Introduction

Viral Replication
Taxonomy

About the Virus

    A disease of Equidae(horses, donkeys, ponies, mules and zebras), Equine Infectious Anemia Virus(EIAV) is a member of the Family Retroviridae, as an enveloped single stranded RNA virus.4  The most important factor of the family Retroviridae is the possession of the replicative enzyme reverse transcriptase.This family of viruses have the ability to convert their single stranded RNA genome into a double stranded DNA molecule for the integration into the host genome via the reverse transcriptase enzyme. This allows the virus to persist in the host indefinitely with or without clinical signs/disease.4

      

The genus lentivirus has become the most intensively studied viruses of all time due to the fact that this genera contains the human immunodeficiency virus(HIV)acquired immunodeficiency syndrome(AIDS) in humans.2  There has been testing of drugs and vaccines on veterinary models in hopes of finding preventative and therapeutic treatments for humans and animals.2  For this reason, there is a lot of interest in animal lentiviruses.


    EIAV has the simplest genetic organization of any lentivirus with the genome acting as a template for the synthesis of m-RNA species that then code for a variety of regulatory, structural and enzymatic functions. There are three major proteins recognized by equids infected with EIAV. These are the major core protein (p26, a gag gene product), and the surface unit and transmembrane proteins (gp90 and gp45, env gene products).4  
   
    EIAV is prone to mutation because of the lack of proof-reading ability by the enzyme reverse transcriptase. A minimum of one error to the virus’s genome occurs during each replication leading to multiple related but variant individual virus populations within one infected equid. This allows the virus to rapidly respond to environmental selection pressure as expressed by the host’s immune system.4

 Reverse Transcriptase

 r

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Viral Replication

    Virus replication occurs in mature tissue macrophages.  This is only a small amount of the virus that is present in the body as there is a huge amount of cell free virus that circulates in the blood.6

    Replication begins with reverse transcription of the virion RNA into a double stranded DNA.  This double stranded DNA is integrated into the host DNA and then used for transcription in the host.2

    The virus will attach to many different cellular receptors depending upon which envelope glycoproteins are present.  Most of the time the virus envelope and the cell membrane fuse which allows the virion core to enter the cytoplasm of a cell, and in fewer instances the entry involves receptor-mediated endocytosis into the cell.2



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Taxonomy


    Genera of Family Retroviridae and represented in Table 1

     Table 1 

Alpharetrovirus

Betaretrovirus

Gammaretrovirus

Deltaretrovirus

Epsilonretrovirus

Lentivirus

Spumavirus

    The properites of Retroviruses include:

        Enveloped virions which are 80-100 nm in diameter.  They have a 3 layered structure: innermost genome, capsid in the middle and an outermost envelople with glycoproteins.
       All have gag, pol and env genes and some acquire an oncogene which usually makes their replication defective.  Lentiviruses have a complex array of up to 6 accessory genes.
       Viral reverse transcriptase which transcribes DNA from virion RNA and then integrates this RNA into cellular DNA as a provirus.
       In productive infections the virion assembles at and buds from the plasma membrane.2

    Species of the Genus Lentivirus are represented in Table 2

    Table 2

Human immunodeficiency viruses 1&2

Simian immunodeficiency virus

Maedi/visna virus

Caprine arthritis-encephalitis virus

Feline immunodeficiency virus

Equine infectious anemia virus

Bovine immunodeficiency virus

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