Experimental visna in foetal Icelandic sheep.

Visna is a chronic neurological disease of sheep caused by a slow infection with a naturally occurring ovine retrovirus. The present study was focused on the influence of age on the pathogenesis of this infection and involved intrauterine inoculation of 7 sheep foetuses at about 90 days of gestation with slaughter at birth, 6 to 7 weeks later. The results were compared with prior observations in newborn lambs and adult sheep infected with the same visna virus strain.Pathological lesions of the CNS in foetal sheep were of similar character, localization and severity as those in both newborn and adult sheep. The distribution of virus, the generally low titres of free infectious virus, and the requirement for explantation to isolate virus in some instances, were similar in foetal and in older sheep. However, the more frequent virus isolations from foetal and newborn sheep indicate a slightly greater permissiveness for virus replication. In general, age apparently has little effect on the course of this slow infection.