Pathogenesis and clinical signs:
V- MV undergoes a primary replication in lung macrophages. which carry virus to the target organs, the brain, lung tissue, udder and/or joints according to the isolate.
Target organs become chronically inflamed after 2- 6 years of T cell responses to bouts of replication and the sheep rapidly lost condition and fall behind during flock movements.
In Visna inflammation results in demyelination with subacute meningitis around the ventricles and choroid plexus. Posterior paresis progresses for up to one year when the sheep can no longer stand.
In Maedi the alveolar septa become infiltrated by lymphocytes and macrophages and the smooth muscle becomes hypertrophic (compensatory hypertrophy). Sheep become increasingly dyspnoeic and deaths occur after 6 months. At post-mortem the lungs are heavy, rubbery and do not collapse.