Week 11

• Retroviridae characteristics

  enveloped RNA viruses formed from budding through the cell membrane

  Diploid genome: two copies of single stranded, positive sense RNA

  Integrate into genome of host (integrase) - may alter or acquire host sequences or may activate or inactivate particular host genes located near integration site
• Retroviridae morphology

  Spherical virions surrounded by envelope consisting of lipid membrane bilayer, surface studded by projections of envelope glycoprotein

  Spherical layer of protein beneath envelope

  Internal capsid protein (nucleocapsid) encloses RNA (reverse transcriptase, integrase, protease)
• Retroviridae virion properties

  Easily inactivated by lipid solvents or detergents and by heating

  Resistant to UV or xrays

  Gag gene encodes virus core proteins (capsid, nucleoprotein, matrix)

  Pol gene encodes reverse transcriptase and integrase

  Env gene (virion envelope proteins, surface, transmembrane)

  Pro gene (protease)
• Retroviridae replication

  1. Receptor binding and membrane fusion
  2. Internalization and uncoating 
  3. Reverse transcription of RNA genome (Form linear dsDNA)
  4. Nuclear entry of dsDNA 
  5. Integration of linear dsDNA into host genome to form provirus 
  6. Transcription of provirus to form viral RNA 
  7. Splicing and nuclear export of RNA to cytoplasm 
  8. Translation of RNA to form precursor proteins 
  9. Assembly of virions and packaging of viral RNA genome 
  10. Budding and release of the virions 
  11. Proteolytic and processing of precursors and maturation of the virions
• Outcomes of retrovirus infection

  Transformation of malignancy

  Chronic infection

  Latent infection

  Budding or lysis
• Acute transformation

  Retroviruses contain viral oncogenes
• Chronic transformation

  Retroviruses insert/mutate through random integration into genome of the host
• Oncogenesis

  Cellular proto-oncogenes are required for normal cell grwoth and differentiations

  Retroviruses acquire proto-oncogenes during replication and become oncogenes

  Undergo mutations, usually remove regulatory mechanisms

  Lead to transformation of infected cells by acting as growth factor, receptor, intracellular signal transducer, intracellular transcription factor
• Alpharetrovirus

  Simple retroviruses

  Genome contains gag, pro, pol, and env genes

  Mostly endogenous (transmitted horizontally) and exogenous (transmitted vertically) viruses of chicken

  Avian leukosis, Sarcoma virus, Avian myeloblastosis virus, Rous sarcoma virus
• Gammaretrovirus

  Simple retroviruses

  Leukemia virus, Feline leukemia virus, Feline sarcoma virus, Avian reticuloendotheliosis virus

  Contain endogenous and exogenous retroviruses of mammals and reptiles
• Deltaretrovirus

  Complex retroviruses

  Human T-lymphotropic virus

  Bovine leukemia virus
• Epsilonretrovirus

  Complex retroviruses

  Walley derma sarcoma virus

  Endogenous retroviruses found in fish and reptiles
• Lentivirus

  Complex retroviruses (cylindrical or conical shaped core)

  Slow viruses

  Human immunodeficiency virus

  Feline immunodeficiency virus

  Caprine arthritis-encephalitis virus

  Maedi-visna virus
• Feline Leukemia virus

  Gammaretrovirus affecting domestic cats worldwide

  Genome contains envelope gene (glycoprotein gp70, transmembrane protein p15e), polymerase gene (reverse transcriptase, protease, integrase), and group specific gene (p27 - structural component)
• Epidemiology of feline leukemia virus

  Doesn't survive long outside the host (detroyed readily by disinfectants, soaps, heating, and drying)

  Prevalence low in individually kept cats, multicat households FeLV prevalence >20%

  Cats with FeLV viremia shed infection in saliva, nasal secretions, feces, and milk

  Transmitted through mutual grooming or bites

  Risk factors - young age, high population density, poor hygiene

  Viremic queens - pregnancy results in embryonic death, stillbirth, viremic kittens
• Pathogenesis of FeLV

  SU protein is the main determinant of pathogenicity

  Infection through oral or pharyngeal associated tissue

  Spreads to peripheral tissues through monocytes and lymphocytes

  Viremia develops within few weeks of infection

  Tropism to lymphoid tissue leads to immunosuppression, defect in T cells, and low or no cellular immunity

  Wasting syndrome and atrophy of the thymus (recurrent infections)

  May atrophy or hyperplasia of lymphoid tissue

  Reduced IgM, but normal IgG

  Antibodies lead to antibody-antigen complex deposits -> glomerulonephritis
• p15e

  Blocks T cell reactivity to IL-2 and IL-12

  Blocks production oh hematopoietic cells leading to anemia
• Outcomes of FeLV infections

  Abortive infections

  Regressive infection

  Progressive infection
• FeLV clinical signs

  Immunosuppression

  Regenerative anemia (rarely): secondary opportunistic infections

  Non-regenerative anemia: chronic inflammatory mechanisms, myelodestruction, myelosuppresion, myeloproliferative disease

  Lymphoma

  Leukemia

  bites wounds, swollen lymph nodes, dehydration, gingivitis, abscesses, fever, weight loss, neurological signs
• FeLV diagnosis

  p27 ELISA, Immunofluorescence assay

  Virus isolation in cell culture

  Real-time PCR - detection of provirus

  Reverse transcriptase PCR - detection of viral RNA

  Serology - antibodies measured, results difficult to interpret (antibodies to endogenous FeLV)
• What cats should be tested for FeLV?

  Sick cats, even if tested negative in the past

  Cats with known recent exposure to FeLV infected cats or unknown status

  Cats living in households with other cats infected with FeLV (on an annual basis unless isolated)
• Management of FeLV infected cats

  Strictly confined indoors to prevent spread to other cats and minimize potential exposure of infected cats to other infectious agents
• No FeLV vaccines provide complete protection or prevent infection
• Feline Immunodeficiency virus

  Lentivirus

  Long incubation period

  Attacks immune system and weakens immune response to infections and cancers

  2.5-5% of healthy cats in US are infected

  Cats are vulnerable at any age

  Outdoor, free-roaming cats are greater risk (males), rare in catteries

  Infects domestic and wild cats
• Transmission of feline immunodeficiency virus

  Primarily deep bite wounds and scratches (infected saliva enters the other cats bloodstream)

  Sexual contact

  During passage through birth canal; in utero or milk (very rare)
• FIV pathogenesis

  1. Replicates in lymphocytes (CD4+ T cells, B cells, macrophages)
  2. reduction of IL-2
  3. Lymphopenia
  4. FIV activated CD4+/CD25+ T reg cells deepens immunosuppresion
  5. Increased IL-10 production -> immunosuppression
• FIV clinical signs

  • Persistent fever 
  • Loss of appetite 
  • Lethargy 
  • Gingivitis, stomatitis 
  • Chronic or recurrent skin, urinary bladder, and upper respiratory tract infections 
  • Persistent diarrhea, Poor coats condition
  • Feline lymphocytic-plasmacytic gingivostomatitis 
  • Slow but progressive weight loss, severe wasting (late in disease process) 
  • Cancer, blood diseases may develop, abortions 
  • Seizures, behavior changes
• FIV diagnostic tests

  ELISA: detects antigens and anti-FIV antibodies, most common, rapid and reliable, doesn't work well with very early detection

  Immunoblotting: use antibodies to detect

  PCR: detects viral RNA, check for false positives, useful for detecting infections in kittens born from FIV positive mothers
• FIV treatment

  No treatment to eliminate the virus

  Depends on proper health management

  Some antiviral medications, not 100% effective in improving immune system

  Treat systemically: antibiotics, appetite stimulants, immunomodulators, dental extractions

  Nutritional support and fluid therapy
• FIV preventions

  Isolate infected cats

  Spaying and neutering outdoors cats (decreased aggressive behavior)
• FIV vaccination

  Does not provide full protection against all strains

  FIV antibody positive