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VIRUSES



Introduction

Retroviruses are persistent, non-cytopathic, systemic viruses that give rise to secondary disease, such as tumors, immune-complex disease, or immunosuppression.

Morphology

  • Fragile, enveloped RNA viruses with roughly spherical spike proteins
  • Genome has 3 genes:
    • gag: group-specific antigen coding gene, encodes capsid proteins
    • pol: encodes reverse transcriptase (RT) and integrase
    • env: encodes envelope spikes, and can be used in diagnosis and subunit vaccines
  • Both ends of genome show a promoter (LTR: long terminal repeat)

Virulence and Pathogenesis

  • Replication involves integrating into the host cell genome:
    • Uncoating to release RNA and RT
    • +RNA transcribed to -DNA by RT
    • -DNA becomes circular dsDNA and is integrated into host chromosome by integrase
    • DNA codes for viral proteins using cellular organelles and enzymes
    • Because of this technique, virus replication is much slower, and retroviruses can remain latent
  • Tumor production takes 2 forms:
    • Viruses can carry oncogenes within their genome --> tumor production occurs quickly
    • Insertion of the viral LTR switches on proto-oncogenes in the host cell genome --> tumor production can take years

Types and Subtypes

7 Genera, including these of veterinary relevance:

  1. Mammalian Type C retrovirus
  2. Mammalian Type D retrovirus
  3. Avian Type C retrovirus
  4. Spumavirus
    • Feline and bovine syncitial viruses (common, nonpathogenic)
  5. BLV-HTLV retroviruses
    • Bovine Leukosis Virus
  6. Lentiviruses

Antigenicity

  • Group-specific antigens (gag's) are shared by all isolates of each virus
    • This can be exploited by diagnostic tests
  • Lentiviruses show variation by mutation, making vaccination difficult

Retroviruses


Lentiviruses