, 83 EHV-2 and EHV-5 are now known, from recent nucleotide sequence analysis of their genomic DNAs, to be members of the Rhadinovirus genus of the Gammaherpesvirinae subfamily.3, 75 By amino acid sequence homology, EHV-2 and EHV-5 are genetically colinear with one another and closely related to the gammaherpesviruses of humans [Epstein-Barr herpesvirus (46 per cent homology) and human herpesvirus 8], of nonhuman primates [saimiriine herpesvirus 2 (52 per cent homology)], and, presumably, of other domesticated livestock (e.g., bovine herpesvirus 4; alcelaphine herpesvirus 1 and 2 and ovine herpesvirus 2 of malignant catarrhal fever of cattle).36, 75, 76 EHV-2 and EHV-5 are genetically and antigenically distant from the three alphaherpesviruses (EHV-1, EHV-3, and EHV-4) of the horse.13, 29, 41, 53, 55, 57, 68, 70 No cross-neutralization has been reported between EHV-2 or EHV-5 and EHV-1, EHV-3, or EHV-4.3, 55

The equid gammaherpesviruses have typical herpesvirus morphology and virion architecture.56 The 184 kbp doublestranded DNA genome of EHV-2 has a G+C content of 57,5 per cent and a buoyant density of 1,717 g/cm.3, 53, 5, 83 It is a non-isomerizing genome that comprises a large (149 kbp), central unique sequence flanked at both ends by long (17,5 kbp), direct terminal repeats.14, 76 The unique region of EHV-2 also contains an unrelated pair of internal, short inverted repeats at separate locations.76 By contrast, the EHV-5 genome (52 per cent G + C) is 179 kbp in size and lacks both the terminal and internal sequence repeats.2 A striking feature of the EHV-2 genome is that nearly a third of its DNA sequence, including the large direct terminal repeat, appears not to encode protein.76 EHV-2 and EHV-5 are distinct, but closely related, equid herpesviruses, sharing an overall 60 per cent amino acid sequence identity.3, 36, 6975

The genome of EHV-2 is predicted to encode 77 distinct polypeptides.76 The proteins and glycoproteins of both EHV-2 and EHV-5 have been characterized.1 Many of the EHV-2 and EHV-5 structural proteins co-migrate in SDS-PAGE gels and contain both type-common and type-specific epitopes.1 Significant differences exist between the electrophoretic mobilities (Mr) of EHV-2 and EHV-5 glycoproteins.1 The proteins of the two viruses cross-react strongly in ELISA, Western blot, and radioimmunoprecipitation, but less so in infectivity neutralization assays.1

The two gammaherpesviruses can be distinguised, however, by differences in:

  • the Mr values of their proteins in SDS-PAGE gels;
  • their dna restriction endonuclease cleavage patterns;
  • the titres of homologous versus heterologous antisera for neutralizing the two viruses, and
  • the intensity of homologous versus heterologous nucleic acid probe hybridization to Southern blots of their DNA fragments.1, 2, 12, 14

In cell culture, EHV-2 and EHV-5 exhibit a relatively broad host range and a long replication cycle.35, 40, 43, 53, 56, 70, 83 Because much of the viral infectivity remains avidly...

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