Equine infectious anaemia

Equine infectious anaemia

R F COOK AND C J ISSEL
Equine infectious anaemia

Introduction

Equine infectious anaemia" (EIA) is a disease of members of the Equidae caused by infection with a lentivirus. Although the majority of infections appear to be clinically inapparent, the EIA virus (EIAV) can be found in the blood of infected equine hosts for life.4, 7, 17, 33, 39 The persistent nature of infections with EIAV is correlated with an inherently high rate of genetic variability in this macrophage tropic lentivirus.22, 24, 32, 38, 47 In part, the clinical response of infected equids is dependent on undefined host susceptibility factors and on the genetic makeup of the infecting virus strain. Acute clinical disease is characterized by pyrexia, lassitude, and thrombocytopenia. The most frequently diagnosed form of the disease, the chronic form, is characterized by recurring febrile episodes concomitant with thrombocytopenia, anaemia, weight loss, weakness and, sometimes, dependent oedema.4, 7, 17, 33, 39

Infection of equids with EIAV has been documented in most areas of the world. The disease has been recognized in Europe since 184325 and in the USA, where it is known as swamp fever, since 1896. At present over a million horses (of an estimated six million) are tested annually in the USA, resulting in the discovery of approximately 1 000 new cases. In South Africa the disease was described in horses and donkeys by De Kock in 1923,12 based only on transmission studies by blood, symptomatology and pathology, but all subsequent attempts to demonstrate antibodies or to isolate the virus failed. Whether the early diagnoses were incorrect or whether the virus has since disappeared is uncertain, but the infection is not known to be endemic in domesticated or free-living equids2 in sub-Saharan Africa. Exhaustive serological surveys, however, have not been performed. It is interesting to note that EIAV was the first lentivirus for which a serological test was proven to be an accurate indicator of infection.17

Aetiology

The EIAV shares many morphologic, antigenic and genetic features with visna/maedi and human immunodeficiency lentiviruses (HIV).4, 7, 17, 33, 39 In fact, these features of EIAV were used to help define the causative agent of acquired immunodeficiency syndrome (AIDS), as serum from EIAV infected horses cross-reacted and recognized the major core protein of HIV.31

Equine infectious anaemia" virus is a macrophage tropic lentivirus that has the simplest genetic organization of any lentivirus described to date.7, 17, 33 The proviral genome of EIAV (Figure 62.1) serves as a template for synthesis of m-RNA species that code for a variety of regulatory, structural and enzymatic functions (Table 62.1). The three major proteins recognized by equids infected with EIAV are the major core protein (p26, a gag gene product), and the surface unit and transmembrane proteins (gp90 and gp45, respectively, env gene products). Infected hosts, however, mount immune responses to all structural and nonstructural proteins of EIAV.

In common with all retroviruses, EIAV is prone to mutation, mainly because the replicative enzyme reverse transcriptase has no proofreading capacity.1 As at least one error is introduced into the viral genome during each replication cycle, the population of viruses present in an infected horse is composed of a large number of related but variant individuals. These are referred to as a quasispecies or ‘viral swarm’. 14 The existence of such a population permits EIAV to respond rapidly to any environmental selection pressure, including that exerted by the immune system. The importance of variation in the viral life cycle is demonstrated by the fact that each febrile episode experienced by an EIAV-infected animal is caused by a different antigenic variant.21, 22, 24, 32, 37, 47 Although variation occurs in all gene regions, some of which are termed hypervariable,24, 47 there are enough conserved group-specific determinants found in the three major antigens of EIAV (p26, gp90 and gp45) to make the production of effective diagnostic reagents possible.6, 35

Epidemiology

The EIAV is restricted to members of the Equidae and cases of infection have been observed in horses, ponies, mules and donkeys. From the limited studies that have been conducted, infections with EIAV do not appear to be endemic in southern Africa. Transmission of EIAV occurs generally through transfer of blood between equine hosts. During acute disease episodes, EIAV has been found in many secretions and/or excretions but contact transmission rarely occurs. Explosive outbreaks of EIA have been traced to iatrogenic transmission, although this can be avoided by adopting sanitary practices designed to prevent transfer of blood-borne pathogens.4, 7, 17, 33, 39 In nature, haematophagous insects, especially tabanids (horse flies and deer flies), mediate transmission of EIAV by mechanically transferring blood between equids in close proximity. 11, 13, 15, 17, 20 The chance of transmission is related to the amount of EIAV in the blood, which is highest during febrile episodes.15, 17 The distance separating infected and susceptible equids is an important factor in transmission. 13 In most cases, a distance of 180 metres is considered adequate to break transmission. In free-roaming equine populations with EIA, stallions appear to have an exceptionally high infection rate, suggesting...

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