Equine Herpesviruses 4 (Equine Rhinopneumonitis Virus) and 1 (Equine Abortion Virus)

Abstract

The horse (Equus caballus) is unusual, in that it is host to three distinct alphaherpes viruses, designated as equine herpesvirus 1 (EHV1; equine abortion virus), EHV4 (equine rhinopneumonitis virus), and EHV3 (equine coital exanthema virus). EHV4 and EHVl are closely related viruses that, prior to 1981, were considered a single virus type or subtypes of a single virus. There was an assumption that this single virus, “EHV1,” caused an acute respiratory tract disease called “rhinopneumonitis,”which established a latent infection and then, as an unpredictable sequel in the case of a pregnant mare, underwent reactivation followed by viremic spread to the fetus, causing abortion. This chapter is concerned primarily with EHV4 and EHV1, with brief consideration of AHV3, highlighting its relationship to EHV1. Earlier, the reviews of “EHV1” may be consulted for a historic perspective and earlier references. However, each of these reviews carries ambiguities related to a belief that only a single virus type “EHV1,” rather than two distinct viruses, was involved. This chapter mainly focuses on the advances in our understanding of the molecular biology, including genetic and antigenic characteristics, of EHV4 and EHVl since 1981. A brief overview of the clinical syndromes and pathogenesis, epidemiology, laboratory diagnosis, vaccines, and immunity of EHV4 and EHVl is also given in this chapter.