Visna-Maedi Virus
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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