Visna-Maedi Virus

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VIRUSES
RETROVIRIDAE



Pathogenesis

  • Primary replication in lung macrophages
  • These then carry infection to brain, lung, udder and/or joints
  • T cells react: Type IV immune complex disease
  • Target organs become chronically inflamed after 2-6 years
  • Visna:
    • Inflammation results in demyelination with subacute meningitis around ventricles and choroid plexus
    • Posterior paresis progresses for up to a year until sheep can no longer stand
  • Maedi:
    • Alveolar septa become infiltrated by lymphocytes and macrophages
    • Smooth muscle hypertrophy
    • PM: lungs are heavy, rubbery and do not collapse (for more, see here)
  • All result in loss of condition
  • Infected animals remain antibody and virus positive

Epidemiology

  • Transfer via aerosol, milk or colostrum
  • Increased risk in winter housing

Diagnosis

  • Clinical signs
  • Serology: AGDT/ELISA for antibody

Control

  • Seropositive and progeny must be removed from flock
  • Farm can be re-accredited after 2 years of clear tests