The aetiological agent of Visna is an exogenous retrovirus which replicates
by means of a proviral DNA intermediate (Haase & Varmus, I973). The virus replicates
in sheep cell cultures causing acute cytopathic effects (c.p.e.). In contrast, virus replication
is slow in sheep. This limitation of replication in vivo could not be overcome by infection
of foetal lambs or infection of lambs immunosuppressed by foetal thymectomy or treatment
with antilymphocyte serum