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)

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

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
   • Feline Leukemia Virus (FeLV)
   • Feline sarcoma virus (rare)
   • Porcine type C oncovirus (rare)
2. Mammalian Type D retrovirus
   • Sheep Pulmonary Adenomatosis
3. Avian Type C retrovirus
   • Avian Leukosis Virus
4. Spumavirus
   • Feline and bovine syncitial viruses (common, nonpathogenic)
5. BLV-HTLV retroviruses
   • Bovine Leukosis Virus
6. Lentiviruses
   • Feline Immunodeficiency Virus (FIV)
   • Bovine Immunodeficiency Virus
   • Visna-Maedi Virus of Sheep
   • Caprine Arthrirtis Encephalitis Virus
   • Equine Infectious Anemia
   • Jembrana of Bali cattle
   • Human Immunodeficiency Virus (HIV)