Visna-Maedi Virus

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
  • For more on joint pathology, see here
• 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