RETROVIRIDAE. Part II- LENTIVIRUSES

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RETROVIRIDAE. Part II- LENTIVIRUSES

• PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS
• Department of Pathology and Infectious Diseases,
  The Royal Veterinary College,
• Royal College Street,
• London NW1 OTU.
• E-mail Web site

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ObjectiveStudents should be able to

• describe the differing immune-complex or
  dysfunction diseases associated with these
  viruses Visna Maedi and caprine
  arthritis/encephalitis virus, equine infectious
  anaemia, feline immunodeficiency virus (FIV),
  bovine immunodeficiency virus.
• compare and contrast the diagnosis and
  pathogenesis of FIV and FeLV.

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FELINE IMMUNODEFICIENCY VIRUS

• Once considered to be a major cause of
  lymphadenopathy and immunosuppression in cats,
  but now field work shows this is untrue. The
  virus is neither a death sentence to cats. Nor
  zoonotic.

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Pathogenesis and clinical signs
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1)Regressive transient infection with no CD4
decline - subclinical, most UK cases.
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2) Typical, progressive CD4 decline, 3 idealised
stages

• Stage 1. High circulating virus for 1-10 weeks.
  Lymphoid depletion as a result of virus infection
  and destruction of double positive CD4CD8 Tcells.
  Thymic aplasia (most important in kittens).
• Stage 2. Asymptomatic carrier stage with a
  decrease in circulating virus effected by CD8
  cells and polyclonal antibody. Paracortical T
  cells and lymphoid follicles expand in lymph
  nodes, spleen and thymus, this may be visible as
  white nodules.

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3)Rapidly progressing infection with high virus
and rapid depletion of CD4 cells in 40 of cats
after vaginal exposure
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Clinical signs of virulent viruses

• Weight loss, pyrexia, anaemia, stomatitis,
  abortion, chronic rhinitis, recurrent cystitis,
  vomiting, recurrent skin infections, diarrhoea,
  abnormally high level of B cell lymphomas - in
  combinations but not all together.

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Control

• No vaccine. Early experimental vaccines
  exacerbated disease. If a cat is healthy and
  positive get a confirmatory test done at Glasgow.
  If the cat is positive then keep it isolated.
  Addie now considers the cats protection league's
  policy of killing any healthy ELISA positive cat
  to be unnecessary.

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BOVINE IMMUNODEFICIENCY VIRUS

• MAFF consider it to be absent from UK and of
  little clinical relevance.

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VISNA-MAEDI VIRUS (V-M V)(or Maedi-Visna)

• The virus Isolates are neurotropic or
  pneumotropic and cause chronic wasting disease in
  sheep. Variants which escape neutralisation
  arise during the infection (as with HIV and
  Equine Anaemia Virus). Visna Icelandic for
  wasting Maedi Icelandic for dyspnoea.

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Immunity and epidemiology

• Infected animals remain carriers. Transmission
  is via aerosol, milk, or colostrum. The crowded
  6 month winter housing as practised in Iceland
  allowed spread not only of V-MV but also of
  Johne's disease and SPA.
• Infection was introduced into the UK by French
  exotic breeds and carriers occur in one third of
  flocks containing imported sheep (particularly
  Texel).

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CAPRINE ARTHRITIS/ENCEPHALITIS VIRUS

• This is caused by a distinct virus which is
  serologically related to V-MV but has antigenic
  and genetic differences in its envelope. It
  causes an encephalitis of kids or an insidious
  polyarthritis of adults.

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EQUINE INFECTIOUS ANAEMIA (EIA) (Swamp fever)

• EIA is an exotic type III immune-complex
  haemolytic disease of horses of great importance
  in the USA. The disease is notifiable in the UK.
  

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Immunity and epidemiology

• Animals can be virus-carriers for several years
  despite having antibody. Transmission is by the
  mechanical transfer of leucocytes via mosquitoes,
  flies, syringe-needles, semen and milk.

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Control

• UK. Horses can only be imported from virus-free
  premises. However EIA is widespread in damp
  areas of Germany and Italy and so could enter the
  UK.

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Summary of both lectures

• The retroviruses all contain RT and all integrate
  and mutate, but do not cause acute disease. This
  replication results in latency and persistence
  and chronic diseases eg tumours,
  immunosuppression and immune complex disease.
• FeLV, but not FIV, is the most important
  immunosuppressive viral diseases of cats in the
  UK and suspect cats are blood tested for antigen.

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Summary of both lectures(cont.)

• The viruses are labile and cell associated and so
  close contact (eg in cat colonies or winter
  housed sheep), or sources of leucocytes, eg
  blood, milk and semen, transfer them
• Vaccines are not usual except to FeLV.

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Summary of both lectures(cont.)

• Visna maedi virus of sheep is endemic and causes
  a T-cell based disease whereas EIA is exotic and
  causes complement-mediated haemolysis. Both
  involve bouts of variant viruses and immune
  complex disease.
• The bovine viruses are of little importance in
  the UK to date.