GENERAL INTRODUCTION: ARTERIVIRIDAE

Introduction

Equine viral arteritis (EVA) is a reproductive and respiratory disease of equids (horses, donkeys, mules, and zebras) caused by equine arteritis virus (EAV).⁶⁵ EVA is an acute contagious disease principally characterized by fever, dependent oedema, respiratory signs, neonatal mortality and abortion in pregnant mares.¹⁸¹ Most importantly, EAV establishes persistent infection of variable duration in the reproductive tract in the majority of stallions infected with the virus (carrier state).^{181, 187, 188}

The older veterinary literature dating back to the late nineteenth century carries several reports of a disease clinically indistinguishable from EVA, which was referred to by various terms, such as infectious cellulitis-pinkeye, fièvre typhoide du cheval and Pferdestaupe.^{26, 49, 156, 159} It was not until 1953, however, following an outbreak of respiratory illness and abortion on a Standardbred breeding farm near Bucyrus, Ohio, USA, that EVA was defined as a separate equine viral disease and the aetiological agent (i.e., EAV) was isolated for the first time.⁶⁵ Little international significance was attached to EVA for many years until 1984, when it occurred on a widespread scale in Kentucky, involving an estimated 41 Thoroughbred breeding farms.⁹⁹ Since then, periodic outbreaks of EVA have been reported in the United States and a number of countries around the world. The major concern over the potential risk of the virus spreading to susceptible Thoroughbred populations in the USA and abroad resulted in the imposition of stringent measures governing the international movement of horses between most countries.¹⁸² Many of these restrictions continue to impact on the international trade of horses and equine semen. This disease may result in significant economic loss to the equine breeding industry due to the occurrence of abortion in pregnant mares, neonatal mortality, and the establishment of the carrier state in stallions.

Of the changing trends that have taken place in the horse industry worldwide over the past 40 to 50 years, two have contributed significantly to the  global dissemination  of EAV.¹⁷⁶ First, the continued growth in the volume of international movement of horses for performance and breeding purposes has dramatically enhanced the risk of the spread of the virus. The second factor has been acceptance of the use of frozen semen for breeding by most major horse breed registries worldwide. As a result, the international trade in equine semen has expanded and outbreaks of EVA have been linked directly to the use of imported infective  semen from particular carrier stallions.^{20, 175}

EVA can have economic consequences for both the breeding and performance sectors of the horse industry.¹⁸⁰ A range of financial losses can result directly from outbreaks of the disease on breeding farms. These include losses due to abortion and/or disease and death in young foals; decreased commercial value of stallions that become carriers of the virus; reduced demand to breed to carrier stallions because of the associated inconvenience and added expense; denied export markets for carrier stallions or EAV-infective semen; and reduced export markets for fillies, mares, colts, geldings and non-carrier stallions which are EAV seropositive. An outbreak of EVA at a racecourse, equestrian event or sale can have considerable impact. Such occurrences can result in direct financial losses through disruption of training schedules, reduced entries and even fixture cancellations. At the international level, EVA continues to have a significant effect on trade in horses and semen, with all the major horse breeding countries, except the USA, denying entry to EAV carrier stallions and EAV-infective semen.

During the past several decades, there have been significant advances in characterization of the molecular biology of EAV including identification of neutralization and virulence determinants of the virus, the molecular epidemiology of EAV infection, and host-pathogen interactions including the genetic basis of persistent infection in stallions and the cellular and molecular basis of EAV persistent infection in the stallion reproductive tract. The authors of this chapter have authored or co-authored a significant number of recent research and review articles on EAV and EVA^{2, 5, 17-19, 21, 36-41, 161, 162} and, thus, some of the published material is reused in this book chapter, and the relevant references are cited in the text. The readership is encouraged to read the original publications and citations therein for more information.

Aetiology

Equine arteritis virus is an enveloped, positive-sense, single-stranded RNA (ssRNA) virus that belongs to the family Arteriviridae (genus...